WEBVTT 00:00:00.000 --> 00:00:03.970 align:middle line:84% GERALD SCHNEIDER: OK, what were we talking about last time? 00:00:03.970 --> 00:00:05.130 align:middle line:90% Neuronal migration. 00:00:05.130 --> 00:00:17.710 align:middle line:90% 00:00:17.710 --> 00:00:22.900 align:middle line:84% And we talked about the shepherd's crook cell. 00:00:22.900 --> 00:00:28.360 align:middle line:84% Let's just-- I won't load that slide. 00:00:28.360 --> 00:00:33.850 align:middle line:84% Why was it so important to find that cell that 00:00:33.850 --> 00:00:39.530 align:middle line:84% could be identified at such early stages? 00:00:39.530 --> 00:00:41.470 align:middle line:84% You see, some people didn't believe 00:00:41.470 --> 00:00:48.310 align:middle line:84% that you could get nuclear translocation in the midbrain 00:00:48.310 --> 00:00:52.480 align:middle line:84% when the thickness of the neural tube was fairly grayed. 00:00:52.480 --> 00:00:55.880 align:middle line:84% They thought you would need radial glial guidance. 00:00:55.880 --> 00:01:01.480 align:middle line:84% So what other explanation could there be for those pictures? 00:01:01.480 --> 00:01:04.420 align:middle line:84% Well, with the Golgi method, it's 00:01:04.420 --> 00:01:09.970 align:middle line:84% possible that they could be staining glia and then 00:01:09.970 --> 00:01:14.050 align:middle line:84% seeing bumps on the glia, misinterpreting the pictures. 00:01:14.050 --> 00:01:15.970 align:middle line:84% They had to be able to reconstruct 00:01:15.970 --> 00:01:17.050 align:middle line:90% the entire sequence. 00:01:17.050 --> 00:01:19.090 align:middle line:84% And remember what they did, they took-- 00:01:19.090 --> 00:01:20.920 align:middle line:84% if this was the surface of the tectum, 00:01:20.920 --> 00:01:23.260 align:middle line:90% this is the ventricular layer. 00:01:23.260 --> 00:01:27.140 align:middle line:84% And here's one of these elongated cells. 00:01:27.140 --> 00:01:30.430 align:middle line:90% Here's a cell body. 00:01:30.430 --> 00:01:33.475 align:middle line:84% Then very early on they saw that-- 00:01:33.475 --> 00:01:38.150 align:middle line:90% 00:01:38.150 --> 00:01:41.030 align:middle line:84% sorry, I should start here, and where they started. 00:01:41.030 --> 00:01:43.760 align:middle line:90% 00:01:43.760 --> 00:01:44.810 align:middle line:90% Let's do that. 00:01:44.810 --> 00:01:47.770 align:middle line:90% 00:01:47.770 --> 00:01:49.720 align:middle line:84% The adult shepherd's crook cell is 00:01:49.720 --> 00:01:53.380 align:middle line:84% a neuron here with an axon that goes out like that. 00:01:53.380 --> 00:01:56.120 align:middle line:84% It always has that characteristic. 00:01:56.120 --> 00:01:59.230 align:middle line:84% It's not clear why it goes up a little bit before it goes out. 00:01:59.230 --> 00:02:03.190 align:middle line:84% But it always has the shepherd's crook. 00:02:03.190 --> 00:02:05.200 align:middle line:84% I'm not drawing the dendrites and everything. 00:02:05.200 --> 00:02:07.630 align:middle line:84% Because what's critical about identifying that cell 00:02:07.630 --> 00:02:11.000 align:middle line:90% was the shape of the axon. 00:02:11.000 --> 00:02:14.080 align:middle line:90% So that was the adult, OK. 00:02:14.080 --> 00:02:16.600 align:middle line:84% And then they went back successively 00:02:16.600 --> 00:02:20.920 align:middle line:90% to earlier stages [INAUDIBLE]. 00:02:20.920 --> 00:02:24.730 align:middle line:84% And they saw that a cell with the shepherd's crook. 00:02:24.730 --> 00:02:28.030 align:middle line:84% But then they saw the connection there. 00:02:28.030 --> 00:02:30.040 align:middle line:84% It might have disconnected from the ventricle 00:02:30.040 --> 00:02:33.170 align:middle line:90% already at that stage. 00:02:33.170 --> 00:02:39.160 align:middle line:84% And then they went back a little bit earlier and they still 00:02:39.160 --> 00:02:40.810 align:middle line:90% of the shepherd's crook. 00:02:40.810 --> 00:02:46.030 align:middle line:84% And then the cell body was down here and it was connected here. 00:02:46.030 --> 00:02:48.910 align:middle line:84% And then even earlier, it looks like the first one 00:02:48.910 --> 00:02:49.690 align:middle line:90% I drew there. 00:02:49.690 --> 00:02:53.080 align:middle line:90% So they went backwards in time. 00:02:53.080 --> 00:02:56.890 align:middle line:84% They couldn't have been always mistaking 00:02:56.890 --> 00:02:58.120 align:middle line:90% this for a glial cell. 00:02:58.120 --> 00:03:01.060 align:middle line:90% 00:03:01.060 --> 00:03:04.210 align:middle line:90% So that was the idea. 00:03:04.210 --> 00:03:07.630 align:middle line:84% In other words, the mechanism of migration 00:03:07.630 --> 00:03:10.510 align:middle line:84% in the chick midbrain for that particular cell 00:03:10.510 --> 00:03:12.520 align:middle line:90% was nuclear translocation. 00:03:12.520 --> 00:03:25.380 align:middle line:90% 00:03:25.380 --> 00:03:28.090 align:middle line:90% OK, so that's where we left off. 00:03:28.090 --> 00:03:30.150 align:middle line:84% Now let's talk about cell migration 00:03:30.150 --> 00:03:32.430 align:middle line:84% in the neocortex in the cerebellum 00:03:32.430 --> 00:03:36.660 align:middle line:84% where nuclear translocation is not the mechanism. 00:03:36.660 --> 00:03:38.370 align:middle line:84% There are many similarities though 00:03:38.370 --> 00:03:39.675 align:middle line:90% in what the cells are doing. 00:03:39.675 --> 00:03:46.030 align:middle line:90% 00:03:46.030 --> 00:03:51.510 align:middle line:84% But it's pretty clear now that in the neocortex when 00:03:51.510 --> 00:03:55.350 align:middle line:84% cells migrate, they move along radial glial cells. 00:03:55.350 --> 00:03:58.230 align:middle line:84% And many of them move along the same radial glia. 00:03:58.230 --> 00:04:00.570 align:middle line:84% They go up to a structure called the cortical plate. 00:04:00.570 --> 00:04:02.230 align:middle line:90% We'll look at pictures of that. 00:04:02.230 --> 00:04:08.760 align:middle line:84% And finally you get the six layers of the neocortex. 00:04:08.760 --> 00:04:14.550 align:middle line:84% And they show an inside out pattern of movement 00:04:14.550 --> 00:04:18.750 align:middle line:84% as seen with these labeling methods. 00:04:18.750 --> 00:04:21.959 align:middle line:84% They were actually known before it was really verified. 00:04:21.959 --> 00:04:26.190 align:middle line:84% I put verified rather than demonstrated initially, 00:04:26.190 --> 00:04:28.200 align:middle line:84% because these patterns were known 00:04:28.200 --> 00:04:32.610 align:middle line:84% before the autoradiography methods. 00:04:32.610 --> 00:04:34.260 align:middle line:84% But they were more tedious methods. 00:04:34.260 --> 00:04:37.080 align:middle line:84% They used, for example, vitamin A poisoning 00:04:37.080 --> 00:04:38.190 align:middle line:90% at different stages. 00:04:38.190 --> 00:04:44.610 align:middle line:84% And vitamin A in high doses will kill the immature cells. 00:04:44.610 --> 00:04:47.040 align:middle line:84% So they could wipe out whole layers of neocortex 00:04:47.040 --> 00:04:48.830 align:middle line:90% when they were developing. 00:04:48.830 --> 00:04:52.770 align:middle line:84% You can do the same thing with heat application. 00:04:52.770 --> 00:04:55.080 align:middle line:84% It's not intense enough to kill every cell. 00:04:55.080 --> 00:04:58.800 align:middle line:84% But it'll kill the developing ones. 00:04:58.800 --> 00:05:01.230 align:middle line:84% But it was these labeling methods that were the clearest. 00:05:01.230 --> 00:05:04.470 align:middle line:90% 00:05:04.470 --> 00:05:08.940 align:middle line:84% So this is a picture from the [INAUDIBLE] graduate textbook. 00:05:08.940 --> 00:05:15.660 align:middle line:90% 00:05:15.660 --> 00:05:25.880 align:middle line:84% It's just depicting an early stage of cortical development 00:05:25.880 --> 00:05:28.860 align:middle line:84% in the middle of the cell migration period. 00:05:28.860 --> 00:05:32.270 align:middle line:84% The hamster is born when this is still happening. 00:05:32.270 --> 00:05:34.940 align:middle line:84% In fact, this is about the stage they're born at. 00:05:34.940 --> 00:05:36.740 align:middle line:84% OK, then they're showing a column there 00:05:36.740 --> 00:05:42.500 align:middle line:84% through the neural tube in the left hemisphere of the brain. 00:05:42.500 --> 00:05:45.800 align:middle line:84% They're showing that neurons can be 00:05:45.800 --> 00:05:48.860 align:middle line:84% seen moving along those blue cells, which 00:05:48.860 --> 00:05:50.780 align:middle line:90% are the glial cells, OK. 00:05:50.780 --> 00:05:57.680 align:middle line:90% 00:05:57.680 --> 00:06:00.710 align:middle line:90% So here you see this cell. 00:06:00.710 --> 00:06:01.940 align:middle line:90% And here you see the blowup. 00:06:01.940 --> 00:06:04.100 align:middle line:84% And then you see the blue cell there 00:06:04.100 --> 00:06:07.280 align:middle line:84% which is attached here at the pia. 00:06:07.280 --> 00:06:08.630 align:middle line:90% And it's-- I'm sorry-- 00:06:08.630 --> 00:06:11.090 align:middle line:84% here at the ventricle and here at the pia. 00:06:11.090 --> 00:06:13.670 align:middle line:84% The cell body for that radial glial cell 00:06:13.670 --> 00:06:16.490 align:middle line:84% is down here in the ventricular layer. 00:06:16.490 --> 00:06:21.550 align:middle line:90% 00:06:21.550 --> 00:06:25.390 align:middle line:84% OK, now that mitoses are happening here 00:06:25.390 --> 00:06:27.950 align:middle line:90% at the ventricle. 00:06:27.950 --> 00:06:29.660 align:middle line:84% But not only right in the ventricle, 00:06:29.660 --> 00:06:33.610 align:middle line:84% but also in a layer just deep to the ventricle where cells have, 00:06:33.610 --> 00:06:36.070 align:middle line:84% even when they've detached from the ventricle, 00:06:36.070 --> 00:06:39.370 align:middle line:84% you get another layer of mitotic cells there. 00:06:39.370 --> 00:06:42.490 align:middle line:84% That's called a subventricular layer. 00:06:42.490 --> 00:06:45.490 align:middle line:84% The artist just couldn't-- he just assumed, well, 00:06:45.490 --> 00:06:46.510 align:middle line:90% this is above this. 00:06:46.510 --> 00:06:48.400 align:middle line:84% This must be the subventricular layer. 00:06:48.400 --> 00:06:51.370 align:middle line:84% He wasn't-- because he didn't know, 00:06:51.370 --> 00:06:54.540 align:middle line:90% and somebody didn't check it. 00:06:54.540 --> 00:06:56.550 align:middle line:84% So I'm pointing out, a lot of textbooks 00:06:56.550 --> 00:06:59.490 align:middle line:90% make mistakes in anatomy. 00:06:59.490 --> 00:07:05.100 align:middle line:84% OK, this is from a study at Columbia University 00:07:05.100 --> 00:07:09.120 align:middle line:84% where they were first able to see migration in tissue culture 00:07:09.120 --> 00:07:11.220 align:middle line:84% with movement in the cell lung and glial cell. 00:07:11.220 --> 00:07:15.150 align:middle line:84% And here you have, see that it's moving along. 00:07:15.150 --> 00:07:17.430 align:middle line:84% In the 31 minutes they were looking at it here, 00:07:17.430 --> 00:07:18.540 align:middle line:90% it moved that far. 00:07:18.540 --> 00:07:22.010 align:middle line:90% 00:07:22.010 --> 00:07:30.140 align:middle line:84% OK, so this now is showing different stages of development 00:07:30.140 --> 00:07:31.340 align:middle line:90% in the mouse or rat. 00:07:31.340 --> 00:07:37.430 align:middle line:84% I think this is probably rat, although they're very similar. 00:07:37.430 --> 00:07:41.510 align:middle line:84% So VZ here means ventricular zone. 00:07:41.510 --> 00:07:44.180 align:middle line:84% So the neural tube is very thin here. 00:07:44.180 --> 00:07:46.070 align:middle line:84% It's basically a ventricular zone 00:07:46.070 --> 00:07:50.030 align:middle line:84% in a little bit of marginal zone there. 00:07:50.030 --> 00:07:53.630 align:middle line:84% And you see they're depicting mitosis happening there 00:07:53.630 --> 00:07:54.500 align:middle line:90% at the ventricle. 00:07:54.500 --> 00:07:59.620 align:middle line:90% 00:07:59.620 --> 00:08:03.610 align:middle line:84% Then a little later you see some fibers appearing here 00:08:03.610 --> 00:08:07.030 align:middle line:84% in an intermediate zone and some cells appearing up here. 00:08:07.030 --> 00:08:08.050 align:middle line:90% There's just a few. 00:08:08.050 --> 00:08:11.350 align:middle line:84% They call that the pre-plate period. 00:08:11.350 --> 00:08:13.330 align:middle line:84% There's a few cells right at the top. 00:08:13.330 --> 00:08:18.730 align:middle line:84% But still, the main thing going on is a lot of mitoses here. 00:08:18.730 --> 00:08:21.160 align:middle line:84% The cell division is symmetrical. 00:08:21.160 --> 00:08:25.660 align:middle line:84% So one stem cells becoming two stem cells, 00:08:25.660 --> 00:08:27.850 align:middle line:84% becoming four stem cells, and so forth. 00:08:27.850 --> 00:08:32.860 align:middle line:84% And the longer that happens, the bigger the area of cortex. 00:08:32.860 --> 00:08:37.720 align:middle line:84% So in species like us, when we have such a large neocortex, 00:08:37.720 --> 00:08:43.210 align:middle line:84% that stage is going to go along a lot longer 00:08:43.210 --> 00:08:45.070 align:middle line:90% and the cortex will become-- 00:08:45.070 --> 00:08:46.000 align:middle line:90% increase in area. 00:08:46.000 --> 00:08:49.460 align:middle line:90% 00:08:49.460 --> 00:08:53.380 align:middle line:84% OK, now when you get asymmetric division and one cell 00:08:53.380 --> 00:09:00.520 align:middle line:84% becomes post-mitotic, it attaches to an elongated cell 00:09:00.520 --> 00:09:03.400 align:middle line:84% that has now been, it's been shown 00:09:03.400 --> 00:09:06.850 align:middle line:84% pretty convincingly is not a neuron but a glial cell, 00:09:06.850 --> 00:09:08.980 align:middle line:84% just like they showed in the previous picture. 00:09:08.980 --> 00:09:11.830 align:middle line:84% Here you see they're depicting a glial cell 00:09:11.830 --> 00:09:16.840 align:middle line:84% with a cell migrating along it here and another one up here. 00:09:16.840 --> 00:09:19.390 align:middle line:90% 00:09:19.390 --> 00:09:22.510 align:middle line:84% There's not one radial glial cell for every neuron. 00:09:22.510 --> 00:09:26.740 align:middle line:84% There's a lot of neurons are using the same cells. 00:09:26.740 --> 00:09:30.040 align:middle line:84% But basically all the cells that will end up in one column 00:09:30.040 --> 00:09:33.340 align:middle line:84% through the cortex are moving along one 00:09:33.340 --> 00:09:34.480 align:middle line:90% of the radial glial cells. 00:09:34.480 --> 00:09:37.220 align:middle line:90% 00:09:37.220 --> 00:09:42.020 align:middle line:84% The intermediate zone is acquiring more axons here. 00:09:42.020 --> 00:09:45.170 align:middle line:84% And that will become the white matter of the cortex 00:09:45.170 --> 00:09:46.865 align:middle line:90% of the ventricular layer. 00:09:46.865 --> 00:09:51.050 align:middle line:90% 00:09:51.050 --> 00:09:54.800 align:middle line:84% The cells migrate up to this bunch of cells here. 00:09:54.800 --> 00:09:56.460 align:middle line:84% And that's called the cortical plate. 00:09:56.460 --> 00:10:01.700 align:middle line:84% That's the developing layers of the neocortex. 00:10:01.700 --> 00:10:03.710 align:middle line:84% That's a little later going to end up 00:10:03.710 --> 00:10:06.380 align:middle line:84% like this where you just have a thin ventricular 00:10:06.380 --> 00:10:08.630 align:middle line:84% layer-- they're not even showing the cells here-- then 00:10:08.630 --> 00:10:12.320 align:middle line:84% the white matter, then the layers of cells in the cortex. 00:10:12.320 --> 00:10:16.000 align:middle line:90% 00:10:16.000 --> 00:10:18.970 align:middle line:84% They're showing largest pyramidal cell layer 6. 00:10:18.970 --> 00:10:23.890 align:middle line:84% It would actually be in layer 5, smaller cells here 00:10:23.890 --> 00:10:28.300 align:middle line:84% in layer 4, more pyramidal cells and granule cells in layers 2 00:10:28.300 --> 00:10:30.430 align:middle line:84% and 3, and very few cells up in layer 1, 00:10:30.430 --> 00:10:34.152 align:middle line:84% but lots of dendrites and axonal connections. 00:10:34.152 --> 00:10:35.860 align:middle line:84% But they occur throughout all the layers. 00:10:35.860 --> 00:10:38.870 align:middle line:90% 00:10:38.870 --> 00:10:41.450 align:middle line:84% OK, now the other interesting thing about this 00:10:41.450 --> 00:10:46.940 align:middle line:84% is that when these cells move up, they don't end up-- 00:10:46.940 --> 00:10:48.980 align:middle line:84% they're not dropped off down here. 00:10:48.980 --> 00:10:52.220 align:middle line:84% They move past all the other cells in the cortical plate 00:10:52.220 --> 00:10:54.920 align:middle line:90% and they end up at the top. 00:10:54.920 --> 00:10:59.310 align:middle line:84% So the latest born cells are more superficial. 00:10:59.310 --> 00:11:02.450 align:middle line:84% And that's why we call it an inside out 00:11:02.450 --> 00:11:03.920 align:middle line:90% pattern of migration. 00:11:03.920 --> 00:11:06.890 align:middle line:84% The cells on the inside here of the cortical plate 00:11:06.890 --> 00:11:09.740 align:middle line:84% were the first ones born, the first ones 00:11:09.740 --> 00:11:11.870 align:middle line:90% to reach the cortical plate. 00:11:11.870 --> 00:11:14.030 align:middle line:84% And the later ones pile up on top. 00:11:14.030 --> 00:11:17.408 align:middle line:90% 00:11:17.408 --> 00:11:19.325 align:middle line:84% That's what we mean by the inside out pattern. 00:11:19.325 --> 00:11:23.500 align:middle line:90% 00:11:23.500 --> 00:11:26.770 align:middle line:84% So that means in the cortex here, 00:11:26.770 --> 00:11:29.920 align:middle line:84% layer 6 cells where the first born. 00:11:29.920 --> 00:11:32.530 align:middle line:84% And we could see that in audioradiographic pictures 00:11:32.530 --> 00:11:36.520 align:middle line:84% by labeling, injecting tritiated thymidine to label 00:11:36.520 --> 00:11:42.220 align:middle line:84% mitotic cells [INAUDIBLE] inject the pregnant female 00:11:42.220 --> 00:11:44.980 align:middle line:84% or inject her fetus with the tritiated thymidine 00:11:44.980 --> 00:11:46.130 align:middle line:90% at different stages. 00:11:46.130 --> 00:11:48.640 align:middle line:84% And if you do it very, very early and then just 00:11:48.640 --> 00:11:51.490 align:middle line:84% wait until the pups are born and they grow up, 00:11:51.490 --> 00:11:57.070 align:middle line:84% you will see the radioactive cells here in layer 6. 00:11:57.070 --> 00:12:00.100 align:middle line:84% And if you do it very late, in the hamster would 00:12:00.100 --> 00:12:03.220 align:middle line:84% have to be postnatal, you would see them up here 00:12:03.220 --> 00:12:04.450 align:middle line:90% in layers 2 and 3. 00:12:04.450 --> 00:12:08.090 align:middle line:90% 00:12:08.090 --> 00:12:10.840 align:middle line:90% Excuse me? 00:12:10.840 --> 00:12:18.330 align:middle line:84% MZ means marginal zone, the most superficial layer 00:12:18.330 --> 00:12:19.170 align:middle line:90% of the neural tube. 00:12:19.170 --> 00:12:24.690 align:middle line:90% 00:12:24.690 --> 00:12:29.130 align:middle line:84% OK, so now let's look at a slightly different thing that 00:12:29.130 --> 00:12:32.190 align:middle line:84% happens in the cerebellar cortex. 00:12:32.190 --> 00:12:34.720 align:middle line:84% We'll look at the proliferative cells 00:12:34.720 --> 00:12:36.870 align:middle line:84% and the post-mitotic granule cells. 00:12:36.870 --> 00:12:41.400 align:middle line:90% 00:12:41.400 --> 00:12:47.740 align:middle line:84% Now, this is-- and they have taken these-- 00:12:47.740 --> 00:12:51.550 align:middle line:90% we mentioned this briefly from-- 00:12:51.550 --> 00:12:54.710 align:middle line:90% OK, this is the new slide. 00:12:54.710 --> 00:12:59.300 align:middle line:84% Then I'll repeat pictures from an earlier lecture. 00:12:59.300 --> 00:13:02.320 align:middle line:84% The cells move from this specialized region 00:13:02.320 --> 00:13:03.160 align:middle line:90% of the inner plate. 00:13:03.160 --> 00:13:04.460 align:middle line:90% I call it the [INAUDIBLE]. 00:13:04.460 --> 00:13:06.250 align:middle line:84% It's a thick area, looks like a lip when 00:13:06.250 --> 00:13:09.760 align:middle line:90% you look at it in dissection. 00:13:09.760 --> 00:13:13.480 align:middle line:84% And they migrate into the roof plate. 00:13:13.480 --> 00:13:17.570 align:middle line:84% And they form a proliferative layer there. 00:13:17.570 --> 00:13:19.360 align:middle line:84% So that's different from the [INAUDIBLE].. 00:13:19.360 --> 00:13:21.280 align:middle line:84% It's a little bit like the cell ventricular, where 00:13:21.280 --> 00:13:22.820 align:middle line:84% cells move away from the ventricle, 00:13:22.820 --> 00:13:24.640 align:middle line:90% but they remain proliferative. 00:13:24.640 --> 00:13:27.580 align:middle line:84% But they're there right next to the ventricular layer. 00:13:27.580 --> 00:13:29.810 align:middle line:84% Here they're moving further away. 00:13:29.810 --> 00:13:32.665 align:middle line:84% But they keep undergoing mitosis. 00:13:32.665 --> 00:13:36.450 align:middle line:90% 00:13:36.450 --> 00:13:39.000 align:middle line:90% And they really proliferate. 00:13:39.000 --> 00:13:43.080 align:middle line:84% I mean, those granule cells end up-- 00:13:43.080 --> 00:13:46.950 align:middle line:90% 00:13:46.950 --> 00:13:48.480 align:middle line:84% enormous numbers of them, they're 00:13:48.480 --> 00:13:50.610 align:middle line:84% probably as many granule cells in the cerebellum 00:13:50.610 --> 00:13:52.193 align:middle line:84% as there are in the rest of the brain. 00:13:52.193 --> 00:13:54.960 align:middle line:90% 00:13:54.960 --> 00:13:57.240 align:middle line:84% OK, so this is the earlier picture. 00:13:57.240 --> 00:14:00.640 align:middle line:84% If you look here, this is the earlier picture, 00:14:00.640 --> 00:14:03.540 align:middle line:84% the neural tube that developed early in development where 00:14:03.540 --> 00:14:06.030 align:middle line:84% you see the inner plate and basal plate here, 00:14:06.030 --> 00:14:09.810 align:middle line:84% and the stretched out roof plate of a-- typical of hindbrain. 00:14:09.810 --> 00:14:14.340 align:middle line:84% The layer of proliferative cells that we call [INAUDIBLE] 00:14:14.340 --> 00:14:15.000 align:middle line:90% is there. 00:14:15.000 --> 00:14:18.000 align:middle line:84% And cells are migrating into the roof plate, 00:14:18.000 --> 00:14:19.548 align:middle line:84% and other cells are migrating down. 00:14:19.548 --> 00:14:21.090 align:middle line:84% The ones that migrate down, remember, 00:14:21.090 --> 00:14:23.130 align:middle line:90% become cells of the pons. 00:14:23.130 --> 00:14:28.137 align:middle line:84% They also become the inferior [INAUDIBLE] structure. 00:14:28.137 --> 00:14:29.970 align:middle line:84% There's several structures that they become. 00:14:29.970 --> 00:14:32.310 align:middle line:84% We're concerned right now with these that move 00:14:32.310 --> 00:14:34.380 align:middle line:90% into the roof plate, OK? 00:14:34.380 --> 00:14:41.850 align:middle line:90% 00:14:41.850 --> 00:14:44.210 align:middle line:84% This is the location that we're talking about, 00:14:44.210 --> 00:14:47.090 align:middle line:84% the rostral end of the hindbrian. 00:14:47.090 --> 00:14:52.010 align:middle line:84% And when animals like hamsters are born, 00:14:52.010 --> 00:14:54.050 align:middle line:84% the whole cerebellum is very small. 00:14:54.050 --> 00:14:55.580 align:middle line:84% I should show you some more pictures 00:14:55.580 --> 00:15:02.060 align:middle line:84% of that, because most of the development is postnatal. 00:15:02.060 --> 00:15:04.040 align:middle line:84% Now, this is a picture from Cajal. 00:15:04.040 --> 00:15:05.990 align:middle line:84% And here's one of the things he saw. 00:15:05.990 --> 00:15:09.830 align:middle line:84% If he looked at the cerebellar cortex real early on, 00:15:09.830 --> 00:15:15.350 align:middle line:84% it was not only much thinner, but he saw a dense layer 00:15:15.350 --> 00:15:17.840 align:middle line:90% of cells at the surface. 00:15:17.840 --> 00:15:20.240 align:middle line:84% And he knew from studies of the cerebellum 00:15:20.240 --> 00:15:23.430 align:middle line:84% that those weren't there in the adult. 00:15:23.430 --> 00:15:27.530 align:middle line:84% This was something peculiar to the developing animal 00:15:27.530 --> 00:15:32.300 align:middle line:84% right around birth and after birth in the mouse. 00:15:32.300 --> 00:15:39.510 align:middle line:84% And he saw in using Golgi methods and other methods-- 00:15:39.510 --> 00:15:40.400 align:middle line:90% what happened here? 00:15:40.400 --> 00:15:44.680 align:middle line:90% 00:15:44.680 --> 00:15:46.120 align:middle line:90% OK. 00:15:46.120 --> 00:15:48.652 align:middle line:84% He saw these cells with various shapes. 00:15:48.652 --> 00:15:50.860 align:middle line:84% Some of them were just sit around cells, some of them 00:15:50.860 --> 00:15:51.830 align:middle line:90% had processes. 00:15:51.830 --> 00:15:57.020 align:middle line:90% 00:15:57.020 --> 00:16:01.140 align:middle line:84% The deeper ones tended to have two processes like that. 00:16:01.140 --> 00:16:07.070 align:middle line:84% And then he also saw some with a third process going down. 00:16:07.070 --> 00:16:09.200 align:middle line:84% And then especially later in development 00:16:09.200 --> 00:16:13.610 align:middle line:84% he would notice that the cell body 00:16:13.610 --> 00:16:18.230 align:middle line:84% appeared to be moving down, the process that 00:16:18.230 --> 00:16:21.050 align:middle line:90% was going ventrally. 00:16:21.050 --> 00:16:23.060 align:middle line:90% So the P is up here. 00:16:23.060 --> 00:16:24.830 align:middle line:84% This is the layer of proliferative cells. 00:16:24.830 --> 00:16:26.720 align:middle line:84% He's just showing you a few of them. 00:16:26.720 --> 00:16:31.790 align:middle line:84% It's packed very, very densely with cells. 00:16:31.790 --> 00:16:35.870 align:middle line:84% These big cells down here are very large cells 00:16:35.870 --> 00:16:38.150 align:middle line:84% of the proliferative layer of the cerebellum, 00:16:38.150 --> 00:16:39.740 align:middle line:90% the Purkinje cells. 00:16:39.740 --> 00:16:42.110 align:middle line:90% Those did not develop that way. 00:16:42.110 --> 00:16:46.130 align:middle line:84% Those came from the ventricular layer down here. 00:16:46.130 --> 00:16:50.690 align:middle line:84% And they moved up, much like we saw in the spinal cord 00:16:50.690 --> 00:16:51.710 align:middle line:90% and neocortex. 00:16:51.710 --> 00:16:54.570 align:middle line:90% 00:16:54.570 --> 00:16:55.590 align:middle line:90% They're large cells. 00:16:55.590 --> 00:16:57.450 align:middle line:84% So those cells are all there already. 00:16:57.450 --> 00:17:02.430 align:middle line:84% They developed early, just typical of larger cells. 00:17:02.430 --> 00:17:04.020 align:middle line:90% Many small cells develop late. 00:17:04.020 --> 00:17:05.440 align:middle line:84% And that's what's happening here. 00:17:05.440 --> 00:17:08.910 align:middle line:84% So here's the picture he put together. 00:17:08.910 --> 00:17:12.720 align:middle line:84% These are the cells that move from around the clip. 00:17:12.720 --> 00:17:18.062 align:middle line:84% They grew processes when they became post-mitotic. 00:17:18.062 --> 00:17:19.770 align:middle line:84% And many of the cells that are undergoing 00:17:19.770 --> 00:17:21.359 align:middle line:90% mitosis, [INAUDIBLE]. 00:17:21.359 --> 00:17:22.950 align:middle line:84% But when they became post-mitotic 00:17:22.950 --> 00:17:26.430 align:middle line:84% and begin to differentiate, they grew lateral processes. 00:17:26.430 --> 00:17:28.140 align:middle line:84% Now, this would be the way they would 00:17:28.140 --> 00:17:30.570 align:middle line:90% look in a frontal section. 00:17:30.570 --> 00:17:32.190 align:middle line:84% Because these cells look different 00:17:32.190 --> 00:17:33.990 align:middle line:84% depending on the plane you look at them in. 00:17:33.990 --> 00:17:37.450 align:middle line:90% 00:17:37.450 --> 00:17:42.310 align:middle line:84% The lateral processes here become-- 00:17:42.310 --> 00:17:45.790 align:middle line:90% both of them are axons. 00:17:45.790 --> 00:17:49.690 align:middle line:84% The cell has axons going in two directions. 00:17:49.690 --> 00:17:51.670 align:middle line:84% And we call them parallel fibers, 00:17:51.670 --> 00:17:54.220 align:middle line:84% because there's enormous numbers of fibers all parallel 00:17:54.220 --> 00:17:55.990 align:middle line:84% with each other, all doing the same thing. 00:17:55.990 --> 00:17:58.780 align:middle line:90% 00:17:58.780 --> 00:17:59.830 align:middle line:90% Now, what about this? 00:17:59.830 --> 00:18:01.100 align:middle line:90% There's a process going down. 00:18:01.100 --> 00:18:03.430 align:middle line:90% Well, that's an axon too. 00:18:03.430 --> 00:18:05.230 align:middle line:90% But the cell doesn't stay there. 00:18:05.230 --> 00:18:08.110 align:middle line:84% It doesn't stay there with three axons coming out of it. 00:18:08.110 --> 00:18:11.845 align:middle line:84% Instead the cell body moves down the axon. 00:18:11.845 --> 00:18:14.680 align:middle line:90% 00:18:14.680 --> 00:18:17.840 align:middle line:84% This process is always a little further down. 00:18:17.840 --> 00:18:20.440 align:middle line:84% So here you see a cell let's move down further. 00:18:20.440 --> 00:18:23.650 align:middle line:84% Here it's moved clear down to the level of the Purkinje cell. 00:18:23.650 --> 00:18:27.730 align:middle line:84% And then as it moves past the Purkinje cells here, 00:18:27.730 --> 00:18:31.810 align:middle line:90% it begins to differentiate. 00:18:31.810 --> 00:18:36.820 align:middle line:84% And finally you end up with cells like these. 00:18:36.820 --> 00:18:38.290 align:middle line:84% The one furthest to the left would 00:18:38.290 --> 00:18:43.660 align:middle line:84% be the more mature granule cell of the cerebellum, always 00:18:43.660 --> 00:18:47.530 align:middle line:84% located below the Purkinje cell layer. 00:18:47.530 --> 00:18:51.790 align:middle line:84% And it's now growing these specialized little dendrites 00:18:51.790 --> 00:19:00.220 align:middle line:84% that will receive input fibers just like the spinocerebellar 00:19:00.220 --> 00:19:06.295 align:middle line:84% tracts or the axons from coming from the pons. 00:19:06.295 --> 00:19:09.530 align:middle line:90% 00:19:09.530 --> 00:19:15.130 align:middle line:84% So there are the major input layer of the cerebellum, OK. 00:19:15.130 --> 00:19:21.040 align:middle line:84% The axon then of the mature cell goes up 00:19:21.040 --> 00:19:24.490 align:middle line:84% to the superficial layer above the Purkinje cell 00:19:24.490 --> 00:19:28.330 align:middle line:84% and forms parallel fibers, each cell 00:19:28.330 --> 00:19:29.650 align:middle line:90% forming one parallel fiber. 00:19:29.650 --> 00:19:36.800 align:middle line:90% 00:19:36.800 --> 00:19:40.280 align:middle line:84% So this just summarizes what we were talking about. 00:19:40.280 --> 00:19:42.650 align:middle line:84% The cells migrate away from the pial surface. 00:19:42.650 --> 00:19:44.640 align:middle line:90% They leave their axons behind. 00:19:44.640 --> 00:19:48.590 align:middle line:84% It's like a spider moving down its web. 00:19:48.590 --> 00:19:52.520 align:middle line:84% They move past the big Purkinje cells to the adult location. 00:19:52.520 --> 00:19:56.580 align:middle line:84% That's where they grow their dendrites. 00:19:56.580 --> 00:19:59.160 align:middle line:84% And the cerebellum is an amazing structure. 00:19:59.160 --> 00:20:02.790 align:middle line:84% Because it's a very large cortical sheath. 00:20:02.790 --> 00:20:04.740 align:middle line:84% And yet no matter where you look at it, 00:20:04.740 --> 00:20:06.913 align:middle line:90% you'll see the same structures. 00:20:06.913 --> 00:20:09.330 align:middle line:84% No matter where you look in the cerebellum, this same kind 00:20:09.330 --> 00:20:12.720 align:middle line:90% of thing going on. 00:20:12.720 --> 00:20:16.250 align:middle line:84% It's more uniform in structure than the neocortex. 00:20:16.250 --> 00:20:22.190 align:middle line:90% 00:20:22.190 --> 00:20:24.380 align:middle line:84% So there at the end we were starting out 00:20:24.380 --> 00:20:25.940 align:middle line:90% to talk about migration. 00:20:25.940 --> 00:20:29.070 align:middle line:84% We saw the migration, how that happens. 00:20:29.070 --> 00:20:31.550 align:middle line:84% Then we talk about differentiation 00:20:31.550 --> 00:20:36.620 align:middle line:84% when the cells are growing their dendrites and axons. 00:20:36.620 --> 00:20:39.710 align:middle line:84% We know that the axon often develops first. 00:20:39.710 --> 00:20:43.110 align:middle line:84% We saw that for the shepherd's crook cell on the tectum. 00:20:43.110 --> 00:20:45.440 align:middle line:84% The axon grew out even before the cell body 00:20:45.440 --> 00:20:48.050 align:middle line:90% reached its final position. 00:20:48.050 --> 00:20:50.580 align:middle line:84% We saw the same thing for the granule cells. 00:20:50.580 --> 00:20:53.510 align:middle line:84% Even though the movement was very different, 00:20:53.510 --> 00:20:57.050 align:middle line:84% the axons grew earlier, long before the cell 00:20:57.050 --> 00:21:00.260 align:middle line:90% was in its final position. 00:21:00.260 --> 00:21:03.080 align:middle line:84% Now when these processors are growing, 00:21:03.080 --> 00:21:05.870 align:middle line:84% they have an active tip that's broader 00:21:05.870 --> 00:21:07.370 align:middle line:90% than the trunk of the axon. 00:21:07.370 --> 00:21:08.600 align:middle line:90% We call it the growth cone. 00:21:08.600 --> 00:21:11.510 align:middle line:84% I'm going to show you some pictures of those, including 00:21:11.510 --> 00:21:13.370 align:middle line:90% video clips. 00:21:13.370 --> 00:21:17.880 align:middle line:84% And where they go depends on a number of factors. 00:21:17.880 --> 00:21:21.710 align:middle line:84% One of them is we call selective adhesion. 00:21:21.710 --> 00:21:25.850 align:middle line:84% The active growth cone, which is you can imagine like my hand 00:21:25.850 --> 00:21:26.540 align:middle line:90% here. 00:21:26.540 --> 00:21:28.610 align:middle line:84% These little processes extend out. 00:21:28.610 --> 00:21:31.940 align:middle line:84% If you imagine now that the tips, the tips of the fingers 00:21:31.940 --> 00:21:35.168 align:middle line:84% here are more adhesive to something on the surfaces 00:21:35.168 --> 00:21:35.960 align:middle line:90% they're growing on. 00:21:35.960 --> 00:21:38.270 align:middle line:84% And they can't grow except on surfaces. 00:21:38.270 --> 00:21:41.240 align:middle line:84% They don't grow through fluid space. 00:21:41.240 --> 00:21:44.120 align:middle line:84% These grab on to the more adhesive substrates. 00:21:44.120 --> 00:21:45.680 align:middle line:90% And then they get pulled along. 00:21:45.680 --> 00:21:47.495 align:middle line:90% That's how the axon grows. 00:21:47.495 --> 00:21:51.640 align:middle line:90% 00:21:51.640 --> 00:21:53.530 align:middle line:84% So this is one of Cajal's pictures 00:21:53.530 --> 00:21:57.480 align:middle line:84% of stages of nerve fiber development. 00:21:57.480 --> 00:22:01.360 align:middle line:84% So you see the cell layer beginning 00:22:01.360 --> 00:22:05.720 align:middle line:90% to grow its axon first. 00:22:05.720 --> 00:22:07.690 align:middle line:84% So you can see it only has an axon. 00:22:07.690 --> 00:22:09.700 align:middle line:84% There you see his picture of the growth cone. 00:22:09.700 --> 00:22:12.010 align:middle line:84% And Cajal described this just from Golgi methods 00:22:12.010 --> 00:22:13.645 align:middle line:84% without seeing them in tissue culture. 00:22:13.645 --> 00:22:17.100 align:middle line:90% 00:22:17.100 --> 00:22:19.740 align:middle line:84% And then when it reaches its adult location, 00:22:19.740 --> 00:22:23.940 align:middle line:84% it begins to mature, forms its terminal arborization, 00:22:23.940 --> 00:22:26.280 align:middle line:90% can also grow collaterals. 00:22:26.280 --> 00:22:28.200 align:middle line:84% Those can happen at various stages. 00:22:28.200 --> 00:22:30.510 align:middle line:84% And then the cell body will be changing too, 00:22:30.510 --> 00:22:31.710 align:middle line:90% growing its dendrites. 00:22:31.710 --> 00:22:34.155 align:middle line:84% They usually grow somewhat later than the axon. 00:22:34.155 --> 00:22:36.697 align:middle line:90% 00:22:36.697 --> 00:22:37.780 align:middle line:90% So that's Cajal's picture. 00:22:37.780 --> 00:22:40.370 align:middle line:90% 00:22:40.370 --> 00:22:43.680 align:middle line:84% And here's a particular kind of-- it's 00:22:43.680 --> 00:22:48.280 align:middle line:84% a freeze fracture picture of a growth cone showing you 00:22:48.280 --> 00:22:50.900 align:middle line:84% that there's a lot of things going on inside. 00:22:50.900 --> 00:22:53.020 align:middle line:84% In fact, there's many actin filaments there 00:22:53.020 --> 00:22:55.420 align:middle line:90% which are contractile proteins. 00:22:55.420 --> 00:22:57.580 align:middle line:84% There's many organelles, including 00:22:57.580 --> 00:22:59.710 align:middle line:90% mitochondria and vesicles. 00:22:59.710 --> 00:23:04.480 align:middle line:84% And these little extensions are what we call filipodia, OK. 00:23:04.480 --> 00:23:13.110 align:middle line:90% 00:23:13.110 --> 00:23:14.880 align:middle line:84% Filipodium would be the singular. 00:23:14.880 --> 00:23:17.690 align:middle line:90% 00:23:17.690 --> 00:23:20.240 align:middle line:84% I haven't finished all the flash cube 00:23:20.240 --> 00:23:22.100 align:middle line:84% terms for the development lectures, 00:23:22.100 --> 00:23:23.270 align:middle line:90% but I'm working on them. 00:23:23.270 --> 00:23:24.860 align:middle line:90% So I will post that. 00:23:24.860 --> 00:23:28.718 align:middle line:90% 00:23:28.718 --> 00:23:30.260 align:middle line:84% But the thing to remember about these 00:23:30.260 --> 00:23:32.720 align:middle line:84% is that they're transient structures. 00:23:32.720 --> 00:23:37.190 align:middle line:90% 00:23:37.190 --> 00:23:40.970 align:middle line:84% These little structures extend, and then they pull back, 00:23:40.970 --> 00:23:43.170 align:middle line:90% and they extend and pull back. 00:23:43.170 --> 00:23:45.470 align:middle line:90% So it's a very active structure. 00:23:45.470 --> 00:23:47.045 align:middle line:84% And you'll see that in the pictures. 00:23:47.045 --> 00:23:50.000 align:middle line:90% 00:23:50.000 --> 00:23:51.500 align:middle line:90% They're volatile structures. 00:23:51.500 --> 00:23:54.430 align:middle line:90% 00:23:54.430 --> 00:23:58.480 align:middle line:84% And remember that at their tips, they're more adhesive. 00:23:58.480 --> 00:24:02.590 align:middle line:84% And when they contract then if the tip is adhering 00:24:02.590 --> 00:24:05.320 align:middle line:84% to a substrate, the growth cone will tend 00:24:05.320 --> 00:24:07.618 align:middle line:90% to be pulled in that direction. 00:24:07.618 --> 00:24:09.160 align:middle line:84% You say, but if there's a lot of them 00:24:09.160 --> 00:24:12.920 align:middle line:84% pulling in every direction, how can they move at all? 00:24:12.920 --> 00:24:16.360 align:middle line:84% Well, it seems-- we think it moves as the result 00:24:16.360 --> 00:24:18.550 align:middle line:84% the vector sum of all the tensions on it. 00:24:18.550 --> 00:24:21.970 align:middle line:90% 00:24:21.970 --> 00:24:24.250 align:middle line:84% And we know some of the molecules involved, 00:24:24.250 --> 00:24:26.050 align:middle line:90% cell adhesion molecules. 00:24:26.050 --> 00:24:28.820 align:middle line:84% One of them we call NCAM neuronal cell adhesion 00:24:28.820 --> 00:24:29.320 align:middle line:90% molecule. 00:24:29.320 --> 00:24:31.780 align:middle line:84% But there's a number of adhesive molecules like that. 00:24:31.780 --> 00:24:35.500 align:middle line:90% 00:24:35.500 --> 00:24:38.800 align:middle line:84% OK, now one way that this is studied, 00:24:38.800 --> 00:24:42.220 align:middle line:84% it's very difficult to see these things 00:24:42.220 --> 00:24:44.920 align:middle line:84% if we want to look at them in the live action 00:24:44.920 --> 00:24:46.870 align:middle line:90% in the real brain. 00:24:46.870 --> 00:24:50.140 align:middle line:84% That has been done, but with great difficulty. 00:24:50.140 --> 00:24:52.690 align:middle line:84% Only recently if we have methods we can do that. 00:24:52.690 --> 00:24:55.450 align:middle line:84% A lot of the work has been in tissue culture. 00:24:55.450 --> 00:25:01.510 align:middle line:84% And this is just a picture of cultures from-- 00:25:01.510 --> 00:25:03.670 align:middle line:84% this could be either a dorsal root ganglion 00:25:03.670 --> 00:25:06.670 align:middle line:90% or a synthetic ganglion. 00:25:06.670 --> 00:25:10.810 align:middle line:84% It's probably a dorsal root ganglion placed in culture. 00:25:10.810 --> 00:25:12.490 align:middle line:84% And in this particular picture, they're 00:25:12.490 --> 00:25:16.960 align:middle line:84% contrasting a culture where there was no particular growth 00:25:16.960 --> 00:25:19.000 align:middle line:84% factor, and another one where they 00:25:19.000 --> 00:25:20.830 align:middle line:84% added this growth factor, neuronal growth 00:25:20.830 --> 00:25:25.210 align:middle line:84% factor, the first growth factor to be discovered, NGF we call 00:25:25.210 --> 00:25:29.470 align:middle line:84% it, the first member of the family of growth factors 00:25:29.470 --> 00:25:32.620 align:middle line:90% we call the neurotrophins. 00:25:32.620 --> 00:25:37.210 align:middle line:84% And so here with the right medium and with growth factor, 00:25:37.210 --> 00:25:41.440 align:middle line:84% you can see a proliferation of axons coming out 00:25:41.440 --> 00:25:45.260 align:middle line:90% of that clump of cells. 00:25:45.260 --> 00:25:48.520 align:middle line:84% And we can study the growth cones 00:25:48.520 --> 00:25:51.250 align:middle line:84% and other properties of growth in tissue culture 00:25:51.250 --> 00:25:54.018 align:middle line:90% in a flat plate. 00:25:54.018 --> 00:25:55.810 align:middle line:84% And there have been many studies like that. 00:25:55.810 --> 00:25:57.670 align:middle line:84% And I'll show you now some video clips. 00:25:57.670 --> 00:26:20.850 align:middle line:90% 00:26:20.850 --> 00:26:22.680 align:middle line:84% OK, this is just-- the first one just 00:26:22.680 --> 00:26:23.920 align:middle line:90% shows the large growth cone. 00:26:23.920 --> 00:26:26.430 align:middle line:84% We might even have to turn out all the lights here, 00:26:26.430 --> 00:26:28.320 align:middle line:84% because it's the little dim there. 00:26:28.320 --> 00:26:31.800 align:middle line:84% It's that shadowy thing in the back there. 00:26:31.800 --> 00:26:34.710 align:middle line:84% Here's the trunk, and the axon has got some little processes 00:26:34.710 --> 00:26:35.340 align:middle line:90% coming out. 00:26:35.340 --> 00:26:37.330 align:middle line:84% And it's moving across the screen. 00:26:37.330 --> 00:26:39.703 align:middle line:90% Can you see it there? 00:26:39.703 --> 00:26:40.620 align:middle line:90% This is the end of it. 00:26:40.620 --> 00:26:42.600 align:middle line:84% And you see there's a filipodium, 00:26:42.600 --> 00:26:44.580 align:middle line:84% there's a whole bunch of filipodium. 00:26:44.580 --> 00:26:45.660 align:middle line:90% So let's watch it again. 00:26:45.660 --> 00:26:46.770 align:middle line:90% It starts here. 00:26:46.770 --> 00:26:49.650 align:middle line:84% And just look at what's happening there. 00:26:49.650 --> 00:26:59.120 align:middle line:90% 00:26:59.120 --> 00:27:01.430 align:middle line:84% You'll see them extend, and then they disappear, 00:27:01.430 --> 00:27:04.520 align:middle line:90% and they extend again. 00:27:04.520 --> 00:27:06.560 align:middle line:84% And the axon is gradually elongated. 00:27:06.560 --> 00:27:09.380 align:middle line:84% So what's happening there is it's-- 00:27:09.380 --> 00:27:11.570 align:middle line:84% we're seeing the action at the growth cone. 00:27:11.570 --> 00:27:14.150 align:middle line:84% There's a lot of things going on in the cell, too, of course. 00:27:14.150 --> 00:27:16.220 align:middle line:84% And one thing is it's adding membrane. 00:27:16.220 --> 00:27:18.810 align:middle line:90% Where's the membrane come from? 00:27:18.810 --> 00:27:20.480 align:middle line:84% Well, it comes from the cell body. 00:27:20.480 --> 00:27:25.850 align:middle line:84% And it's being transported down this axon in little vesicles. 00:27:25.850 --> 00:27:29.480 align:middle line:90% The vesicles contain things too. 00:27:29.480 --> 00:27:32.930 align:middle line:84% But the membrane of the vesicle becomes the membrane 00:27:32.930 --> 00:27:34.670 align:middle line:90% of the axon, all right. 00:27:34.670 --> 00:27:48.800 align:middle line:90% 00:27:48.800 --> 00:27:51.010 align:middle line:84% OK, let's just look at the next one. 00:27:51.010 --> 00:27:54.260 align:middle line:84% Like I said, I don't have to go back through all this. 00:27:54.260 --> 00:27:58.160 align:middle line:84% OK, this time you're going to see two axons that 00:27:58.160 --> 00:28:00.800 align:middle line:84% don't belong together because they don't occur together 00:28:00.800 --> 00:28:02.540 align:middle line:90% in the real brain. 00:28:02.540 --> 00:28:06.620 align:middle line:84% One's from the retina, one's from the dorsal root ganglion. 00:28:06.620 --> 00:28:10.670 align:middle line:90% See we're magnifying in 2,400. 00:28:10.670 --> 00:28:12.800 align:middle line:84% But that, of course, it probably much more 00:28:12.800 --> 00:28:15.590 align:middle line:84% than that because of the size of this screen, then 00:28:15.590 --> 00:28:17.960 align:middle line:90% the 60X acceleration, OK. 00:28:17.960 --> 00:28:23.690 align:middle line:90% 00:28:23.690 --> 00:28:28.010 align:middle line:84% OK, one here, one here, watch what happens. 00:28:28.010 --> 00:28:29.410 align:middle line:90% This one is retracting. 00:28:29.410 --> 00:28:31.850 align:middle line:84% You'll see it's still hanging on there a little bit. 00:28:31.850 --> 00:28:34.800 align:middle line:84% But it is going to withdraw those in the cell. 00:28:34.800 --> 00:28:36.350 align:middle line:84% This is called growth cone collapse. 00:28:36.350 --> 00:28:39.860 align:middle line:90% 00:28:39.860 --> 00:28:42.680 align:middle line:84% Perhaps calling it retraction of the growth cone 00:28:42.680 --> 00:28:44.660 align:middle line:90% is a little more descriptive. 00:28:44.660 --> 00:28:47.180 align:middle line:84% They're both nice descriptive terms. 00:28:47.180 --> 00:28:48.350 align:middle line:90% But that's very common. 00:28:48.350 --> 00:28:52.130 align:middle line:84% When two axons are growing in culture 00:28:52.130 --> 00:28:55.250 align:middle line:84% and they meet each other, the growth cone of one 00:28:55.250 --> 00:28:59.570 align:middle line:84% touches the growth cone or the axon of another. 00:28:59.570 --> 00:29:04.520 align:middle line:84% They either can cross over or one of them will retract. 00:29:04.520 --> 00:29:06.020 align:middle line:90% The growth cone can retract. 00:29:06.020 --> 00:29:08.490 align:middle line:84% It's when it's filipodia touch the other. 00:29:08.490 --> 00:29:12.380 align:middle line:84% And we find that depends on which cells they are. 00:29:12.380 --> 00:29:17.540 align:middle line:84% Now, if you have retinal axons meeting a dorsal root axon, 00:29:17.540 --> 00:29:21.360 align:middle line:84% you usually get retraction when they touch each other. 00:29:21.360 --> 00:29:25.950 align:middle line:84% If it's two dorsal root axons that touch each other, 00:29:25.950 --> 00:29:28.460 align:middle line:90% they still often retract. 00:29:28.460 --> 00:29:32.390 align:middle line:84% But they retract more than 50% of the time. 00:29:32.390 --> 00:29:34.640 align:middle line:84% But they often also cross over each other. 00:29:34.640 --> 00:29:37.830 align:middle line:90% 00:29:37.830 --> 00:29:40.110 align:middle line:84% Now, when they retract, what happens? 00:29:40.110 --> 00:29:42.240 align:middle line:90% They pull back. 00:29:42.240 --> 00:29:44.403 align:middle line:84% And then after a very brief pause, 00:29:44.403 --> 00:29:45.570 align:middle line:90% they'll start growing again. 00:29:45.570 --> 00:29:47.130 align:middle line:84% And usually when they start growing again, 00:29:47.130 --> 00:29:48.525 align:middle line:90% they change direction slightly. 00:29:48.525 --> 00:29:53.460 align:middle line:90% 00:29:53.460 --> 00:29:55.560 align:middle line:84% That's the most common thing to happen. 00:29:55.560 --> 00:29:59.213 align:middle line:90% 00:29:59.213 --> 00:30:01.005 align:middle line:84% So now we'll look at three of them growing. 00:30:01.005 --> 00:30:06.780 align:middle line:90% 00:30:06.780 --> 00:30:10.971 align:middle line:84% This one here, there's one there, and there's one there. 00:30:10.971 --> 00:30:12.660 align:middle line:90% Can you see them? 00:30:12.660 --> 00:30:15.630 align:middle line:90% This one is going to branch. 00:30:15.630 --> 00:30:19.800 align:middle line:84% It's seeing a lot of adhesion here and here, less here, 00:30:19.800 --> 00:30:23.670 align:middle line:90% and it ends up branching. 00:30:23.670 --> 00:30:27.180 align:middle line:84% These two are touching each other. 00:30:27.180 --> 00:30:32.160 align:middle line:90% And one of them is retracting. 00:30:32.160 --> 00:30:33.360 align:middle line:90% This one's just pausing. 00:30:33.360 --> 00:30:37.430 align:middle line:90% 00:30:37.430 --> 00:30:39.350 align:middle line:90% And then it keeps going. 00:30:39.350 --> 00:30:41.660 align:middle line:84% And this one is continuing to retract. 00:30:41.660 --> 00:30:44.120 align:middle line:84% You see the body, the thickest part of the growth cone, 00:30:44.120 --> 00:30:45.590 align:middle line:90% is pulling way back. 00:30:45.590 --> 00:30:47.940 align:middle line:84% It's lost all its processes here. 00:30:47.940 --> 00:30:50.990 align:middle line:84% And this, the distance of retraction 00:30:50.990 --> 00:30:53.294 align:middle line:90% can vary quite a bit. 00:30:53.294 --> 00:30:55.664 align:middle line:90% STUDENT: [INAUDIBLE] 00:30:55.664 --> 00:30:59.380 align:middle line:90% 00:30:59.380 --> 00:31:02.930 align:middle line:84% GERALD SCHNEIDER: It's properties of the membrane. 00:31:02.930 --> 00:31:07.090 align:middle line:84% But we also now know that there are secreted substances around, 00:31:07.090 --> 00:31:10.870 align:middle line:84% that these things can be secreting molecules. 00:31:10.870 --> 00:31:14.410 align:middle line:84% One of the molecules that causes that, they call collapsin. 00:31:14.410 --> 00:31:18.800 align:middle line:84% Because it literally affects the membrane of the other cell 00:31:18.800 --> 00:31:21.220 align:middle line:90% and can induce the retraction. 00:31:21.220 --> 00:31:24.040 align:middle line:84% Now, in the literature it's commonly 00:31:24.040 --> 00:31:29.050 align:middle line:84% believed that there's no way to predict 00:31:29.050 --> 00:31:30.400 align:middle line:90% how far they're going to go. 00:31:30.400 --> 00:31:32.350 align:middle line:84% But in fact, I've studied that literature. 00:31:32.350 --> 00:31:35.560 align:middle line:84% And I found out that it's actually somewhat systematic, 00:31:35.560 --> 00:31:38.560 align:middle line:84% that depending on the axon properties, 00:31:38.560 --> 00:31:39.640 align:middle line:90% you'll get various-- 00:31:39.640 --> 00:31:42.520 align:middle line:84% not only different frequencies of retraction, 00:31:42.520 --> 00:31:44.920 align:middle line:84% that is the probability it'll retract varies-- 00:31:44.920 --> 00:31:49.220 align:middle line:84% but also the distance it'll retract varies. 00:31:49.220 --> 00:31:51.400 align:middle line:84% And I found out that that-- and I did this 00:31:51.400 --> 00:31:52.690 align:middle line:90% by computer simulation. 00:31:52.690 --> 00:31:54.610 align:middle line:84% I didn't bring any of my simulation pictures. 00:31:54.610 --> 00:31:57.850 align:middle line:84% But I showed that the distance that retracts 00:31:57.850 --> 00:32:00.730 align:middle line:84% makes an enormous difference in the pattern of axons 00:32:00.730 --> 00:32:02.530 align:middle line:90% you get in the culture. 00:32:02.530 --> 00:32:04.390 align:middle line:84% The short distances will give you 00:32:04.390 --> 00:32:07.210 align:middle line:84% axons that are growing like they're glued 00:32:07.210 --> 00:32:09.160 align:middle line:90% to each other, fasciculated. 00:32:09.160 --> 00:32:12.460 align:middle line:84% And the large distances, you get non-fasciculation. 00:32:12.460 --> 00:32:15.608 align:middle line:84% I'll show you pictures of what those things look like, yes. 00:32:15.608 --> 00:32:18.300 align:middle line:90% STUDENT: [INAUDIBLE] 00:32:18.300 --> 00:32:21.920 align:middle line:84% GERALD SCHNEIDER: This is just artifact debris. 00:32:21.920 --> 00:32:25.086 align:middle line:90% STUDENT: [INAUDIBLE] 00:32:25.086 --> 00:32:27.470 align:middle line:90% 00:32:27.470 --> 00:32:30.860 align:middle line:84% GERALD SCHNEIDER: That they're moving on a substrate, OK. 00:32:30.860 --> 00:32:33.830 align:middle line:84% They're moving on a plate that's coated 00:32:33.830 --> 00:32:38.690 align:middle line:90% with polylysine or something. 00:32:38.690 --> 00:32:43.490 align:middle line:84% And the substrate you grow them on will affect how they grow. 00:32:43.490 --> 00:32:44.510 align:middle line:90% That's important. 00:32:44.510 --> 00:32:48.600 align:middle line:90% 00:32:48.600 --> 00:32:50.610 align:middle line:84% OK, this is, I think this is the last one. 00:32:50.610 --> 00:32:52.770 align:middle line:84% It shows a growth cone coming from the left. 00:32:52.770 --> 00:32:55.500 align:middle line:84% And it's contacting a fibroblast. 00:32:55.500 --> 00:32:58.590 align:middle line:90% That's a peripheral cell. 00:32:58.590 --> 00:33:02.550 align:middle line:84% Now, it's a sympathetic neuron, a sympathetic ganglion neuron. 00:33:02.550 --> 00:33:07.680 align:middle line:84% And this could be happening in the natural situation. 00:33:07.680 --> 00:33:10.950 align:middle line:84% These are peripheral cells contacting each other. 00:33:10.950 --> 00:33:18.420 align:middle line:90% 00:33:18.420 --> 00:33:19.820 align:middle line:90% So here's the axon. 00:33:19.820 --> 00:33:21.050 align:middle line:90% There's the growth cone. 00:33:21.050 --> 00:33:23.270 align:middle line:90% Here's the fibroblast. 00:33:23.270 --> 00:33:27.590 align:middle line:90% And it's met a barrier. 00:33:27.590 --> 00:33:30.350 align:middle line:90% And what it does is it keeps-- 00:33:30.350 --> 00:33:31.550 align:middle line:90% stays very active. 00:33:31.550 --> 00:33:33.830 align:middle line:84% And, see, it's moving along the surface. 00:33:33.830 --> 00:33:36.450 align:middle line:84% It feels itself along the surface. 00:33:36.450 --> 00:33:38.300 align:middle line:84% And, in fact, it's turned here now 00:33:38.300 --> 00:33:41.220 align:middle line:84% and it's growing along the surface this way. 00:33:41.220 --> 00:33:41.720 align:middle line:90% Yes. 00:33:41.720 --> 00:33:43.930 align:middle line:90% STUDENT: [INAUDIBLE] 00:33:43.930 --> 00:33:44.820 align:middle line:90% 00:33:44.820 --> 00:33:48.190 align:middle line:84% GERALD SCHNEIDER: From the cell body. 00:33:48.190 --> 00:33:50.020 align:middle line:84% That's a very important question. 00:33:50.020 --> 00:33:57.630 align:middle line:84% These growing cells are metabolically very active, OK. 00:33:57.630 --> 00:34:02.670 align:middle line:84% And it is shipping by anterograde transport 00:34:02.670 --> 00:34:06.450 align:middle line:84% mechanisms the vesicles down the axon, OK. 00:34:06.450 --> 00:34:09.469 align:middle line:90% 00:34:09.469 --> 00:34:11.940 align:middle line:84% Does it happen in [INAUDIBLE] is the question. 00:34:11.940 --> 00:34:14.150 align:middle line:90% And the answer is yes. 00:34:14.150 --> 00:34:19.050 align:middle line:84% We will talk a little bit about regeneration soon. 00:34:19.050 --> 00:34:20.489 align:middle line:84% In the peripheral nervous system, 00:34:20.489 --> 00:34:22.280 align:middle line:84% especially when you have an injury, 00:34:22.280 --> 00:34:25.199 align:middle line:84% you will get a lot of growth like this. 00:34:25.199 --> 00:34:27.080 align:middle line:90% Axons will attempt to regrow. 00:34:27.080 --> 00:34:29.570 align:middle line:84% In many cases, they will reform something 00:34:29.570 --> 00:34:31.880 align:middle line:84% approximating their original connections. 00:34:31.880 --> 00:34:54.894 align:middle line:90% 00:34:54.894 --> 00:34:57.120 align:middle line:84% Let's see how we're doing on time here. 00:34:57.120 --> 00:35:01.730 align:middle line:84% OK, so now I'm going to describe two different kinds of growth 00:35:01.730 --> 00:35:04.010 align:middle line:84% [INAUDIBLE] that was discovered in the optic tract. 00:35:04.010 --> 00:35:06.980 align:middle line:84% But now we know it happens also in the fibers growing 00:35:06.980 --> 00:35:09.830 align:middle line:84% into neocortex, a number of other systems 00:35:09.830 --> 00:35:12.770 align:middle line:90% where you get similar things. 00:35:12.770 --> 00:35:15.440 align:middle line:84% Growth that looks different grows at a different rate, 00:35:15.440 --> 00:35:17.750 align:middle line:90% has different properties. 00:35:17.750 --> 00:35:19.580 align:middle line:84% So the properties of these growth cones 00:35:19.580 --> 00:35:22.490 align:middle line:84% must be different in two different stages 00:35:22.490 --> 00:35:26.165 align:middle line:84% of the same cell growing it's axon. 00:35:26.165 --> 00:35:29.120 align:middle line:90% 00:35:29.120 --> 00:35:31.280 align:middle line:84% And then we a little bit about what they're doing. 00:35:31.280 --> 00:35:35.360 align:middle line:84% They compete for space to terminate. 00:35:35.360 --> 00:35:37.580 align:middle line:84% We know they're influenced by growth factors. 00:35:37.580 --> 00:35:40.490 align:middle line:84% And these growth factors can play more than one role. 00:35:40.490 --> 00:35:43.370 align:middle line:84% I'll describe a little bit of that in this class. 00:35:43.370 --> 00:35:45.830 align:middle line:84% When we talk about it, we spend sessions 00:35:45.830 --> 00:35:48.510 align:middle line:90% on that in the second term. 00:35:48.510 --> 00:35:50.790 align:middle line:84% And, of course, if they're coming from the retina, 00:35:50.790 --> 00:35:53.170 align:middle line:84% they have other problems to solve. 00:35:53.170 --> 00:35:55.262 align:middle line:84% They need to terminate in a very specific way. 00:35:55.262 --> 00:35:56.970 align:middle line:84% In fact if the're coming from the retina, 00:35:56.970 --> 00:35:58.730 align:middle line:84% they terminate in a topographic way. 00:35:58.730 --> 00:36:01.970 align:middle line:84% The same is true for those axons, say, the spinothalamic, 00:36:01.970 --> 00:36:02.960 align:middle line:90% or the medial lem-- 00:36:02.960 --> 00:36:04.760 align:middle line:84% spinothalamic or medial lemnicus, 00:36:04.760 --> 00:36:09.170 align:middle line:84% they have to terminate in the right place, 00:36:09.170 --> 00:36:11.540 align:middle line:84% because there's a topographic map, a map of the body 00:36:11.540 --> 00:36:12.830 align:middle line:90% surface for the retina. 00:36:12.830 --> 00:36:17.740 align:middle line:90% 00:36:17.740 --> 00:36:20.500 align:middle line:84% And when they're elongating, we know they're very specific. 00:36:20.500 --> 00:36:23.425 align:middle line:84% And now we know there's different kinds of guidance. 00:36:23.425 --> 00:36:27.710 align:middle line:90% 00:36:27.710 --> 00:36:30.620 align:middle line:84% There's chemical guidance of more than one kind. 00:36:30.620 --> 00:36:32.390 align:middle line:84% There can be diffusing chemicals that 00:36:32.390 --> 00:36:34.310 align:middle line:90% can attract them or repel them. 00:36:34.310 --> 00:36:36.020 align:middle line:84% And you'd have to write this down. 00:36:36.020 --> 00:36:39.110 align:middle line:90% We'll go over it later. 00:36:39.110 --> 00:36:40.640 align:middle line:84% They can be attracted or repelled 00:36:40.640 --> 00:36:42.500 align:middle line:90% by a diffusing molecule. 00:36:42.500 --> 00:36:45.110 align:middle line:90% Or they can also-- 00:36:45.110 --> 00:36:48.080 align:middle line:84% they just by this, there can be molecules on the surfaces 00:36:48.080 --> 00:36:49.010 align:middle line:90% they're contacting. 00:36:49.010 --> 00:36:53.930 align:middle line:84% And those molecules can attract them or repel them also. 00:36:53.930 --> 00:36:57.470 align:middle line:84% We know the glial cells play specific roles in axon growth. 00:36:57.470 --> 00:36:59.930 align:middle line:84% We saw the importance of the glial cell 00:36:59.930 --> 00:37:01.190 align:middle line:90% in neuronal migration. 00:37:01.190 --> 00:37:03.230 align:middle line:84% Well, they also play very important roles 00:37:03.230 --> 00:37:04.850 align:middle line:90% in the growth of axons. 00:37:04.850 --> 00:37:09.290 align:middle line:84% The main environment that the axons are growing through 00:37:09.290 --> 00:37:12.715 align:middle line:90% is a forest of glial cells. 00:37:12.715 --> 00:37:14.840 align:middle line:84% So when the axons grow into the optic tectum coming 00:37:14.840 --> 00:37:17.060 align:middle line:84% from the eye, for example, what do they meet? 00:37:17.060 --> 00:37:19.910 align:middle line:90% 00:37:19.910 --> 00:37:22.550 align:middle line:84% They meet processes of neurons too. 00:37:22.550 --> 00:37:24.680 align:middle line:84% A lot of those processes are growing through, 00:37:24.680 --> 00:37:28.310 align:middle line:84% it's a forest of radial cells, some of which will become glia, 00:37:28.310 --> 00:37:30.185 align:middle line:90% others will become neurons. 00:37:30.185 --> 00:37:33.185 align:middle line:90% 00:37:33.185 --> 00:37:35.810 align:middle line:84% Then I'll stay a little bit more about competitive interactions 00:37:35.810 --> 00:37:39.650 align:middle line:84% among axons and the various factors. 00:37:39.650 --> 00:37:43.130 align:middle line:84% I've mentioned a few of them already, 00:37:43.130 --> 00:37:46.430 align:middle line:84% influencing, growing, or regenerating, 00:37:46.430 --> 00:37:50.390 align:middle line:84% or just axons growing through collateral sprouting. 00:37:50.390 --> 00:37:53.530 align:middle line:90% 00:37:53.530 --> 00:37:56.330 align:middle line:84% OK, so here's a picture from our research, 00:37:56.330 --> 00:38:00.800 align:middle line:84% about two months of growth in the optic tract. 00:38:00.800 --> 00:38:03.240 align:middle line:84% First, we talk about the period of elongation. 00:38:03.240 --> 00:38:06.070 align:middle line:90% 00:38:06.070 --> 00:38:09.040 align:middle line:84% And the cells have this kind of appearance. 00:38:09.040 --> 00:38:13.750 align:middle line:84% Then they have little enlargements 00:38:13.750 --> 00:38:17.020 align:middle line:84% along their way, little tiny processes, and then 00:38:17.020 --> 00:38:19.150 align:middle line:90% an active growth cone. 00:38:19.150 --> 00:38:22.420 align:middle line:90% They don't branch very much. 00:38:22.420 --> 00:38:23.260 align:middle line:90% They might fork. 00:38:23.260 --> 00:38:28.330 align:middle line:84% But generally they won't form much branching at all. 00:38:28.330 --> 00:38:30.760 align:middle line:84% And then in a hamster, they're born at this stage 00:38:30.760 --> 00:38:31.840 align:middle line:90% in the optic tract. 00:38:31.840 --> 00:38:34.330 align:middle line:84% And then the axons start sprouting collaterals. 00:38:34.330 --> 00:38:36.250 align:middle line:84% And they don't just sprout collaterals 00:38:36.250 --> 00:38:39.880 align:middle line:84% where they're going to end up with an arbor. 00:38:39.880 --> 00:38:42.610 align:middle line:84% They sprout them all over the place. 00:38:42.610 --> 00:38:44.590 align:middle line:84% Even axons growing through half the tectum, 00:38:44.590 --> 00:38:49.030 align:middle line:84% it'll sprout little collaterals all along, OK you see there. 00:38:49.030 --> 00:38:52.270 align:middle line:84% But then the axon will start to focalize. 00:38:52.270 --> 00:38:54.280 align:middle line:84% It'll pull in a lot of those collaterals. 00:38:54.280 --> 00:38:55.900 align:middle line:90% And one of them will grow. 00:38:55.900 --> 00:38:58.180 align:middle line:84% It might not be the one at the end, 00:38:58.180 --> 00:39:01.330 align:middle line:84% like this one I'm showing growing along the way. 00:39:01.330 --> 00:39:04.100 align:middle line:90% 00:39:04.100 --> 00:39:06.580 align:middle line:84% And then finally we'll withdraw the other collaterals. 00:39:06.580 --> 00:39:08.140 align:middle line:90% it might form more than one. 00:39:08.140 --> 00:39:10.900 align:middle line:84% As I show here, it's forming collaterals in another place 00:39:10.900 --> 00:39:14.830 align:middle line:84% also, as many axons have more than one place where 00:39:14.830 --> 00:39:17.350 align:middle line:90% they terminate. 00:39:17.350 --> 00:39:20.110 align:middle line:84% And then the differentiation continues. 00:39:20.110 --> 00:39:22.690 align:middle line:84% And axons might end up, for example, terminating 00:39:22.690 --> 00:39:24.350 align:middle line:90% in one layer and not others. 00:39:24.350 --> 00:39:28.730 align:middle line:84% So it simply withdraws its branches in some places 00:39:28.730 --> 00:39:30.430 align:middle line:84% and continues growing them in others. 00:39:30.430 --> 00:39:34.460 align:middle line:90% 00:39:34.460 --> 00:39:39.200 align:middle line:84% When it withdraws, sometimes we call it a pruning, 00:39:39.200 --> 00:39:42.420 align:middle line:84% a self pruning it's going through. 00:39:42.420 --> 00:39:48.440 align:middle line:84% We can also prune them by making lesions, damaging them. 00:39:48.440 --> 00:39:50.090 align:middle line:84% And then the hamster, this is all 00:39:50.090 --> 00:39:52.370 align:middle line:84% happening when the eyes are still closed. 00:39:52.370 --> 00:39:54.920 align:middle line:84% After the eyes open, there's a maturation 00:39:54.920 --> 00:39:56.300 align:middle line:90% of arbors that continues. 00:39:56.300 --> 00:40:00.170 align:middle line:84% And we know from various studies that that maturation can 00:40:00.170 --> 00:40:02.490 align:middle line:90% be affected by visual input. 00:40:02.490 --> 00:40:04.280 align:middle line:84% We don't know much about it in the tectum. 00:40:04.280 --> 00:40:09.200 align:middle line:84% But it's well known in other parts of the visual system. 00:40:09.200 --> 00:40:13.520 align:middle line:84% So finally you end up with the mature arbors. 00:40:13.520 --> 00:40:16.890 align:middle line:84% So these are the two modes, elongation and arborization. 00:40:16.890 --> 00:40:19.610 align:middle line:84% We know that there are different proteins found in the cells 00:40:19.610 --> 00:40:21.050 align:middle line:84% during these two modes of growth. 00:40:21.050 --> 00:40:23.030 align:middle line:84% I did some initial studies of this 00:40:23.030 --> 00:40:26.780 align:middle line:84% with Ken Moya, who's been working in France since then. 00:40:26.780 --> 00:40:28.100 align:middle line:90% But he's an American citizen. 00:40:28.100 --> 00:40:31.540 align:middle line:84% He came to MIT to do his graduate work. 00:40:31.540 --> 00:40:33.500 align:middle line:84% He worked with me and [INAUDIBLE] 00:40:33.500 --> 00:40:35.660 align:middle line:90% at Harvard Medical School. 00:40:35.660 --> 00:40:38.390 align:middle line:84% And he looked at different stages 00:40:38.390 --> 00:40:40.790 align:middle line:84% of the development of these axons 00:40:40.790 --> 00:40:42.590 align:middle line:84% and found out that the cells were 00:40:42.590 --> 00:40:45.980 align:middle line:84% making protein and transporting different proteins 00:40:45.980 --> 00:40:47.280 align:middle line:90% at different stages. 00:40:47.280 --> 00:40:48.980 align:middle line:84% So then elongation, for example, it 00:40:48.980 --> 00:40:53.030 align:middle line:84% was transporting a lot of NCAM, neuronal cell adhesion 00:40:53.030 --> 00:40:54.260 align:middle line:90% molecule. 00:40:54.260 --> 00:40:57.830 align:middle line:84% There are other proteins related to synapse formation that begin 00:40:57.830 --> 00:40:59.870 align:middle line:90% to appear during arborizaiton. 00:40:59.870 --> 00:41:03.950 align:middle line:84% We know also receptors can change and respond 00:41:03.950 --> 00:41:06.920 align:middle line:90% to various growth factors. 00:41:06.920 --> 00:41:08.540 align:middle line:90% We weren't able to specify that. 00:41:08.540 --> 00:41:11.600 align:middle line:84% But more of that has been found since that time. 00:41:11.600 --> 00:41:16.160 align:middle line:84% We know that a particular molecule, BCL2, 00:41:16.160 --> 00:41:22.400 align:middle line:84% is very important early on, prevents the cell from dying. 00:41:22.400 --> 00:41:24.900 align:middle line:84% It's [INAUDIBLE] apoptotic factor. 00:41:24.900 --> 00:41:28.100 align:middle line:84% And now we also know it plays a role in axon outgrowth. 00:41:28.100 --> 00:41:31.490 align:middle line:84% And there's many other proteins too. 00:41:31.490 --> 00:41:35.300 align:middle line:84% So with any elongation, what we study was the anatomy. 00:41:35.300 --> 00:41:40.730 align:middle line:84% We saw that they grew rapidly, 80 to 100 microns in an hour. 00:41:40.730 --> 00:41:42.350 align:middle line:84% That might not seem so rapid to you. 00:41:42.350 --> 00:41:44.090 align:middle line:90% It's about 2 millimeters a day. 00:41:44.090 --> 00:41:46.175 align:middle line:84% But for a little cell, that's pretty rapid. 00:41:46.175 --> 00:41:50.000 align:middle line:90% 00:41:50.000 --> 00:41:58.750 align:middle line:90% In case you-- that's a micron. 00:41:58.750 --> 00:42:00.850 align:middle line:90% It's a Greek letter mu. 00:42:00.850 --> 00:42:06.120 align:middle line:84% But because I didn't bother to put the Greek letter, I'm not-- 00:42:06.120 --> 00:42:08.320 align:middle line:90% so I just use u. 00:42:08.320 --> 00:42:11.400 align:middle line:84% So 80 to 100 microns an hour, about 2 millimeters a day. 00:42:11.400 --> 00:42:14.550 align:middle line:84% And at that stage of growth, when more and more axons 00:42:14.550 --> 00:42:16.890 align:middle line:84% appear in the tract, they grow up against each other. 00:42:16.890 --> 00:42:19.770 align:middle line:90% They form a bundle or ribbon. 00:42:19.770 --> 00:42:21.390 align:middle line:90% We call that fasciculation. 00:42:21.390 --> 00:42:26.130 align:middle line:84% If they're fasciculated, they form fascicles, they group 00:42:26.130 --> 00:42:27.930 align:middle line:90% together. 00:42:27.930 --> 00:42:29.640 align:middle line:90% And they're largely unbranched. 00:42:29.640 --> 00:42:30.990 align:middle line:90% And they have this-- 00:42:30.990 --> 00:42:34.020 align:middle line:84% during elongation, they have these swellings and filipodia 00:42:34.020 --> 00:42:34.890 align:middle line:90% that occur. 00:42:34.890 --> 00:42:37.960 align:middle line:84% It's almost like the whole axon is a growth cone. 00:42:37.960 --> 00:42:40.710 align:middle line:84% Because they have-- although there's the more active growth 00:42:40.710 --> 00:42:43.680 align:middle line:84% cone at the end, they have many little filipodia along the way. 00:42:43.680 --> 00:42:46.210 align:middle line:90% 00:42:46.210 --> 00:42:55.650 align:middle line:84% OK, so that we're talking only about that top, this area. 00:42:55.650 --> 00:42:58.410 align:middle line:90% That's the elongation. 00:42:58.410 --> 00:43:01.560 align:middle line:84% And then when they aroborize, they slow down. 00:43:01.560 --> 00:43:04.680 align:middle line:84% And my calculations for the hamster optic tract, 00:43:04.680 --> 00:43:06.600 align:middle line:84% and the [INAUDIBLE] body when they arborize, 00:43:06.600 --> 00:43:12.030 align:middle line:84% where they grow at about a tenth the rate, now that's 00:43:12.030 --> 00:43:16.320 align:middle line:84% the average rate for a group of exons. 00:43:16.320 --> 00:43:20.760 align:middle line:84% So [INAUDIBLE] the actual rate of an individual exon 00:43:20.760 --> 00:43:24.150 align:middle line:84% is not what I was able to measure. 00:43:24.150 --> 00:43:26.100 align:middle line:84% We also know that when they grow like that, 00:43:26.100 --> 00:43:28.098 align:middle line:90% they don't bundle up. 00:43:28.098 --> 00:43:28.890 align:middle line:90% They grow separate. 00:43:28.890 --> 00:43:30.210 align:middle line:90% They stay away from each other. 00:43:30.210 --> 00:43:32.040 align:middle line:84% They space themselves out so they're 00:43:32.040 --> 00:43:34.638 align:middle line:84% non-fasciculated with their branching. 00:43:34.638 --> 00:43:36.180 align:middle line:84% They're undergoing a lot of branches. 00:43:36.180 --> 00:43:39.150 align:middle line:84% So whereas one-- in elongation, they're 00:43:39.150 --> 00:43:40.650 align:middle line:90% supporting one growth cone. 00:43:40.650 --> 00:43:42.600 align:middle line:84% When they're arborizing, the same axon 00:43:42.600 --> 00:43:46.890 align:middle line:84% can be supporting many growth cones. 00:43:46.890 --> 00:43:48.300 align:middle line:90% Maybe that's why they slow down. 00:43:48.300 --> 00:43:51.570 align:middle line:90% 00:43:51.570 --> 00:43:54.450 align:middle line:84% And then we know that it's widespread, a lot of branching. 00:43:54.450 --> 00:43:56.490 align:middle line:84% But they're branches are rather rudimentary. 00:43:56.490 --> 00:43:59.070 align:middle line:90% And then later they vocalize. 00:43:59.070 --> 00:44:02.850 align:middle line:84% They grow mainly in one place, and they lose the others, OK. 00:44:02.850 --> 00:44:07.690 align:middle line:90% 00:44:07.690 --> 00:44:12.010 align:middle line:84% And in that period of early branch formation, 00:44:12.010 --> 00:44:15.940 align:middle line:84% you can get branches appearing in places where the axon will 00:44:15.940 --> 00:44:18.400 align:middle line:90% not terminate later. 00:44:18.400 --> 00:44:21.115 align:middle line:84% And sometimes they even seem to be growing arbors there, 00:44:21.115 --> 00:44:22.240 align:middle line:90% but then they're withdrawn. 00:44:22.240 --> 00:44:24.340 align:middle line:84% For example, the optic tract will 00:44:24.340 --> 00:44:26.740 align:middle line:84% grow into the ventral posterior nucleus. 00:44:26.740 --> 00:44:28.330 align:middle line:84% That's a somatosensory structure, 00:44:28.330 --> 00:44:30.250 align:middle line:84% but they don't terminate there normally. 00:44:30.250 --> 00:44:34.090 align:middle line:84% They grow in, and then they pull out. 00:44:34.090 --> 00:44:37.570 align:middle line:84% Lateral posterior nucleus gets a lot of axons in development, 00:44:37.570 --> 00:44:40.228 align:middle line:84% many fewer in the adult. It does get a few. 00:44:40.228 --> 00:44:41.770 align:middle line:84% We used to think they didn't get any. 00:44:41.770 --> 00:44:43.450 align:middle line:84% But with the sensitive methods, we 00:44:43.450 --> 00:44:46.540 align:middle line:84% do see a few there, but many more in development. 00:44:46.540 --> 00:44:52.450 align:middle line:84% And then it was discovered by Inocenti, 00:44:52.450 --> 00:44:54.620 align:middle line:84% an Italian neuroscientist working in Switzerland, 00:44:54.620 --> 00:44:56.830 align:middle line:84% that this happens in transcortical connections, 00:44:56.830 --> 00:44:59.830 align:middle line:84% in the cat, and probably all mammals, 00:44:59.830 --> 00:45:02.710 align:middle line:84% where you have whole transient connections forming, 00:45:02.710 --> 00:45:04.870 align:middle line:90% and then they disappear. 00:45:04.870 --> 00:45:07.300 align:middle line:84% Sometimes they actually form synaptic connections 00:45:07.300 --> 00:45:10.560 align:middle line:84% in structures that they then withdraw from. 00:45:10.560 --> 00:45:12.310 align:middle line:84% And other times, they're just these little 00:45:12.310 --> 00:45:15.220 align:middle line:84% transient collaterals, as I showed you for the optic tract. 00:45:15.220 --> 00:45:20.680 align:middle line:90% 00:45:20.680 --> 00:45:25.190 align:middle line:84% And when that vocalization is occurring, 00:45:25.190 --> 00:45:31.750 align:middle line:84% that's when the topographic map becomes more precise. 00:45:31.750 --> 00:45:34.510 align:middle line:84% There is a crude topography even at the beginning 00:45:34.510 --> 00:45:37.990 align:middle line:84% when they first start to arborize. 00:45:37.990 --> 00:45:40.780 align:middle line:84% But then it becomes more precise as they focalize. 00:45:40.780 --> 00:45:45.250 align:middle line:90% 00:45:45.250 --> 00:45:47.500 align:middle line:90% We'll start there next time. 00:45:47.500 --> 00:45:48.000 align:middle line:90%