1 00:00:15 --> 00:00:17 Differentiation. For someone who didn't see the 2 00:00:17 --> 00:00:23 board. Yeah? 3 00:00:23 --> 00:00:26 Yeah. So what a cell will become eventually. And differentiation is 4 00:00:26 --> 00:00:30 the process by which it becomes that. 5 00:00:30 --> 00:00:34 OK. Good. So these terms are essential for you to know for 6 00:00:34 --> 00:00:38 today's lecture. And if they are a little murky for 7 00:00:38 --> 00:00:42 you then you should really make sure they become clear. 8 00:00:42 --> 00:00:46 OK. I'm going to talk to you today about stem cells. 9 00:00:46 --> 00:00:50 This is the how-to module, our second how-to module. We have 10 00:00:50 --> 00:00:54 two lectures, stem cells and cloning. 11 00:00:54 --> 00:00:59 And what I want to do today is to go beyond the media, 12 00:00:59 --> 00:01:04 beyond the hype and tell you about stem cells, what we really know and 13 00:01:04 --> 00:01:09 why there is such a fuss about them. And to put it in perspective of 14 00:01:09 --> 00:01:14 where you are in the course, in our game board of life we're 15 00:01:14 --> 00:01:19 moving right along past foundations up through formation and now into 16 00:01:19 --> 00:01:24 our second how-to module. So let's start with a question. 17 00:01:24 --> 00:01:30 And you have a handout. If you do not have a handout, Shamsah, 18 00:01:30 --> 00:01:34 could you be mail person? And on the other side it looks like 19 00:01:34 --> 00:01:39 we need another mail person. You will realize that I typically 20 00:01:39 --> 00:01:43 do give you a handout of the most important slides so that you don't 21 00:01:43 --> 00:01:48 have to grapple with things and that you don't, in fact, 22 00:01:48 --> 00:01:53 have to print out the PowerPoint handout before you come. 23 00:01:53 --> 00:01:58 So you should always just check. OK. So what is a stem cell? 24 00:01:58 --> 00:02:04 Well, a stem cell is something that has two important capacities. 25 00:02:04 --> 00:02:10 Firstly, it can self-renew. That's a very popular term. 26 00:02:10 --> 00:02:17 It can make more of itself. And, secondly, it can give rise to 27 00:02:17 --> 00:02:23 a number of different, one or more different cell fates. 28 00:02:23 --> 00:02:39 So it can give rise -- 29 00:02:39 --> 00:02:44 -- to one or more differentiated cell types. Schematically, 30 00:02:44 --> 00:02:49 I've represented it for you in this way. So here is something that's 31 00:02:49 --> 00:02:54 called an uncommitted stem cell. And you should be familiar with the 32 00:02:54 --> 00:02:59 term uncommitted, naive and so on. 33 00:02:59 --> 00:03:03 And this uncommitted stem cell can undergo cell division to make more 34 00:03:03 --> 00:03:07 of itself. And it can also go on to become something that's termed in 35 00:03:07 --> 00:03:11 the field a "progenitor cell". It's just a term. It means a cell 36 00:03:11 --> 00:03:15 that is going to give rise to something else. 37 00:03:15 --> 00:03:19 And I've depicted it here as a determine progenitor. 38 00:03:19 --> 00:03:23 It knows what it's going to become. And this term uncommitted stem cell, 39 00:03:23 --> 00:03:27 the uncommitted part is a little misleading. I'll talk about that 40 00:03:27 --> 00:03:31 more in a moment. This determined progenitor then goes 41 00:03:31 --> 00:03:35 on to give rise to a whole bunch of one or more, actually, 42 00:03:35 --> 00:03:40 differentiated cell types. And the number of cell types that a 43 00:03:40 --> 00:03:44 stem cell can go on to give rise to is a measurement of its potency. 44 00:03:44 --> 00:03:48 That is the definition of the term potency. So this is the notion of 45 00:03:48 --> 00:03:53 the stem cell concept. You have this. It's the first 46 00:03:53 --> 00:03:57 figure on your handout. And you'll find this in many 47 00:03:57 --> 00:04:02 newspaper articles and books. So what's so special about this and 48 00:04:02 --> 00:04:08 why are there newspaper articles and newspaper articles and newspaper 49 00:04:08 --> 00:04:14 articles and television programs and covers of Time Magazine about stem 50 00:04:14 --> 00:04:20 cells? What's the hype? Well, the deal is that it is 51 00:04:20 --> 00:04:26 believed by some that stem cells are going to be some kind of 52 00:04:26 --> 00:04:31 universal repair kit. That somehow because stem cells can 53 00:04:31 --> 00:04:35 give rise often to differentiated cell types and make more of 54 00:04:35 --> 00:04:40 themselves that they are in a position to repair parts of the body 55 00:04:40 --> 00:04:44 when the body needs repair. You have a heart attack, you throw 56 00:04:44 --> 00:04:48 in some stem cells, repair the heart, you have a brain 57 00:04:48 --> 00:04:53 lesion, you throw in some stem cells and you repair the damage. 58 00:04:53 --> 00:04:57 And if you go through the newspapers, as I did very, 59 00:04:57 --> 00:05:02 very briefly, it's very easy to find many, many different headlines. 60 00:05:02 --> 00:05:05 One of the big ones last week, I'll talk about this at the end, 61 00:05:05 --> 00:05:09 is that the Massachusetts House passed a law expanding the use of 62 00:05:09 --> 00:05:13 stem cells in research. Hopkins is beginning human trials 63 00:05:13 --> 00:05:17 with donor adult stem cells to repair muscle damage from heart 64 00:05:17 --> 00:05:21 attack. This is an enormous amount of interest in these cells because 65 00:05:21 --> 00:05:25 of this notion of universal repair kit-ness. There are also 66 00:05:25 --> 00:05:29 advertisements. The Stem Cell Bank and the Cord 67 00:05:29 --> 00:05:33 Blood Registry would love you to pay them to save your child's umbilical 68 00:05:33 --> 00:05:38 cord stem cells, or the blood cells from a newborn's 69 00:05:38 --> 00:05:43 umbilical cord in the possibility that you'll use these later on to 70 00:05:43 --> 00:05:47 save the life of this child or somebody else. 71 00:05:47 --> 00:05:52 And the other reason that stem cells have got so much hype is that, 72 00:05:52 --> 00:05:57 as we'll discuss towards the end of the lecture, they involve the use, 73 00:05:57 --> 00:06:02 or are likely to involve the use of human embryos. 74 00:06:02 --> 00:06:07 And that's a real fire point. But let's step back and let's talk 75 00:06:07 --> 00:06:12 about stem cells and biologically what are these things and what do 76 00:06:12 --> 00:06:18 you need them for? And the thing we have to consider 77 00:06:18 --> 00:06:23 is the question of organ maintenance. Now, I've told you several times, 78 00:06:23 --> 00:06:29 oh, you see, it popped up. There you go. We have to give these frogs 79 00:06:29 --> 00:06:34 a moment to pop up. OK. That must have been somebody's 80 00:06:34 --> 00:06:38 frog that popped up. I've told you several times that 81 00:06:38 --> 00:06:42 our bodies consist of about ten to the twelfth cells. 82 00:06:42 --> 00:06:46 That's a lot of cells. But it's actually much worse than 83 00:06:46 --> 00:06:50 that because it is not the same ten to the twelfth cells that you 84 00:06:50 --> 00:06:54 started with, once you got there, that you keep throughout your life. 85 00:06:54 --> 00:06:58 The cells in your body are constantly dying and are constantly 86 00:06:58 --> 00:07:02 being replenished. And, in fact, if you look at your 87 00:07:02 --> 00:07:06 ten to the twelfth cells, over the course of your lifetime you 88 00:07:06 --> 00:07:10 will make many times ten to the twelfth cells. 89 00:07:10 --> 00:07:14 You're not going to replace every cell in your body because some of 90 00:07:14 --> 00:07:19 them life forever, but for many populations of cells in 91 00:07:19 --> 00:07:23 your body you will replace those organs completely. 92 00:07:23 --> 00:07:27 OK? So we're talking about making lots and lots of cells. 93 00:07:27 --> 00:07:32 And the issue really is one of organ and tissue maintenance. 94 00:07:32 --> 00:07:39 And if one considers how organs and 95 00:07:39 --> 00:07:44 tissues are maintained, they fall into three categories 96 00:07:44 --> 00:07:50 loosely. Those that have a high turnover rate where cells do not 97 00:07:50 --> 00:07:55 live for very long and need to be replenished rather frequently to 98 00:07:55 --> 00:08:01 maintain the size and function of the organ. 99 00:08:01 --> 00:08:05 Those organs that have a low turnover rate and those that are 100 00:08:05 --> 00:08:10 said to be static. Organs with a high turnover rate 101 00:08:10 --> 00:08:14 include, as I'll tell you a moment, the blood, some of your hair. 102 00:08:14 --> 00:08:19 Organs with a low turnover rate include things like your liver and 103 00:08:19 --> 00:08:24 pancreases. And organs that were thought to be static, 104 00:08:24 --> 00:08:28 but in fact that may not be true, was the nervous system. And now we 105 00:08:28 --> 00:08:34 know that that's not true. So let's talk about how we get to 106 00:08:34 --> 00:08:40 these terms. High turnover. Low turnover. Let me just write it 107 00:08:40 --> 00:08:47 out. Turnover. And static natures of organ 108 00:08:47 --> 00:08:53 maintenance. And the way we do this, or the way this has been done is by 109 00:08:53 --> 00:09:00 an assay that's called a pulse-chase assay. 110 00:09:00 --> 00:09:05 And a pulse-chase assay, and this is the second slide on your 111 00:09:05 --> 00:09:10 handout, goes like this. In a cell population one can add a 112 00:09:10 --> 00:09:15 nucleotide analog bromodeoxyuridine. It's uridine but it's incorporated 113 00:09:15 --> 00:09:21 into DNA because it's got that deoxy there. And bromodeoxyuridine is a 114 00:09:21 --> 00:09:26 nucleotide that is incorporated and can then be detected using 115 00:09:26 --> 00:09:30 various stains. So you can tell which cells have 116 00:09:30 --> 00:09:34 incorporated BrdU by staining them appropriately. 117 00:09:34 --> 00:09:38 So you can tell which cells are dividing, which cells have gone 118 00:09:38 --> 00:09:41 through S phase by whether they are labeled or not. 119 00:09:41 --> 00:09:45 And if you give a cell population a short pulse, if you give BrdU for 120 00:09:45 --> 00:09:49 just a short time, this is called a pulse, 121 00:09:49 --> 00:09:52 it gets incorporated into the DNA, and then the BrdU is gone. And so 122 00:09:52 --> 00:09:56 you label your cell population. And then you follow the cells over 123 00:09:56 --> 00:10:00 a long time without any added label. 124 00:10:00 --> 00:10:03 And so the only cells you look at are the ones that were labeled 125 00:10:03 --> 00:10:07 during this pulse period. And if you do that, you see that 126 00:10:07 --> 00:10:11 your initially labeled cell population lives for a while, 127 00:10:11 --> 00:10:15 and then the cells start to disappear because they die. 128 00:10:15 --> 00:10:19 And you can monitor the half-life a population by counting how long 129 00:10:19 --> 00:10:23 cells live for. OK? So this is a pulse-chase 130 00:10:23 --> 00:10:27 analysis. And this will give you the cell turnover time of 131 00:10:27 --> 00:10:31 a particular organ. And this is very useful in trying to 132 00:10:31 --> 00:10:35 figure out whether cells have a high turnover rate or a low turnover rate. 133 00:10:35 --> 00:10:39 Now, you can do something else with this. You can look at your labeled 134 00:10:39 --> 00:10:43 cell population, and you can actually look not just 135 00:10:43 --> 00:10:48 at the numbers of cells but you can look at what those cells become. 136 00:10:48 --> 00:10:52 And sometimes your initially labeled cell population will be the 137 00:10:52 --> 00:10:56 final form, the differentiated state of the cells, but sometimes it won't. 138 00:10:56 --> 00:11:00 And then it's very informative to follow that labeled cell population 139 00:11:00 --> 00:11:04 and see what they do. This is your third handout, 140 00:11:04 --> 00:11:08 your third slide on your handout. And if you follow them you might see 141 00:11:08 --> 00:11:12 that this labeled cell population, during the chase, differentiates 142 00:11:12 --> 00:11:15 into something. And that's very interesting. 143 00:11:15 --> 00:11:19 And then later on those cells will die and you'll be able to measure a 144 00:11:19 --> 00:11:23 half-life as previously, but this is very interesting because 145 00:11:23 --> 00:11:27 it tells you that this labeled cell population was not the same as the 146 00:11:27 --> 00:11:31 final fate of the cells. And it implies that there is some 147 00:11:31 --> 00:11:35 kind of precursor cell, some kind of progenitor in your 148 00:11:35 --> 00:11:39 labeled cell population that's giving rise to the differentiated 149 00:11:39 --> 00:11:43 cells. And it implies that these initially labeled cells either are 150 00:11:43 --> 00:11:47 stem cells or are progenitors derived from stem cells that are 151 00:11:47 --> 00:11:51 responsible for repopulating a tissue as the tissue or organ needs 152 00:11:51 --> 00:11:56 to be maintained. So these are two assays that are 153 00:11:56 --> 00:12:00 very useful and are used to, have been used to define high 154 00:12:00 --> 00:12:04 turnover organs. So what are some of these organs 155 00:12:04 --> 00:12:08 that have, or tissues that have high turnover? 156 00:12:08 --> 00:12:18 In fact, they've been used, 157 00:12:18 --> 00:12:23 these assays have been used to define many tissues, 158 00:12:23 --> 00:12:28 those with high and those with low turnover. But it's the high 159 00:12:28 --> 00:12:33 turnover ones I want to talk about. The red blood cells in your body 160 00:12:33 --> 00:12:38 have a turnover time, half-time of about 120 days. 161 00:12:38 --> 00:12:42 And if you do the calculation, because you have a lot of red blood 162 00:12:42 --> 00:12:46 cells, you're making more than a billion cells a day. 163 00:12:46 --> 00:12:50 OK? As you are sitting here, you are probably, during the course 164 00:12:50 --> 00:12:54 of this lecture, making several million new red blood 165 00:12:54 --> 00:12:58 cells. Your intestine, some of the cells in your intestine 166 00:12:58 --> 00:13:02 that are responsible for food absorption turnover every 167 00:13:02 --> 00:13:07 three to five days. Your skin cells, 168 00:13:07 --> 00:13:11 the epidermis of your skin turns over with a half-life of about 14 169 00:13:11 --> 00:13:15 days. And your hair, depending on where it comes from, 170 00:13:15 --> 00:13:20 turns over with a half-life of about 14 days to about four years. 171 00:13:20 --> 00:13:27 And the sense in many of these organs, and I'll go through this a 172 00:13:27 --> 00:13:34 bit more in a moment, is that this high turnover and the 173 00:13:34 --> 00:13:41 replenishment of these cells as they disappear is driven by a stem cell 174 00:13:41 --> 00:13:48 population. And I'm going to abbreviate stem cells SC. 175 00:13:48 --> 00:13:55 And I'm going to put forth the hypothesis that stem cells replenish 176 00:13:55 --> 00:14:00 these organs. Now, in some cases you can actually 177 00:14:00 --> 00:14:04 look at the organ and you can do your pulse-chase, 178 00:14:04 --> 00:14:07 just like I told you, and you can see that this must be so. 179 00:14:07 --> 00:14:11 And an example that we've talked about previously, 180 00:14:11 --> 00:14:14 not in this context but previously is the testes, 181 00:14:14 --> 00:14:17 the spermatagonia are the cells sitting right at the edge of the 182 00:14:17 --> 00:14:21 seminiferous tubule, which we'd label with BrdU. 183 00:14:21 --> 00:14:24 And as you do your chase you would see those cells as labeled cells 184 00:14:24 --> 00:14:28 moving towards the lumen and differentiating into the 185 00:14:28 --> 00:14:31 stem cells. OK? And so you can actually see those 186 00:14:31 --> 00:14:35 cells giving rise to the differentiated cells. 187 00:14:35 --> 00:14:39 Another tissue where this is very beautiful is in the epidermis of the 188 00:14:39 --> 00:14:43 skin. So the epidermis of the skin comprises multiple layers stacked up 189 00:14:43 --> 00:14:46 on top of each other. And as you get towards the outside 190 00:14:46 --> 00:14:50 of the epidermis, the cells in the outer layers are 191 00:14:50 --> 00:14:54 dead and they form the layer that we feel as skin. But, 192 00:14:54 --> 00:14:58 in fact, there are a lot of cells that are living underneath 193 00:14:58 --> 00:15:02 the outer layer. In the very deepest layers of the 194 00:15:02 --> 00:15:06 epidermis reside a dividing population of cells that are, 195 00:15:06 --> 00:15:10 that include the stem cells, and I'll show you an assay for some of 196 00:15:10 --> 00:15:14 this in a little bit. These dividing cells, 197 00:15:14 --> 00:15:19 if you follow them, give rise to the cells in the layers above. 198 00:15:19 --> 00:15:23 So they either are the stem cells or they are progenitor cells that 199 00:15:23 --> 00:15:27 are replenishing the epidermis. Now, in the blood, I told you that 200 00:15:27 --> 00:15:31 you can do a pulse-chase assay. And you can very accurately measure 201 00:15:31 --> 00:15:35 the half-life of red blood cells. And it's clear that red blood cells 202 00:15:35 --> 00:15:39 must come from some precursor. In ourselves, in most vertebrates, 203 00:15:39 --> 00:15:42 in many vertebrates red blood cells do not have nuclei so they cannot 204 00:15:42 --> 00:15:46 divide themselves. And so there must be some kind of 205 00:15:46 --> 00:15:50 progenitor. But the progenitors of red blood cells and, 206 00:15:50 --> 00:15:53 in fact, many other cells in the blood lie deep within the bone 207 00:15:53 --> 00:15:57 marrow, the cavity of the bones. And so you really cannot observe 208 00:15:57 --> 00:16:01 them well to come to the conclusions that I've just given you about the 209 00:16:01 --> 00:16:05 testes and the epidermis of the skin. 210 00:16:05 --> 00:16:10 So you have to do some kind of other assay to figure out where these 211 00:16:10 --> 00:16:15 cells are coming from. And over the years a huge body of 212 00:16:15 --> 00:16:20 data has given rise to this notion. In the bone marrow of the long 213 00:16:20 --> 00:16:25 bones, and some of the shorter bones, too, there is some kind of 214 00:16:25 --> 00:16:30 pluripotential or pluripotent hematopoietic stem cell, 215 00:16:30 --> 00:16:35 that I've abbreviated HSC. And this HSC gives rise, 216 00:16:35 --> 00:16:39 through a series of progenitors which then go on to differentiate, 217 00:16:39 --> 00:16:44 into all of the cell types that are found in the blood. 218 00:16:44 --> 00:16:49 The red blood cells. The so-called white blood cells. 219 00:16:49 --> 00:16:54 The immune cells. A huge spectrum of differentiated cell types. 220 00:16:54 --> 00:16:58 And this hematopoietic stem cell has been very, very useful in helping 221 00:16:58 --> 00:17:03 people who have got blood associated illnesses in that it can, 222 00:17:03 --> 00:17:07 after removal of diseased blood cells, it can repopulate the tissue 223 00:17:07 --> 00:17:12 and help people get better. And it's done so, and I'll go 224 00:17:12 --> 00:17:16 through how this is done both in an assay sense and point out that this 225 00:17:16 --> 00:17:21 is exactly what's done in humans, it's done so in a bone marrow 226 00:17:21 --> 00:17:25 transplant. So in assays, let's just write down a couple of 227 00:17:25 --> 00:17:34 things here. 228 00:17:34 --> 00:17:38 I've told you one assay already for stem cells. 229 00:17:38 --> 00:17:49 OK? So assays for stem cells ask 230 00:17:49 --> 00:17:55 how do you know if you've got a stem cell? Well, you can do that. 231 00:17:55 --> 00:18:01 You can ask this using the pulse-chase by direct observation 232 00:18:01 --> 00:18:07 with your pulse-chase. You can also do it by a repopulation 233 00:18:07 --> 00:18:13 assay. And the notion in a repopulation assay is that you try 234 00:18:13 --> 00:18:18 to restore a particular cell type by putting in what you think is the 235 00:18:18 --> 00:18:24 precursor of that cell type. Now, in order to do these assays 236 00:18:24 --> 00:18:30 you usually have to get rid of the normal cells that were there so that 237 00:18:30 --> 00:18:34 you can assay your repopulation. OK? Otherwise, 238 00:18:34 --> 00:18:38 the normal cells can out-compete the cells you're putting in. 239 00:18:38 --> 00:18:42 OK? But that's an aside. So what is a bone marrow transplant? 240 00:18:42 --> 00:18:46 The notion here is that one takes an animal, and this is done in 241 00:18:46 --> 00:18:50 humans, and you irradiate the animal so that you destroy all its bone 242 00:18:50 --> 00:18:54 marrow. This will eventually kill the animal, OK, 243 00:18:54 --> 00:18:58 because you cannot live without a hematopoietic stem cell and without 244 00:18:58 --> 00:19:02 all the cells that derive from that. 245 00:19:02 --> 00:19:06 And so, although this irradiated animal would die, 246 00:19:06 --> 00:19:10 you can rescue it by injecting normal bone marrow. 247 00:19:10 --> 00:19:14 You inject it into a mouse in the tail vein, into people it goes into 248 00:19:14 --> 00:19:19 a central line that goes into your blood system. And then those bone 249 00:19:19 --> 00:19:23 marrow cells, it's extraordinary, know where to go. They home to the 250 00:19:23 --> 00:19:27 bone marrow and they repopulate it and they make again the 251 00:19:27 --> 00:19:32 hematopoietic stem cell and all the lineages that come from that. 252 00:19:32 --> 00:19:36 Or they are the hematopoietic stem cell, they maintain themselves and 253 00:19:36 --> 00:19:40 they give rise to all the other lineages. One of the criteria that 254 00:19:40 --> 00:19:44 I told you about stem cells is that they need to be self-renewing. 255 00:19:44 --> 00:19:49 You can ask whether or not these stem cells are self-renewing in an 256 00:19:49 --> 00:19:53 assay like this, where you take a mouse that you've 257 00:19:53 --> 00:19:57 rescued by bone marrow transplant, and you can ask whether it can serve 258 00:19:57 --> 00:20:01 as a source of marrow of hematopoietic stem cells that will 259 00:20:01 --> 00:20:05 rescue another irradiated mouse. In other words, 260 00:20:05 --> 00:20:09 you could have rescued that first mouse just by giving it a whole 261 00:20:09 --> 00:20:12 spectrum of almost differentiated red blood cells, 262 00:20:12 --> 00:20:16 white blood cells, immune cells and so on. 263 00:20:16 --> 00:20:19 OK? In order to test whether you actually rescued it by giving it a 264 00:20:19 --> 00:20:23 stem cell, you can ask whether or not you can take this rescued mouse 265 00:20:23 --> 00:20:27 and use it to rescue another mouse. And, in fact, you can. 266 00:20:27 --> 00:20:32 And this is consistent with, not unequivocal, but it's consistent 267 00:20:32 --> 00:20:37 with a sense that you have generated more stem cells in the rescued mouse 268 00:20:37 --> 00:20:43 that can go on to rescue another mouse. Another set of experiments 269 00:20:43 --> 00:20:48 that has been really pivotal in understanding stem cells is to try 270 00:20:48 --> 00:20:53 to purify the stem cells and to assay what are these pure stem cells. 271 00:20:53 --> 00:20:59 And part of this is asking how many cells do you need to rescue in a 272 00:20:59 --> 00:21:06 repopulation assay? 273 00:21:06 --> 00:21:10 OK. And so let me make a note here. 274 00:21:10 --> 00:21:19 That when it comes to stem cell 275 00:21:19 --> 00:21:25 isolation the overriding conclusion from many assays done by many labs 276 00:21:25 --> 00:21:32 is that stem cells are rare and they are hard to find. 277 00:21:32 --> 00:21:36 In this experiment, which is number five on your handout, 278 00:21:36 --> 00:21:40 you can assay for how many stem cells you need to rescue a mouse by 279 00:21:40 --> 00:21:44 removing bone marrow from one mouse and staining that bone marrow, 280 00:21:44 --> 00:21:49 if you can, for a stem cell marker. Now, what is a stem cell marker? 281 00:21:49 --> 00:21:53 OK? If I have a stem cell marker I would be able to go and pull out the 282 00:21:53 --> 00:21:57 stem cells. Well, there are no markers. 283 00:21:57 --> 00:22:01 A marker could be a protein or an RNA that is expressed in a 284 00:22:01 --> 00:22:06 particular group of cells that you suspect are the stem cells. 285 00:22:06 --> 00:22:10 OK? This is very much an experimental-driven field. 286 00:22:10 --> 00:22:14 There are markers that always appear, proteins that are always 287 00:22:14 --> 00:22:18 there, antigens on the surface stained with antibodies that are 288 00:22:18 --> 00:22:22 always there where you have a population of cells that is able to 289 00:22:22 --> 00:22:26 rescue a mouse. And so you can use these to 290 00:22:26 --> 00:22:30 hypothesize that this stem cell marker is, in fact, marking 291 00:22:30 --> 00:22:35 the stem cells. And you can go and purify cells that 292 00:22:35 --> 00:22:39 are expressing a particular marker. And the way you do this is by using 293 00:22:39 --> 00:22:43 a really fantastic machine called the Fluorescence Activated Cell 294 00:22:43 --> 00:22:47 Sorter (FACS). I'll go through it very briefly in 295 00:22:47 --> 00:22:51 a moment. But what it is able to do is to purify cells that have got 296 00:22:51 --> 00:22:55 particular fluorescences. And they have these particular 297 00:22:55 --> 00:22:59 fluorescence spectra because of the way you have stained them for 298 00:22:59 --> 00:23:03 particular stem cell markers. And what you get out of your FACS 299 00:23:03 --> 00:23:07 machine is a population that is much more enriched for what I've called 300 00:23:07 --> 00:23:11 stem cells, but they're actually cells that are expressing this 301 00:23:11 --> 00:23:15 punitive stem cell marker. And then you can take these cells 302 00:23:15 --> 00:23:19 and say, well, have I really enriched for the stem 303 00:23:19 --> 00:23:23 cell, by asking how many cells you need to introduce into an irradiated 304 00:23:23 --> 00:23:27 mouse to rescue that mouse. And you can do a dilution assay 305 00:23:27 --> 00:23:31 where you put in one cell, ten cells, a hundred cells. 306 00:23:31 --> 00:23:35 Obviously, when you're down in the one cell range, 307 00:23:35 --> 00:23:39 you do your dilution assay, you get into a Poisson distribution, 308 00:23:39 --> 00:23:43 and you may not have exactly one cell. But you can do this 309 00:23:43 --> 00:23:47 statistically and you can figure out how many cells from your population 310 00:23:47 --> 00:23:51 it takes to rescue a mouse. OK? And the data to date tells us 311 00:23:51 --> 00:23:55 that one cell of the correctly purified population is sufficient to 312 00:23:55 --> 00:23:59 rescue an irradiated mouse. You have to do some tricks. 313 00:23:59 --> 00:24:02 You have to add some extra cells to this one cell. 314 00:24:02 --> 00:24:06 You cannot just put one cell into the mouse and rescue. 315 00:24:06 --> 00:24:09 You have to mix this one cell with some other cells that help it do its 316 00:24:09 --> 00:24:13 work. OK? But this is really an extraordinary assay that tells us 317 00:24:13 --> 00:24:17 we're very close to purifying a hematopoietic stem cell. 318 00:24:17 --> 00:24:20 What is a FACS machine? Very briefly. You can look at this later. 319 00:24:20 --> 00:24:24 Cells that are labeled with fluorescent antibodies are broken up 320 00:24:24 --> 00:24:28 into a stream of tiny droplets each of which will contain one cell. 321 00:24:28 --> 00:24:32 And the cells then pass in a stream through a laser which activates the 322 00:24:32 --> 00:24:37 fluorescence. There's a fluorescence detector that you can 323 00:24:37 --> 00:24:41 program to respond to a particular wavelength. And what happens is 324 00:24:41 --> 00:24:46 that when the detector finds that it has responded to a cell passing 325 00:24:46 --> 00:24:50 through it because of the fluorescence emitted, 326 00:24:50 --> 00:24:55 a charge is given to that cell, and that cell with a particular 327 00:24:55 --> 00:25:00 fluorescence moves down the stream through some deflecting plates where 328 00:25:00 --> 00:25:04 it will be deflected, according to its charge, 329 00:25:04 --> 00:25:08 into a collecting bin. And that way you can really get some 330 00:25:08 --> 00:25:12 very highly purified cell populations. You can purify about 331 00:25:12 --> 00:25:15 300,000 cells an hour using the FACS machine. And we have a lot of them 332 00:25:15 --> 00:25:19 here at MIT and use them. Many laboratories use them for 333 00:25:19 --> 00:25:22 various things. A really cool machine. 334 00:25:22 --> 00:25:26 OK. But we need to move on. So we talked about some assays for 335 00:25:26 --> 00:25:30 stem cells. Talked about the potency of 336 00:25:30 --> 00:25:34 hematopoietic stem cells. I want to tell you something now 337 00:25:34 --> 00:25:39 about the control of stem cells and how a stem cell decides what to do. 338 00:25:39 --> 00:25:43 So if you think about organ maintenance, there is an implicit 339 00:25:43 --> 00:25:48 understanding that the organ knows how much it needs to be maintained. 340 00:25:48 --> 00:25:53 It needs to know how many cells are being lost, it needs to know when to 341 00:25:53 --> 00:25:57 make more cells, and it also needs to know if there 342 00:25:57 --> 00:26:02 has been some catastrophe. If there's been a liver, 343 00:26:02 --> 00:26:07 some kind of a resection of the liver, or if there has been skin 344 00:26:07 --> 00:26:12 wounding, OK, the organ needs to be able to respond. 345 00:26:12 --> 00:26:16 And the stem cells in an organ need to be able to respond to this 346 00:26:16 --> 00:26:21 catastrophe. And this is where this term niche comes in. 347 00:26:21 --> 00:26:26 Niche is one of these fields in the term, terms in the field. 348 00:26:26 --> 00:26:31 It's like the term progenitor. And niche is nothing that you 349 00:26:31 --> 00:26:37 haven't heard about before. What it refers to are the cells, 350 00:26:37 --> 00:26:42 as written up there, that surround the stem cell. 351 00:26:42 --> 00:26:48 And what I'm going to tell you is that the niche cells, 352 00:26:48 --> 00:26:53 the surrounding cells control, by induction, stem cell fate. And 353 00:26:53 --> 00:26:59 I'm also going to point out that there can be some input 354 00:26:59 --> 00:27:04 of the environment. So this is a cartoon that I drew for 355 00:27:04 --> 00:27:08 you. It's number six on your handout. It's six across on your 356 00:27:08 --> 00:27:13 handout. And I have here some stem cells that I've called quiescent. 357 00:27:13 --> 00:27:17 They are not dividing. Or they may be dividing just a little bit just 358 00:27:17 --> 00:27:21 to maintain themselves but not to give rise to any progenitors that 359 00:27:21 --> 00:27:26 will go on to differentiate. And these quiescent cells are 360 00:27:26 --> 00:27:30 maintained in this quiescent state, we believe, because they are 361 00:27:30 --> 00:27:35 surrounded by a group of cells that is maintaining them quiescent. 362 00:27:35 --> 00:27:40 What happens then is that these quiescent cells sometimes get a 363 00:27:40 --> 00:27:45 stimulus, some kind of environmental input, be it wounding, 364 00:27:45 --> 00:27:50 be it some kind of hormonal input from another part of the body that 365 00:27:50 --> 00:27:55 influences the surrounding cells and changes the surrounding cells such 366 00:27:55 --> 00:28:00 that they now change their patterns of gene expression and start 367 00:28:00 --> 00:28:05 secreting stuff, most likely, that changes the 368 00:28:05 --> 00:28:11 activity of the stem cells that they are surrounding. 369 00:28:11 --> 00:28:15 I've shown you arrows there indicating induction from the stem 370 00:28:15 --> 00:28:19 cells, from the surrounding cells to the stem cells. 371 00:28:19 --> 00:28:23 And what this inductive process does, just like in the early embryo 372 00:28:23 --> 00:28:27 is to activate the stem cells to divide and to form progenitor cells 373 00:28:27 --> 00:28:31 which will go on to do their thing and differentiate into 374 00:28:31 --> 00:28:35 various fates. And also, of course, 375 00:28:35 --> 00:28:39 the stem cells are renewed. So this notion of the surrounding 376 00:28:39 --> 00:28:43 cells telling the stem cells what to do is kind of new in the stem cell 377 00:28:43 --> 00:28:48 field. If one is a developmental biologist, it's one of these, 378 00:28:48 --> 00:28:52 OK, kind of observations because we know that cells signal to one 379 00:28:52 --> 00:28:56 another. And it's not surprising that stem cells are told what to do 380 00:28:56 --> 00:29:01 by the cell surrounding them. But there has been not very much 381 00:29:01 --> 00:29:05 information on this. And I want to give you one system 382 00:29:05 --> 00:29:09 where it's been very beautifully looked at and where the contribution 383 00:29:09 --> 00:29:13 of the surrounding cells has been looked at. And this system is the 384 00:29:13 --> 00:29:17 hair follicle. So your hairs come from a sheath of 385 00:29:17 --> 00:29:21 cells that starts off in the epidermis and moves deep down, 386 00:29:21 --> 00:29:25 extends deep down into the deeper layers of the skin called 387 00:29:25 --> 00:29:30 the dermis. And there is a shaft of cells of 388 00:29:30 --> 00:29:35 which these matrix cells down here are important because these matrix 389 00:29:35 --> 00:29:40 cells divide and they give rise to cells that will form the center of 390 00:29:40 --> 00:29:44 the shaft and secrete, or not secrete but synthesis 391 00:29:44 --> 00:29:49 proteins called keratins. The keratins are this stuff. 392 00:29:49 --> 00:29:54 And eventually the cell gets so full of keratin that it dies. 393 00:29:54 --> 00:29:59 And at the same time more cells are coming in the bottom here and this 394 00:29:59 --> 00:30:04 whole shaft, this whole hair shaft is pushed out of the hair follicle. 395 00:30:04 --> 00:30:08 OK? And so that is why your hair grows. It has been pushed out from 396 00:30:08 --> 00:30:13 the bottom. And all the stuff that we are so fond of is dead cells. 397 00:30:13 --> 00:30:18 It's actually protein. It's a very interesting polymer of various 398 00:30:18 --> 00:30:22 keratins. Now, what's very interesting here is that 399 00:30:22 --> 00:30:27 over the years researchers have shown that the hair, 400 00:30:27 --> 00:30:31 well, let me back track for a moment. You know your hair falls out, 401 00:30:31 --> 00:30:34 right? You brush your hair and your hair falls out. 402 00:30:34 --> 00:30:38 OK? Ergo, it must be replenished somehow. And over the years it has 403 00:30:38 --> 00:30:41 been shown that these bulge cells, or cells in a little bulge, which 404 00:30:41 --> 00:30:44 was a rather insignificant little bulge but it's there, 405 00:30:44 --> 00:30:48 sitting on the side of the hair shaft is actually the source of the 406 00:30:48 --> 00:30:51 stem cells for the hair. And another region, I'm going to 407 00:30:51 --> 00:30:54 tell you about in a moment, is down here. It's labeled DP. 408 00:30:54 --> 00:30:58 Bear it in mind as I tell you something and then we'll 409 00:30:58 --> 00:31:02 come back to it. So let me first show you some data 410 00:31:02 --> 00:31:06 that tells us that the bulge cells are the cells that contain or that 411 00:31:06 --> 00:31:11 include the stem cells for making hair. One can do an experiment 412 00:31:11 --> 00:31:16 where one takes these bulge cells and does essentially a repopulation 413 00:31:16 --> 00:31:20 assay, just as in the case of the bone marrow transplant. 414 00:31:20 --> 00:31:25 And one can do this into a strain of mice called nude. 415 00:31:25 --> 00:31:30 So nude mice have got multiple issues. 416 00:31:30 --> 00:31:33 But one of their issues is that they have no fur. OK? 417 00:31:33 --> 00:31:37 And one can take bulge cells from a normal mouse and transplant them 418 00:31:37 --> 00:31:40 into a nude mouse. Here's the control transplant, 419 00:31:40 --> 00:31:44 and you can see there's no fur. And here is a transplant of bulge cells 420 00:31:44 --> 00:31:47 from a normal mouse. And here you see this tuft of fur 421 00:31:47 --> 00:31:51 growing out on the back of the nude mouse. And, in fact, 422 00:31:51 --> 00:31:54 some very cool assays have been done where these transplanted cells are 423 00:31:54 --> 00:31:58 labeled green. And you can look at these tufts of 424 00:31:58 --> 00:32:02 fur and see them growing fluorescently green. 425 00:32:02 --> 00:32:06 So you know that the new fur came from the transplanted cells. 426 00:32:06 --> 00:32:10 OK. So what about niche? Well, you know that your hair falls out. 427 00:32:10 --> 00:32:15 But, actually, there's a lot more to it than that. 428 00:32:15 --> 00:32:19 And it turns out there is a whole hair cycle of when the hair grows 429 00:32:19 --> 00:32:24 and when it doesn't grow. And it goes like this. There's a 430 00:32:24 --> 00:32:28 time called anagen which is a period of growth when these matrix cells 431 00:32:28 --> 00:32:32 down here are dividing and they are pushing out this hair 432 00:32:32 --> 00:32:37 and it's growing. After a while the cells down here 433 00:32:37 --> 00:32:41 stop dividing and the hair follicle doesn't grow anymore, 434 00:32:41 --> 00:32:45 or the hair shaft doesn't grow anymore. This is called catagen. 435 00:32:45 --> 00:32:49 And it's a time that's termed regression. It is when there is no 436 00:32:49 --> 00:32:53 more growth. And if the hair is going to fall out, 437 00:32:53 --> 00:32:57 it might fall out then, or it might stick around and the 438 00:32:57 --> 00:33:02 next step might take place. And this is a step called telogen 439 00:33:02 --> 00:33:06 which is a resting state. And then after that there is a new 440 00:33:06 --> 00:33:10 state, a new anagen where the hair will start growing again. 441 00:33:10 --> 00:33:14 And it may either be a new hair or it may be the one that's there that 442 00:33:14 --> 00:33:19 just grows longer. It depends on the part of the body. 443 00:33:19 --> 00:33:23 OK. Now, what I'm going to tell you is that a region called the 444 00:33:23 --> 00:33:27 dermal papilla induces stem cell activation at certain times during 445 00:33:27 --> 00:33:31 the hair cycle. So look at this. 446 00:33:31 --> 00:33:35 At anagen, during growth, the bulge and this dermal papilla, 447 00:33:35 --> 00:33:39 OK, this DP, it doesn't matter what the name is but just look at the red, 448 00:33:39 --> 00:33:42 are far apart from each other. At catagen they're still far apart, 449 00:33:42 --> 00:33:46 but look at what happens at telogen in this resting stage. 450 00:33:46 --> 00:33:50 The dermal papilla and the bulge are touching one another. 451 00:33:50 --> 00:33:53 And this is the phase, although it's called a resting phase, 452 00:33:53 --> 00:33:57 this is the phase when the hair follicle is actually getting ready 453 00:33:57 --> 00:34:01 for the next big spurt of cell division and differentiation that is 454 00:34:01 --> 00:34:05 going to make the hair longer or make a new one grow. 455 00:34:05 --> 00:34:08 And then, subsequently, the dermal papilla and the bulge 456 00:34:08 --> 00:34:11 stay close together, but eventually they will start 457 00:34:11 --> 00:34:15 pulling apart again. Well, it's been shown very 458 00:34:15 --> 00:34:18 elegantly that this dermal papilla signals to these bulge cells when 459 00:34:18 --> 00:34:22 they're touching one another and tells the bulge stem cells to 460 00:34:22 --> 00:34:25 activate and start making more hair or a new hair follicle. 461 00:34:25 --> 00:34:29 And the signaling is done through a pathway called the wnt 462 00:34:29 --> 00:34:33 signaling pathway. And I point this out because it uses 463 00:34:33 --> 00:34:37 a protein that should be familiar to you, which is our old friend 464 00:34:37 --> 00:34:41 beta-catenin that we talked about way back when as being important for 465 00:34:41 --> 00:34:45 dorsal-ventral patenting in the early embryo. And I point it out to 466 00:34:45 --> 00:34:49 you to make the point that proteins are used over and over again. 467 00:34:49 --> 00:34:53 Regulatory proteins can be used over and over again in the body. 468 00:34:53 --> 00:34:57 OK. So this is a very lovely example of what happens 469 00:34:57 --> 00:35:02 in a stem cell niche. So let's move on and let's ask how 470 00:35:02 --> 00:35:08 plastic adult stem cells are. OK. So here we have some issues. 471 00:35:08 --> 00:35:13 Well, the goal of the medical community is to get a stem cell that 472 00:35:13 --> 00:35:19 can repair every tissue. And it may be that every adult 473 00:35:19 --> 00:35:24 tissue has a stem cell population in it, but these can be difficult to 474 00:35:24 --> 00:35:30 isolate. It's very difficult to isolate these hair stem cells. 475 00:35:30 --> 00:35:35 You can get enough to get tufts of hair on another mouse, 476 00:35:35 --> 00:35:41 but it's hard to get enough of them to repair someone's bald head for 477 00:35:41 --> 00:35:47 example. OK? So the Holy Grail of stem cell-ness is to try to find a 478 00:35:47 --> 00:35:53 cell that is a multi-potent adult progenitor. OK? 479 00:35:53 --> 00:35:59 And this has been the real goal of the adult stem cell field. 480 00:35:59 --> 00:36:07 And the question that has been posed is, is there a multi-potent, 481 00:36:07 --> 00:36:15 a highly plastic, multi-potent also termed plastic, 482 00:36:15 --> 00:36:23 adult stem cell? And if there was then in theory one 483 00:36:23 --> 00:36:29 could use it to repair all organs. So here again is our stem cell 484 00:36:29 --> 00:36:33 concept where some kind of stem cell goes on to give rise to a set of 485 00:36:33 --> 00:36:37 differentiated cells. Now, the set of differentiated 486 00:36:37 --> 00:36:41 cells forms what's called a lineage where all the cell types that come 487 00:36:41 --> 00:36:45 out of a single stem cell are related to one another because they 488 00:36:45 --> 00:36:49 come from a common heritage, from a common progenitor, from a 489 00:36:49 --> 00:36:53 common stem cell. And you can, for different lineages, 490 00:36:53 --> 00:36:57 make different trees of stem cells progenitors and differentiated 491 00:36:57 --> 00:37:02 progeny. And it has been hypothesized that 492 00:37:02 --> 00:37:06 there is some kind of interconvertibility between 493 00:37:06 --> 00:37:11 different stem cells in the adult, and that one stem cell for one 494 00:37:11 --> 00:37:15 lineage can be under the right circumstances converted to a stem 495 00:37:15 --> 00:37:20 cell for a different lineage. And that maybe there's even some 496 00:37:20 --> 00:37:24 kind of master stem cell floating around in the adult that can give 497 00:37:24 --> 00:37:29 rise to all other stem cells. Maybe there is some totipotent or 498 00:37:29 --> 00:37:33 pluripotent very, very highly potent adult stem cell 499 00:37:33 --> 00:37:38 that can give rise to all these other stem cell lineages and can, 500 00:37:38 --> 00:37:43 therefore, be used as this universal repair kit. And this has been 501 00:37:43 --> 00:37:47 assayed. And the prediction from this is that adult stem cells from 502 00:37:47 --> 00:37:52 one lineage, for example the hematopoietic lineage, 503 00:37:52 --> 00:37:57 can contribute to another lineage. For example, the muscle cell 504 00:37:57 --> 00:38:02 lineage. And this prediction has been tested in the following way. 505 00:38:02 --> 00:38:06 It's particularly been tested with hematopoietic stem cells. 506 00:38:06 --> 00:38:10 And the question has been posed can these hematopoietic stem cells 507 00:38:10 --> 00:38:14 contribute to other lineages? Here's a mouse expressing green 508 00:38:14 --> 00:38:19 hematopoietic stem cells, or making green hematopoietic stem 509 00:38:19 --> 00:38:23 cells because of expression of the protein GFP under an appropriate 510 00:38:23 --> 00:38:27 tissue-specific promoter. You can isolate bone marrow cells 511 00:38:27 --> 00:38:32 from the GFP expressing mouse. Transplant them into an irradiated 512 00:38:32 --> 00:38:36 mouse. You get rescue, as we've discussed, but that's not 513 00:38:36 --> 00:38:40 the point of the experiment. The point of the experiment, 514 00:38:40 --> 00:38:44 and this is the second to last slide on your handout. 515 00:38:44 --> 00:38:49 The point of the experiment is to ask not about the bone marrow cells 516 00:38:49 --> 00:38:53 in the rescued mouse, but to ask whether any of the other 517 00:38:53 --> 00:38:57 tissues in the mouse contain these green cells that would have arisen 518 00:38:57 --> 00:39:02 from hematopoietic stem cells. OK. So we're asking whether these 519 00:39:02 --> 00:39:06 hematopoietic stem cells can contribute to lineages other than 520 00:39:06 --> 00:39:10 the blood and the immune system. And this has been an enormous focus 521 00:39:10 --> 00:39:15 of research. And the answer initially appeared to be yes. 522 00:39:15 --> 00:39:19 And this was a cause of tremendous excitement. Here's one piece of 523 00:39:19 --> 00:39:23 data. This is a picture of some cells from a mouse that was rescued 524 00:39:23 --> 00:39:28 with green hematopoietic stem cells. 525 00:39:28 --> 00:39:32 And this is a picture of the muscle from the mice. 526 00:39:32 --> 00:39:36 The blue are nuclei and the green is the GFP that would have come from 527 00:39:36 --> 00:39:40 these transplanted hematopoietic stem cells. And you can see green 528 00:39:40 --> 00:39:45 cells also in the nervous system, nerve cells that appear to come from 529 00:39:45 --> 00:39:49 these hematopoietic stem cells. Well, it turns out alas that this 530 00:39:49 --> 00:39:53 is an artifact of the experiment. And, in fact, what happens is that 531 00:39:53 --> 00:39:57 these green hematopoietic stem cells rescue the mouse in 532 00:39:57 --> 00:40:02 the correct way. But then they also fuse together 533 00:40:02 --> 00:40:06 with other cells. They fuse with muscle cells and 534 00:40:06 --> 00:40:11 with nerve cells. The two cells actually join 535 00:40:11 --> 00:40:16 together. Their cell membranes mingle and you get a kind of hybrid 536 00:40:16 --> 00:40:20 cell that is a mixture of the hematopoietic cell and the regular 537 00:40:20 --> 00:40:25 muscle cell. And this whole notion of the adult hematopoietic stem cell 538 00:40:25 --> 00:40:30 contributing to muscle, to nerve, to various other lineages 539 00:40:30 --> 00:40:34 has completely fallen to disfavor in most fields, in most 540 00:40:34 --> 00:40:38 people's opinion. Although, there is still data, 541 00:40:38 --> 00:40:42 there is still much experimentation to do to really address this, 542 00:40:42 --> 00:40:46 both in the hematopoietic stem cell and other lineages. 543 00:40:46 --> 00:40:49 But no data currently supports that adult stem cells can interconvert 544 00:40:49 --> 00:40:53 one to another. And that leads us to the difficult 545 00:40:53 --> 00:40:57 position of asking, so where is this universal repair 546 00:40:57 --> 00:41:01 kit going to come from? And this is where we move into the 547 00:41:01 --> 00:41:06 field of, I'm going to just leave that now, of embryonic stem cells. 548 00:41:06 --> 00:41:12 So you know now that embryos have got many cells that are multipotent, 549 00:41:12 --> 00:41:17 pluripotent. And indeed the zygote, as we discussed, 550 00:41:17 --> 00:41:22 is totipotent. These cells can give rise to many, many different tissues 551 00:41:22 --> 00:41:28 because during embryogenesis that's the deal. That's what 552 00:41:28 --> 00:41:33 has to happen. And so it came to the attention of 553 00:41:33 --> 00:41:37 people long ago that embryonic stem cells might be a great source of 554 00:41:37 --> 00:41:41 stem cells that can be used as this universal repair kit. 555 00:41:41 --> 00:41:45 Now, I want to tell you, I'm not going to go through this, 556 00:41:45 --> 00:41:49 you can go and look on the PowerPoint, you can go and look on 557 00:41:49 --> 00:41:53 the PDF file on your website. I'm going to tell you about a type 558 00:41:53 --> 00:41:58 of cell, in the last few minutes, called the embryonic stem cell. 559 00:41:58 --> 00:42:07 And what you really need to know 560 00:42:07 --> 00:42:11 about this is it's almost totipotent. And the first slides, 561 00:42:11 --> 00:42:16 I'm not going to go through, show you that this embryonic stem 562 00:42:16 --> 00:42:20 cell type is almost totipotent. Now, what about these embryonic 563 00:42:20 --> 00:42:25 stem cells? They come from, in taking a very early embryo, 564 00:42:25 --> 00:42:30 at a stage called the inner cell mass stage -- 565 00:42:30 --> 00:42:35 -- when there is a little mass of cells in the developing embryo, 566 00:42:35 --> 00:42:40 that's going to give rise to the whole embryo. And taking this cell, 567 00:42:40 --> 00:42:46 removing these inner cell mass cells and putting them into a Petri dish 568 00:42:46 --> 00:42:51 with various culture media, what happens is that the cells from 569 00:42:51 --> 00:42:57 this early embryo divide and they make various clumps. 570 00:42:57 --> 00:42:57 And the clumps are descended from one particular cell. 571 00:42:57 --> 00:42:57 And each of these clumps of cells is called an embryonic stem cell 572 00:42:57 --> 00:42:58 line. Now, what's a cell line? A cell line is a population of 573 00:42:58 --> 00:42:58 cells that can grow continuously in culture. In other words, 574 00:42:58 --> 00:42:59 it self-renews. What's a stem cell line? A cell line, 575 00:42:59 --> 00:42:59 this is on your PDF file so you might want to sit and listen to this 576 00:42:59 --> 00:43:00 and then go and get this afterwards. OK? What is a stem cell line? 577 00:43:00 --> 00:43:09 A stem cell line is a cell line that has the potential to go on and 578 00:43:09 --> 00:43:18 differentiate into various different lineages. OK? 579 00:43:18 --> 00:43:27 And so from this embryo that has been put in a Petri dish and the 580 00:43:27 --> 00:43:36 cells allowed to grow you get out a number of stem cell lines. 581 00:43:36 --> 00:43:45 And the notion is that each of these stem cell lines, 582 00:43:45 --> 00:43:54 if treated with the correct factor that's called here a differentiation 583 00:43:54 --> 00:44:03 factor, can go on to give rise to different tissues. 584 00:44:03 --> 00:44:07 Heart muscle, pancreas, cartilage and so on. And the notion 585 00:44:07 --> 00:44:11 is that different ES cell lines will have different potencies. 586 00:44:11 --> 00:44:15 So some will be really good at making hearts, 587 00:44:15 --> 00:44:19 some will be really good at making muscle and so on. 588 00:44:19 --> 00:44:23 And it's not quite clear how one gets these different ES cell lines, 589 00:44:23 --> 00:44:28 but they exist from the mouse. And some of them exist from the human. 590 00:44:28 --> 00:44:32 And this is where we get into the other major issue of stem cell 591 00:44:32 --> 00:44:36 biology and the reason that there is so much legislature. 592 00:44:36 --> 00:44:40 Because in order to get these human stem cell lines you have to use a 593 00:44:40 --> 00:44:45 human embryo. To get human embryos you use methods of assisted 594 00:44:45 --> 00:44:49 reproductive technology. I sent you some notes on this and 595 00:44:49 --> 00:44:53 you can read this in the book. And the notion is that you remove 596 00:44:53 --> 00:44:57 eggs from a female, you make embryos in vitro, 597 00:44:57 --> 00:45:03 in a test-tube. And then when the embryos are balls 598 00:45:03 --> 00:45:09 of cells you harvest them, you kill them and you turn them into 599 00:45:09 --> 00:45:15 ES line. And the thing that is sticking people right now in the 600 00:45:15 --> 00:45:21 legislature and is something that is really worth thinking about is 601 00:45:21 --> 00:45:27 whether ethically it is OK, guys, it's a 11:54, so stay. 602 00:45:27 --> 00:45:32 Stay. OK. And the thing that is killing people, 603 00:45:32 --> 00:45:36 is really sticking people ethically is whether it's OK to take these 604 00:45:36 --> 00:45:41 human embryos and turn them into cell lines, because this does 605 00:45:41 --> 00:45:45 involve killing an embryo and this is an ethical issue. 606 00:45:45 --> 00:45:50 And I'm going to leave this here, and we'll come back to these ethical 607 00:45:50 --> 00:45:53 issues when we talk about cloning.