Frontiers of Biomedical Engineering

BENG 100 - Lecture 6 - Cell Culture Engineering (cont.)

Chapter 1. Fertilization and Early Development [00:00:00]

Professor Mark Saltzman: Okay, today we’re going to continue our discussion of cellular principles and lead into cell culture technology which will be the subject of the section meeting this afternoon, so just to remind you about the sections. We’ve been reading Chapter 5. We talked last time about some of the basic properties of cells, their basic architecture and what ways cells are the same, in what ways cells from animals, including humans differ from simpler microorganisms like bacteria. Then we talked a little bit about the sort of physics of what holds cells together to form a collection of cells, a tissue and to make up the structure of our body. Today we want to talk in more detail about the question of how can cells–if they’re the same and they have the same kind of construction, and they all contain the same genetic material–how can they develop into a multi cellular organism that has cells that differ so greatly in function–that differ as much as the cells in our brain differ from the cells of our skin or our liver, or our blood. What are the sort of basic principles that lead to these differences between cells?

To start out I’m going to go back to the thinking about development a little bit, embryonic development, and show you a picture here. This is a picture of the female reproductive tract. This is the uterus down here, sort of half of it is shown with the uterine wall, the fallopian tubes, and the ovaries. You know that the ovary produces an egg. An egg is an example of a germ cell. Has only half of the diploid chromosomal content that most of the cells, which are called somatic cells in our body have. Germ line cells sperm and egg are produced by the process of myosis, and that’s reviewed in the book, where it’s a special kind of cell division that results in the reduction of chromosome number from two copies of each chromosome down to one copy of each chromosome. The other germ line cell is the sperm cell.

Fertilization of these two - where these two cells join occurs in the distal part of the fallopian tube. The result of fertilization now is a new cell that is the union of the sperm and the egg, and it’s called the zygote and it contains the diploid number of chromosomes, genes. One copy of the chromosome comes from the egg and one copy of each chromosome comes from the sperm, so this you know about. This one cell, this one fertilized cell which is unique because it’s the - its chromosome contains the combination of the sperm and the egg, develops into an embryo and then on birth develops into a human. Each of us has something like 10¹⁴ cells. We talked about last time that part of the process of going from this single cell to multi cellular many celled organisms like we are is cell division. The cell divides, and divides, and divides many, many times and that’s one of the signature events of embryogenesis until we have many, many cells that make up our bodies.

In that process, cells become different in ways that appear to be highly organized. We have tissues like the brain which are assembled to do functions that are different from any other groups of cells in the body. They work in concert and they all have similarities, the same with all of our other organs. So how does that happen? Well, it really happens throughout development and it happens from the first stages. In fact, there are differences that can be detected upon the first cell division where this zygote divides through the process of mitosis. We talked about mitosis last time; it’s described more completely in the chapter, where two cells are formed from one. Now, these cells have some differences. If you could look at these cells you could find differences between them, there are chemical differences in the content of each of these cells. If mitosis occurs the way that it’s supposed to, the DNA that’s in each of these cells is the same. That’s one of the properties of mitosis, that the DNA gets completely duplicated during the S phase, during DNA synthesis phase.

How could these cells be different then, if they contain the same DNA? What’s a physical mechanism that could lead to differences between these two cells at this very early stage of development? Any ideas? If the DNA is synthesized exactly correctly, so each one gets the right copies of DNA, what other differences could there be?

Student: [inaudible]

Professor Mark Saltzman The size of the cells could be different; maybe mitosis is asymmetrical in some way so that one of the cells ends up being bigger than the other. How could size affect the life of the cell?

Student: [inaudible]

Professor Mark Saltzman Different amount of metabolic activity because one has a greater volume of cytoplasm than the other, for example, so these are exactly the right kinds of ideas. There could be differences in the physics of cell division, this process of separating into two cells such that even though they both have the same chromosomes, they both have the same DNA content, maybe one of the cells entraps something that’s different than the other cells.

That difference could have been generated during the process of fertilization. The sperm - say this is the one sperm cell that’s able to inject its DNA into the egg, well then this cell has a polarity. Now, the top is different from the bottom because the sperm came in this side physically and not the other side. Remember that these cells are relatively large compared to bacteria and so diffusion doesn’t occur very quickly over this length scale. In the time it takes for one cell division to occur, it could be that this cell entraps a different chemical composition of the cytoplasm then this entraps, and that’s a well known concept. The only important thing to realize about that is that these differences start to occur very early in development. Once you have a difference that occurs, two cells or difference, those differences can propagate as the cells continue to divide.

What’s shown in this diagram is the progress of the developing embryo as it travels in time, down the fallopian tube. There’s one division, here it shows it at the 16 cell stage and then here, and this transformation as it goes from 16 cells to more like 64 cells. There’s a change in sort of the shape of the overall embryo as well. It’s no longer just a round spherical mass of cells, but it has some structure. There’s a cluster of cells here, there’s a sheet of cells that forms an outer lining, and what is most noticeable is this cavity, this fluid filled cavity which begins to develop.

Well, this stage of development is called the blastocyst and it’s at this stage late in this blastocyst stage that the developing embryo implants in the uterine wall, and there begins to form an interface with the mother so that it can be nourished during further development. The cells of this surrounding sheet have become different in some way and they develop into the placenta and the extra embryonic tissues. The cells of this cluster inside next to the fluid filled cavity is a region of the blastocyst called the inner cell mass. It’s this group of cells, this subset of cells from the developing embryo that become the embryo, that become the organism, become the human.

We’re going to talk as we go through about the concept of stem cells and how stem cells are related to development and what’s so special about stem cells. We’re going to talk about different kinds of stem cells. One of the differences in stem cell populations that you will hear about is you hear about embryonic stem cells and you hear about adult stem cells. Those are obvious what the differences are, embryonic stem cells are derived from embryos . It’s this - its cells in this region here, this inner cell mass that that’s the source of embryonic stem cells, cells from inner cell mass here. Adult stem cells are acquired in some fashion from an adult organism.

During development now this blastocyst has become implanted. These cells around the outside form the interface, the placenta, where the maternal blood circulation meets the embryonic circulation and nutrients are passed back and forth that way in a very highly regulated and important way. This inner cell mass develops into the embryo, which this here shown at a later stage is beginning to be clear that it’s becoming an organism that looks like us. There’s a region that looks the head, and a region that looks more like the tail. You can see this region here is going to develop into one of the upper limbs, the arms here and the back is different from the front, the spinal cord is developing in the back, whereas, the structures that become our intestinal tract is developing on the other surface. Different kinds of polarity form, there’s a head, and there’s a tail, there’s a back, there’s a front, there’s a left side and there’s a right side. This is one of the kinds of differences that develop - that happens during development and cells somehow know where they are within this developing asymmetrical organism.

Chapter 2. Development of Stem Cells [00:11:52]

How does that happen? It happens through a very regulated, coordinated slow process of what is called differentiation. Cells move from a state of limited differentiation to a state of more differentiation. The zygote or this fertilized egg is a completely undifferentiated cell. We’ll talk about another word for this later, but it’s a cell that’s going to give rise to all the cells of our body. As division happens and the developing organism acquires more and more cells, individual cells become differentiated, they become more and more like their final mature form. Cells that are within the region that becomes the nervous system become less like the zygote and more like the cells of our brain, neurons and glia, and cells of the mature brain. The same way cells that form the limb become more like muscle cells or skin cells, or the structures that become the limb. Well, that process occurs in a series of steps and one of those kinds of steps is shown here.

And this diagram shows what I simply labeled as a stem cell, so I’m not referring to any particular kind of stem cell now, but just a cell that has the stem cell character. What does that mean to have the stem cell character? It means that if I took this cell and isolated and watched it, I’d notice that it had a couple of characteristics. One is that it’s capable of something called asymmetrical division. We talked about division last time. We talked about the parent cell forming two identical daughter cells. An asymmetrical division is not like that, it’s when a parent cell forms two cells that are different in some way. That difference has functional consequences for the daughter cells in that one of the daughter cells becomes what’s called here a committed progenitor cell. It’s no longer a stem cell but it’s a progenitor cell. A progenitor cell, the definition, it just means it can generate the cells that are typical of that tissue or that organ, so it’s capable of becoming these mature classes of cells.

We’ll talk more about this in the context of the brain, but if we were talking about a stem cell in the brain, then the result of this asymmetrical division would be a committed progenitor cell that’s capable of forming cells that are the cell types found in the brain and not cells that are the cell types found in the liver, or the kidney, or the spleen, or muscle. One result of this asymmetrical division is a committed progenitor cell. The result is a cell that’s very similar but it is - very similar to the progenitor cell - but it’s exactly the stem cell. So this stem cell division leads to another stem cell as well as a committed progenitor cell. Now, the differences here may be subtle in terms of chemical composition or if you put these cells under a microscope and looked at their analysis. In terms of function they’re very important because this stem cell which is produced goes back into the population of stem cells and is able to repeat this process to form new committed progenitor cells and to form new stem cells. That’s important because one of the attributes of stem cells is that they remain at their site and capable of reproducing themselves. This process is called self-renewal, so that’s one important process of property stem cells, that they’re capable of self-renewal. The other one is a committed progenitor cell that now somehow has been changed in such a way that it’s going to mature and develop into non-stem cells or the cells that make up our bodies, somatic cells.

Now, what could these differences be? They’re not chromosomal differences because this is the ordinary process of mitosis. So presumably, these two cells have exactly the same DNA content, but something’s been passed onto this one that wasn’t there. This is one of the very important areas that still is not completely understood in stem cell biology. What is the difference that’s generated during an asymmetric cell division? There are types of changes that are known. Some of them are changes in the - not the sequence of DNA, not the sequence of nucleotides in the DNA - but the chemistry of DNA around that the way that it’s packed into a nucleus. So the access that a cell has to certain kinds of genes, or chemical modifications of DNA, not chemical modifications that change the base pairs, that would be a mutation. That can happen but that’s abnormal, but changes in maybe the chemistry of the backbone that holds the nucleotides together. In some cases that backbone gets methylated and those regions of the DNA that are methylated get treated differently by the cell than unmethylated regions.

You go on and you study developmental biology or molecular biology, you’ll learn more about these things. For our purpose just - these are the kinds of changes that can happen. Or it can the kind of change we talked about before, where during division there are some chemicals that are trapped in one cell and not in the other, and that could lead to a difference we already talked about. Or it could be not having to do with the cells themselves but maybe the environment that the cell finds itself in. We talked last time about extracellular matrix and this complex protein-carbohydrate gel that surrounds all cells.

Cell division takes place within an organism. We’ll talk in a minute about stem cells that are involved in generation of blood and they develop and they live in the bone marrow. Well, what if this division takes place in an environment where there’s one kind of extra cellular matrix here and another kind of extracellular matrix here? Then this cell is going to experience something different from this cell. It could be those differences that they experience in their extracellular environment that lead to their choice to either self-renew or to become committed.

So that’s asymmetrical division and that’s a property of stem cells. The other property is that these committed progenitor cells that are formed can turn into something, can turn into more mature cell types. That process of maturation is called differentiation. We’ll talk more about that and I’ve already said something about that. A capability for asymmetric division and the production of cells that become differentiating more mature cells, those are properties of stem cells.

Another concept that’s important in thinking about stem cells is potential. Potential refers to what it sounds like, ‘what potential does this committed progenitor cell have? ’ ‘What potential does this stem cell have?’ Well, one way to think about is that upon this first division, this asymmetric division, this committed progenitor cell has lost some potential. It’s no longer capable of self-renewal to form another stem cell. It’s gone down a path towards maturation that’s very difficult to go back up. So there’s a loss of potential in this division. This stem cell which is reproduced still has the potential to undergo asymmetric division but this one does not.

One sort of, might be kind of simple minded, but one way to think about potential is with the kind of potential that we all experience as we develop from newborns to adults, that a newborn child has lots of different potential. It doesn’t have every potential, it’s a boy or a girl, it’s not going to go back, but it has lots of potential in that eventually when it becomes an adult it could become a cellist, or a biologist, or an auto mechanic, or a biomedical engineer, all those potentials are still there. As you develop, as you’re educated, you retain all those potentials for a certain point and then you make choices and you lose some of those potentials. I’m unlikely to become a concert cellist at this point. It’s not impossible but it’s pretty unlikely. I’ve probably lost my potential to be an outstanding cellist. You all could still do that if you decide too, but it’s going to be harder for you than if you would have started when you were ten, so you’re losing some potential around - along the way. You’re in the process of becoming more mature, more differentiated and you’re losing potential at the same time.

The same thing with these cells here, as these cells undergo continual divisions, they’re changing in ways that make them mature, that make them more like the mature cells of the nervous system, for example, if that’s where they end up being but they’re losing potential as they go through that differentiation process. Now, one of the great hopes of modern biology is that we can figure out how to reverse that process in cells. How we could take cells that are differentiated to some extent and make them de-differentiate, to go back in the process of differentiation so that they gain more potential. Why would that be useful? Well, it would be useful because if I could take cells from the skin, find stem cells in the skin and then de-differentiate them so that they were now capable of becoming liver, or brain, or things that they’re not going to become in their normal site, then that could be a very powerful tool for medicine. So far our ability to de-differentiate cells or find out how to do that is limited. Does this make sense?

Chapter 3. Results of Differentiation [00:23:26]

What is actually changing during this process of differentiation? What’s the difference between this cell which I call a committed progenitor cell and its offspring, and the offspring of that offspring. It goes - going through this process of amplifying divisions, every division increasing the number of cells by a factor of two and these cells becoming more differentiated around the way. I’ve shown the differences here in terms of shape, these are shaped liked octagons and these are shaped like squares, but if I looked at these cells, what would I find that’s really different about them?

What’s different from an immature cell and a mature cell? Well, it’s not the DNA; they all have the same DNA. There might be these, what are called epigenetic differences that I mentioned changes in the structure around DNA, and those changes lead to differences in which fraction of the total genes in the chromosomes are being expressed by a particular cell. This is what makes cells different, the number and quantity of the genes that they express. Out of all the genes that are on the human genome which fraction is this particular cell using, which fraction is it expressing determines what proteins are present in the cell, determines what work or what activities the cell can engage in. What’s changing along here is the - what’s changing along this pathway is the expression pattern of genes in cells.

Let me make this a little bit more explicit by talking about the process of hematopoiesis. Hematopoiesis is the process of generating new blood cells. Hemato means blood and poiesis means generation or formation. You know probably that within your bone marrows there’s populations of cells, there are different kinds of cells within the bone marrow. Some of them have the capability of becoming red blood cells which carry oxygen in the blood. Some of them have the capability of becoming white blood cells, or leukocytes, of which there are many different subsets. Some are called neutrophils and those are responsible for fighting infection. Some are called lymphocytes, B-lymphocytes, T-lymphocytes. We’re going to talk about those in more detail in a couple of weeks when we talk about the immune system because these are the cells that perform and regulate the functions of our immune system that protect us from disease. Some form what are called megakaryocytes which become platelets, which are responsible for clotting, for forming a barrier if your circulatory system gets injured so you don’t bleed.

All these cells come from the bone marrow and biologists have traced the formation of these cells in great detail. In fact, of all the systems of cellular differentiation that are known in our bodies, probably hematopoiesis is the best known. It was the first place where the concept of stem cells was developed, in that if one looks carefully one can find immature cells in the bone marrow. If you isolate those immature cells, some of them are less mature than others, some of them have more potential than others. For example, if I isolated this one called the myeloid cell, it’s capable of forming red blood cells, megakaryocytes and neutrophils. It’s capable of forming all these different cells but it’s not capable of forming lymphocytes. Another stem cell called the lymphoid stem cell is capable of forming the B and T lymphocytes.

Through many decades of study, biologists have teased out certain populations of cells within the bone marrow that are capable of reproducing subsets of cells. Now, in the process of doing that they found some very rare stem cells that are called pluripotent, pluri just means many potencies. Pluripotent stem cells that are capable of self-renewal to generate themselves and are capable of dividing into both bioloid and lymphoid progenitor cells. This is an example of that asymmetric division. This pluripotent stem cell is able to self-renew and it’s able - generating committed progenitors of either the myeloid or the lymphoid lineage. So if I got these two cells they would be less - they would have less potential than these.

Why is it so hard to find stem cells? I mentioned that this process has taken many decades and lots of people studying. Why has it been so hard and why does it continue to be difficult to identify these pluripotent cells within tissues? Well, one reason is that they’re present in very small numbers. Of a million cells in the bone marrow there might only be one of these. This might be 1 in 100,000,000,000 or something like that; don’t write down the numbers I’m just using that for illustration. These cells are rare and so it’s been hard to identify them, that’s part of it. The other part of it is how do you identify them? How do I know when I’ve got this one or this one? In this diagram I’m showing you it’s easy to tell because some are yellow, and some are pink, and some are blue but that’s not the way they come out of the bone marrow. They’re not color-coded, so how you do you find them?

How do you think? If you were searching for unique populations of cells within the bone marrow what tools would you use to look for them? How would you search for these cells? Well, one way would be to isolate individual cells and culture them outside the body and see what they become. That would be a very straightforward functional way to do it, but you could imagine that that’s very labor intensive because you’ve got to separate each individual cell, and you’ve got to nurture it and then keep track and study what it becomes. That turns out to be one really important way that they do it.

The other way that they define - or find stem cells is that over the years of studying them we’ve begun to recognize some of the proteins, the specific proteins that are produced by these characteristic cells. This cell here, which is indicated green, is different than this cell that’s colored red. The difference - the real difference is in what genes its expressing of the total number of genes in the human genome and therefore if it’s expressing these unique - this unique set of genes. If I could find the unique set of proteins that correspond to those genes I could define chemically what the cell is.

It turns out that one of the places that’s been very fruitful to look for proteins that differ between cell populations is on the surface of the cell. We talked about the cell membrane, the plasma membrane separating the inside form the outside. I mentioned a little bit that this membrane is not just lipid bilayer but there’s also proteins that are inserted into the membrane. These proteins have functions that are essential for life of the cell, they transport molecules back and forth across the membrane. They also allow their populations of proteins on the surface of each cell that allow it to interact with its environment, they’re receptors and cell adhesion receptors like I talked about last time. The kinds of proteins that sit on a cell surface and form adhesion junctions with neighboring cells, that’s one class of cells on the surface. There are - that’s one class of proteins on the surface I’m sorry. There are proteins that are responsible for receiving signals, chemical signals. There are proteins, for example, on the surface of some cells that bind insulin and respond to the presence of insulin. We’re going to talk more about these kinds of molecules on the surface next week. For now, just know that different kinds of cells, one of the ways they’re different is that they express different proteins and the population of proteins on the surface of different cells is different.

We’ve learned how to identify and catalog cells according to the composition of proteins on the surface and those proteins that distinguish a cell are often called marker proteins. They’re given names, and we’re able to - often the names are confusing, if you look in the literature you’ll find proteins that are called CD44, CD3, these are differentiation - cluster differentiation antigens is what CD stands for but it really means a particular protein which is present on this cell but not on that cell. So I can use the presence of those to identify cell populations.

One reason it’s been harder to identify stem cells is that they’re so rare; the other reason is that it’s hard to identify the characteristics of them and it’s taken many years to work this out. In the matopoetic system it’s the most well known. In fact, it’s so well known now that we’ve identified proteins that stimulate the development of cells along certain pathways. I talked about one of them a couple of weeks ago, the protein epo, erythropoietin called epo is a naturally occurring protein that is in the bone marrow and it stimulates the development of red blood cells. So it stimulates these myeloid cells to develop along this pathway. It’s a protein that’s produced by other cells in the body and when it’s enriched in a certain area it stimulates more production of red blood cells. Because we’ve learned about that biology we’ve been able to make erythropoietin outside the body and use it as a drug. It can be used - given to people to - who have certain kinds of anemia to stimulate blood cell production in a specific kind of way. There’s lots of these so called signaling proteins that have been identified now. These signaling proteins play important roles in determining how many cells differentiate down particular pathways and they turn out also to be very useful for treating diseases of those pathways.

This pluripotent stem cell from bone marrow is an example of a stem cell, an adult stem cell, a stem cell that could be identified from the blood. Different than the embryonic stem cell we talked about before, so it’s an example of an adult stem cell. It’s also an example of a tissue specific stem cell. Those stem cells from the blood are capable of becoming all those cells in the blood. They’re not capable of becoming other kinds of cells in general.

Now, you’ll hear reports in the literature, you’ll look in the newspaper, you’ll hear about scientists that have found ways to trans-differentiate cells, that is move them from one pathway to the other. To find stem cells that they can move from one kind of pathway to another. That’s been looking at - blood stem cells has been a very fruitful way to look for that. There are some ways that you can take stem cells that normally would only produce blood cells and maintain them in culture, expose them to certain regimens of chemicals, do certain manipulations on these cells and they become capable of producing liver, for example, or brain, or muscle. That’s a lot of the literature of stem cells that you’ll read about, taking a particular source of stem cell and nurturing it in such a way that it gains potentials that it didn’t have necessarily when it was in the body, or exploiting those potentials that it wouldn’t necessarily express. That’s a lot of what stem cell biology is like - is about.

Chapter 4. Stem Cell Lineage [00:36:53]

This diagram here sort of allows me to walk you through some of the terminology of this stem cell development and talk about these concepts. Again, in the context of a specific tissue site, in this case it’s the nervous system. We started the discussion talking about the zygote or the fertilized egg. There’s only one source for that, there’s only one source of that fertilized egg. It’s not self-renewing, in that division of the zygote results in two daughter cells that are no longer the zygote anymore, they’re down some pathway. One way of referring to this cell is in terms of its potential and the zygote obviously has the potential of becoming all of the cells of our body. That’s where they all come from, they all come from the zygote. The word for that is totipotent, totally potent. It has the capability of becoming any kind of cell within the body, in fact, that’s what it does.

Further down the line, for example, in the blastocyst I talked about before, we could obtain cells from this inner cell mass or this cluster of cells that becomes the embryo. Those are called embryonic stem cells, they are self-renewing, and they are pluripotent, meaning they have many potencies. That’s why people are so excited about embryonic stem cells because in nature they become all the cells of the body. If we understood them well enough we could potentially make any particular kind of cell in the body from those pluripotent cells. They’re controversial, I think for obvious reasons, because you have to sacrifice an embryo in order to get them.

Further down this line here are embryonic - or let’s say adult brain stem cells. These are cells that I - that were obtained either from a more developed embryo, past the blastocyst stage. For example, that embryo that I showed on one of the first slides where there’s clearly a head region and a tail region. Now I isolated these cells maybe from the head region of the embryo, the region that’s going to develop into the brain, so that would be a good place to look if you wanted cells that were going to develop into the brain. They’re capable of self-renewal. They’re not quite as potent as the cells from earlier because they’ve now differentiated somewhat.

They might be capable of forming all of the cells of the nervous system; they might still have some potency to form other things that are similar to the nervous system. Maybe they could make skin, maybe they could make other kinds of cells if you treated them the right way. So they’ve lost some capabilities but not many, and these are called multipotent stem cells. They still have broad potential and they’re self-renewing and so there’s much interest in those. Easiest to find them in embryos but sometimes they can be found in adult organisms as well. If you go to the right region of an adult brain you might be able to find cells like this but it’s more difficult. As we’ve found cells from adult organisms that seem to be multipotential and studied them more carefully, some of their potentials turn out to be lost, they’re not exactly the same.

Further down the line here, if we looked in the adult brain or spinal cord and other regions we’d find committed progenitor cells. These are cells that are committed to become nervous tissue. They might self-renew they might not, they have much more limited potential than before. You’re starting to see the pattern as I move further and further away from the embryo from less differentiated to more differentiated, from non-specific regions to more specific regions, I’m getting cells that are easier to obtain because you can obtain them from adult sources but their potential as stem cells is more limited.

There are a couple of tissues that are of particular interest to scientists and clinicians now and bone marrow is one of those. There’s a lot of interest in bone marrow and the stem cells that come from bone marrow and there’s a couple of reasons for this. One is because we understand the bone marrow system so much better than we understand all the other stem cell systems. The other is that it’s possible to get stem cells from patients, from bone marrow. You can collect bone marrow, it’s not a procedure that you would want to do. It involves putting a needle, a fairly large needle into usually one of the pelvic bones and collecting marrow from - which is tissue that’s deep inside those bones. It’s not as easy as getting your blood drawn if you give blood to the Red Cross, for example, but it can be done very safely and wouldn’t it be great if we could identify stem cells that were multipotent from that bone marrow because you could find them potentially for - if I needed a treatment that could - if I had some ailment that could be treated with stem cells then you could get my own - I could get my own stem cells and use them. Or maybe I could donate bone marrow and those stem cells could be given to other people in the same way that blood can be given to other people by matching and making sure that immunologically my cells were compatible with you; there’s a lot of interest in that.

There’s a lot of interest in obtaining stem cells from the blood of the umbilical cord on birth. It turns out that blood within the umbilical cord is also a rich source of stem cells and again specific to a particular patient. There are services now, we don’t know yet how to get those stem cells out of cord blood and how to use them for therapies, but it’s reasonable to think that we might know about this in 30 years. So some parents now are choosing to save the cord blood, have it frozen, locked away somewhere just in case its useful to their child later in life. I’m not endorsing that I’m just saying that that’s something that can be done now.

I digressed a little from this diagram but I think you’ve gotten the picture that as I move to more adult organisms, as I move to more specific regions of the brain, for example, I can still find progenitor cells that have some potential. It’s harder to find, they’re more limited numbers, and in general, more difficult until eventually down this pathway you have fully differentiated cells, cells that are fully mature and performing the function of the mature organ. The two largest populations of cells within the brain are neurons, the ones that actually transmit electrical activity and responsible for the main functions we think of when we think of the brain, and supporting cells called glia, which are responsible for sort of creating the right sort of environment for neurons to function.

Chapter 5. Cell Proliferation [00:44:30]

I wanted to talk about one last concept and this is one the boxes from Chapter 5 and you’ve already been using this in your homework. I just wanted to talk about one last concept and that has to do with cell proliferation. We’ve been talking about single cell or some subset of cells and propagating them so that you get a larger population of cells. Of course this happens all the time in the body. There are cells within your body that are always in the process of division and forming new cells, and sometimes this is for a tissue where cells only have a finite lifetime. The red blood cells that carry oxygen only live within your circulation for about a month and so you have to continually be replacing cells that are dying and so there are cells that are proliferating.

Cell proliferation is a huge issue in cell culture. One of the main things that we use cell culture or maintenance of cells outside the body for is to make more copies of cells. One of the purposes of cell culture is to make many, many more cells under controlled conditions where I can understand what those cells are. So, in general, when cells are proliferating they’re dividing and they’re dividing at a regular rate. What that means is that one way to describe that mathematically is shown here. If X is the number of cells, then the rate of change of X, dX/dt, the rate of change of the cell population, how fast division is taking is proportional to the number of cells I have. This makes sense; the rate at which the cell population is growing, the derivative dX/dt, is proportional to the number of cells I have. The more cells I have the faster they can grow, fewer cells grows more slowly.

This is an example of an exponential growth process and you’re familiar with processes like these. If you solve this differential equation, which you don’t need to do for the course, but I show it here; some of you will understand immediately where this comes from, that means that the number of cells I have at any particular time is equal to the number of cells that I have at some starting time, times e to the power, µ here, where - and it should be µt–looks like there’s a typo in the book, you can’t see it here, maybe, so e^µt or this constant times time.

This constant µ which is the proportionality constant between the number of cells and the rate of growth is a constant that characterizes how fast a cell population is growing. Some will be growing very rapidly, some will be growing less rapidly, and what this equation shows you here is how to relate that growth constant µ or that rate of growth the doubling time of cells. How long does it take for the population of cells to double? One of the interesting properties of cells that are in exponential growth is that the time to increase the cell number by a factor of 2 is always the same. That makes sense if you think about this process of cell multiplication, that I have one cell it becomes 2 cells in a minute, it could become 4 cells in another minute, it could become 8 cells in another minute and that’s all that this set of equations is representing. Questions about that? Good, I’ll see you in section this afternoon.

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