Developmental Biology

Hi. In this video, we're going to talk about development. How organisms start out is that single celled zygote and progress or develop to these complex multi cellular creatures that have specialized tissues, organs, bone structures, all these crazy functions. How does that happen? Well, it turns out that certain processes are fundamental to development across all organisms. Chief among these, of course, is cell division. How does a single cell turn into a multi cellular thing? Well, cell division, obviously right. And you guys might recall, from our discussion of cell division that might Assis is a highly regulated process. And right here we see a chart of the cell cycle or a figure of the cell cycle, and these black bars in it represent the various checkpoints in mitosis, the control mechanisms, the gates, if you will, that regulate the my tonic process. Now, in addition to these checkpoints, we also talked about social control, which is how cells neighbors can regulate their division. So in the course of development, the timing and location of cell division is incredibly tightly regulated as it's crucial for proper development. So these systems involved in this are chiefly these Might Todd IQ control mechanisms and also these social control mechanisms neighbors influencing each other to divide or to halt division. Now, in addition to cell division, cell differentiation is another crucial process. Cell differentiation is how one cell become, or one undifferentiated cell can become a specialized cell like a neuron, for example, the cells lining your stomach and the cells in your brain. Those neurons are very different looking cells, but they all come from the same place. And that is namely stem cells thes undifferentiated cells that, through differentiation give rise to specialize types of cells. And we call that process cell differentiation, and we call the ultimate end of it cell fate. It's the destiny of the cell. What will this sell become? That is the cell fate, right? Is this stem cell to become a neuron, or is it to become a cell of the intestine epithelial cell? Well, that's determined by its fate, and we'll talk more about how self fate is determined later on. Now, in plants, Uh, stem cells are actually located within the plant and remain there and continue to develop throughout the entire life of the plant, and we call these Mary stems, and actually plants have multiple mary stems. Basically anywhere you're going to get new growth. You have Mary stems. So to pretty obvious places are the roots right? The roots need to continue to grow, so you have root Mary stem Mary stems and also the shoot right plants continue to grow upward. They branch out, send out new leaves. You have thes, uh, shoot mayor stems, and we'll talk more about Mary stems when we cover plant development specifically now, animals also use stem cells. They use stem cells. Or I should say we use stem cells to repair our wounds, replace cells, and also to create the cells of the immune system which have to be developed in a very specialized fashion to match a specific immune function. So animals actually keep a supply of stem cells in their bodies. However, unlike plants, we don't have a carte blanche with our stem cells. We can't just continue to produce, you know, anything willy nilly. We use our stem cells for very specific things, and here in this figure, you can see an example of the fertilization event, which will lead to the formation of a zygote and how, over the course of development you will arise or you will give rise to specialize cells that will function in circulatory system or the nervous system like neurons were talking about, or the immune system as we were also talking about. All right, let's flip the page.

during the course of development, cells also interact with each other. Now we've talked a lot about cell interactions in the cell signaling lesson. But during development there's really two main or two major types of interactions that are happening. And those are peregrine cells emitting chemical messengers that are picked up by the receptors of another cell nearby. And also jokester Quran Oops, which is when the ligand bound the surface of one cell interacts with the receptor on another cell. So in these ways, cells influence their neighbors to move around, divide, differentiate and also die, which we'll talk about shortly. Um, differentiating cells have this special way of influencing their neighbors. Thio also differentiate or to behave a certain way, right, those social control mechanisms and we'll talk a little bit more about that later. Aziz. Well, now I mentioned cells moving around, and this is something that is mainly, uh, seen in animals. Not it's not seen in plants, which again will cover in more detail when we specifically talk about animal and plant development. But it's important to note that sells actually have to move around in, um during development to create specialized tissues and thio form specialize structures. Plant cells, on the other hand, are really good at expanding their size and change, which results in changes in the shape and form of the plant. So plant cells air also kind of moving their cells around, just not in the same way that animal cells air literally breaking off from one point and moving a way to another point of part of the embryo in plant cells. The cells will just expand their size and kind of warped the shape of the plant as a result. And we can see unexamined of this cell movement happening in the figure right here. This structure rearranges into this structure, and it does so by these individual cells that we see here actually moving around to arrange this confirmation scene right here. Now, I had mentioned dying as an important part of development before, and indeed, we have talked about a popped Asus, a particular type of programmed cell death, before when we talked about cell signaling. And it turns out that, uh, programmed cell death is incredibly important to development. And a great example of that is in the development of digits on your fingers. So during the course of animal development, you actually have cells between your fingers, like webbing, like the webbed toes of ah, duck, for example. However, during the course of development, those cells that make up the webbing go through a popped assis and die out, resulting in separated digits. So cell death is actually a very important part of life, believe it or not. All right, let's flip the page.

Hello, everyone. In this lesson, we're going to be talking about differential gene expression. Okay, so what is differential? Gene expression Basically differential expression is going to result in multicellular organisms having different types of cells. Obviously you have different types of cells, right? You have skin cells, you have liver cells, you have eyeball cells. You have many different types of cells that do different jobs. But how do they do different jobs if they all possess the same genetic makeup? Well, that's where differential expression is going to come in. So differential expression is when they're different patterns off gene expression and this is going to lead to different cell types. So we're gonna have different types of cells Now, remember, the genetic makeup of every cell in your body is exactly the same. They all have the same DNA, except for maybe a couple mutations between different cells. They should all be exactly the same. So how is this going to happen? What? We're going to have chemical signals inside of our bodies and ourselves that cause differential gene expression. What this means is we're gonna have signals in our body that tell certain genes in certain cells to turn on and to turn off. So you're gonna have some genes that skin cells will never express. And some genes that skin cells will Onley express. So there are genes unique to each type of cell in your body in your genetic makeup and which genes certain cells air going to express is dependent on their chemical signals that they are given. And this gene expression can be modulated or changed or regulated in many different ways. Which genes are being expressed can be changed or determined by many different factors, including transcription a LRA, Gye Lei Shen. This is going to be which genes are transcribed and which ones are not. In certain cells, some genes will be silenced. They won't be allowed to transcribe, and in some cells they will be expressed. They will be told to transcribe into em Arna. Now there's also RNA splicing so this can create certain Marianas, which can create certain FINA types, certain types of proteins. Depending on how that RNA is built now, you can also have translational regulation. Translational regulation is actually determining which Marna is allowed to become a protein, or how many of a certain genes are allowed to become a protein, so this is going to regulate the phenotype off the proteins and of the sell by regulating translation. So this is going to regulate the expression of that cell, and we also have post translational modification. So a protein is made, it is translated, and then it is modified to do a particular job that that particular cell type might utilize these processes air all going to determine differential expression. All of these processes air going to be utilized to give cells unique gene expression and unique FINA types. Remember, gene expression is thief phenotype or the characteristics that air created from the genes that are being expressed? And there are many different ways to modify that expression, including thes four ways, and that's going to create a unique phenotype for each cell type. So also, regulatory factors like transcription factors are going to be utilized to influence which genes are basically turned on and turned off. Now remember, very, very important. I know I already talked about this, but remember that all cells are genetically equivalent. They all have the same genes. They just use different methods to have their unique FINA types. Now we have a visual representation right here of what that might look like. So let's say that these two different cells come from the same multicellular organism, so they're from the same organism, but they obviously look incredibly different, and they probably have very different jobs. Now let's say that this organism on Lee has these three genes I know Unlikely. But let's just go with it for simplification. So let's say it on. Lee has thes three genes, and as you can see, thes, two different cells in these multicellular organisms are expressing these genes differently. You can see that this first cell is expressing Gene a twofold. Let's just say it's expressing gene a two fold, and it's not expressing Jean B whatsoever. And it's expressing Jean C one fold. So obviously it has this particular phenotype because it expressed these genes this way. Perhaps this particular gene be was not transcribed it all. Maybe it was silenced and told not to transcribe. So no Amarna was made whatsoever from this gene. But then let's say that ah lot of Amarna was made for Gene A and a lot of it was translated, so there were many gene products created for Gene A. And maybe only a little bit of Jean C was translated and transcribed, so not as many gene products were made. Things is all going to be dealing with translational and transcription ALS regulation. Now let's look at the other cell. We can see that the other cell did not express Gene a whatsoever. So perhaps Jean A is not transcribed. Maybe it is silenced like the other cell did with Jean B. But we can see the gene B in the second cell is expressed threefold. So this is a really express gene. Maybe it's transcribed a lot. A lot of Marina is created, it is enhanced, and it is translated very quickly and very rapidly. Now we can also see that it does express Jean C two fold as well. This is going to be regulated by its transcription and translation of these genes, and these two different ways that they do these processes off transcription, translation and all these other modulating processes is going to give them their unique phenotype. So this is how we get different cell types now. This is going to lead into the topic of pattern formation. Pattern formation is the complex organization of the different cell fates in your body over space and time, and this is going to be controlled by genes. So basically, pattern formation is dealing with how you're different. Cell types are made, how your tissues and organs are made, and when that happens and where that happens. So, for example, does it happen in utero when the fetus is being created? Or does it happen during puberty? When you're having hormonal changes and different parts of your body are changing their different times and there are different spaces or locations in the body, that pattern formation is happening, and this is all due to differential expression off the genes, which genes are expressed and which ones are not. Now how you develop patterns, how you develop your body organization is greatly controlled by these very important molecules called Morfogen. Morfogen are molecules used to indicate sell position. They indicate sell position via concentration. Grady INTs during pattern formation. What's another word for pattern formation? Another word for formation or pattern formation is more of a genesis, and this is going to be why the name Morfogen comes about because they help with the process of Morphy, Genesis or the building and developing off the body of the organism or the patterns in the organism. These Morfogen zehr very important molecules. Now, I have some examples down here of how these Morfogen zehr gonna work because I know that the concept can be a little bit confusing. So more virgins are utilizing a concentration Grady int. And that Grady int is highest around the cells that admit that particular protein or that particular molecule and cells respond to the particular concentration that they have, whether it's high, medium or low, that cell will recognize Hey, I'm getting a low concentration of this Morfogen. That means I need to do this job and this is based on their location in the body. And this is going to create the specific responses which create specific cell types depending on the concentration of the Morfogen that these cells get is going to tell them the cells, location in the body and what they're supposed to become, what type of tissue and organ they're going to become and basically what job they're going to do. And very important, Morfogen are going to set up the body axes. So your middle, What's top? What's bottom? What's left? What's right? All of this is gonna be determined by these Morfogen molecules. There are a ton of different Morfogen molecules which you will learn about in more advanced biology classes and anatomy classes. But some examples for mammals are retinoic acid. BMP my favorite sonic hedgehog. Yes, funny name, but it is a Morfogen and it is utilized to determine the placement of your fingers in your hand, which is really neat. Now, this example down here is going to be of Drosophila or fruit flies. And I do have two examples of Morfogen at work. The first one that we're going to be looking at is by Coid and the second one we're gonna be looking at is Nano's And this these air just names of the particular Morfogen. Now this is going to be the egg off a Drosophila or excuse me. The zygote of a Drosophila that will become a fruit fly and by Coid and Nano's are utilized to set up the head and the abdomen. So by Coid is at its highest concentration. Oh, sorry about that highest concentration in the head region off the zygote. The region of the zygote that will become the head off the Drosophila fruit fly is gonna have the highest concentration of bike oId, and then by coid, is going to defuse through the zygote. So the lowest concentration off the bike oId Morfogen, is going to be at the abdomen or the tail region off this fruit fly. Now we also have Nano's. Nano's is gonna work in the exact opposite method. Nano's is gonna have its high concentration towards the abdomen region or the tail region off this particular fruit fly. So there's gonna be really high concentration in the cells that will be the tail and then it's going to defuse towards the head so the heads can have really low concentrations. And these two Morfogen working together tell the cells in the Saigo where their position is, whether they're closer to the head or closer to the tail. And this is when the fruit fly is developing. It's obviously not fully developed, but it is developing, and you can see the regions of the head, the thorax and the abdomen, and you can see these different regions with different colors. And this is to represent the different areas of the body that air determined by different Morfogen. You have more than just to chemical signals. There are tons of Morfogen that air utilized to determine the different locations of the body so that the cells know their placement and know their job. And you can see that in this example here, with all these different colors representing the different locations of the different Morfogen concentrations in the different cell types. And that is going to be how the body develops via differential expression and pattern formation. Now let's go on to our next topic.

previously had mentioned the axes of the body during development, and I tried to simplify it by just calling it the head and the butt end. Right? Keep it simple. Side a side turns out, of course, because this is science. There are much more complicated names associated with these body axes. So let's go through a little terminology right now to get our axes straight. So first we have the what's called the anterior posterior access and you can actually see this in our image indicated by this green plain. So here we have our anterior post cheerier access. He anterior end is up here toward the head and the posterior and is down here mhm with that came out ugly. Sorry, guys. Host. Eerie or all right, good enough. Now we also need to be aware of the dorsal ventral axis which is actually represented by this blue plane here, dorsal ven troll access. So the ventral side is toward the belly. So here we have our ventral side and our door soul side. It's toward the back. So think like a dorsal fin on a dolphin. And I'm sorry this is the world's ugliest dolphin ever. But you got the idea, right? All right, so those are two of the major and important body axes to be aware of. So when someone says dorsal or Dorsey Lee, whatever they're talking about something towards the back, ventral toward the belly, Anterior toward the head posterior toward your bum. All right. Now, complex body plans like the human body don't form overnight, right? Rome wasn't built in a day. Development doesn't happen overnight for humans. Takes more like nine months, right? We all know this. So how does that happen? Well, it happens in stages. You see these chemical signals come and go, and they fine tune development. So we start with broad strokes, right? Setting up the major axes. And then we refine that little bit mawr with Gap jeans and then with pair rule genes, we get a little more fine tuned, and then segment polarity genes even a little more fine tuned. And finally, we have the hawks jeans, which we'll talk about in the next page. And if they lead thio, affect your genes. So basically, this process of body formation happens in multiple stages over a long course time. So let's turn the page and talk a little bit more about hocks jeans because those are going to be very important

hot shots. Jeans are a special type of what are called tool kit jeans, and these toolkit jeans are small subset of genes that control and organisms development, a k a. The genes we've been talking about this whole time right those genes important to development, so we often are to get a little more specific. A subset of tool kit genes are these home idiotic genes, which are genes that control the development of specific anatomical structures, and Hawks jeans, which hawks that actually comes from home. Idiotic box. So hawks for short hocks jeans are a type of homoerotic gene homoerotic jeans, and they're highly conserved genes, meaning that they've been around for a really long time through the course of evolution. And they help control development along that anterior to post cheerier access. They're activated after segments form. So might remember there those segment genes that we just mentioned on the previous page. So after all that they come towards the end of development and they help determine the specific structures that will form in a segment. So here we have a nice example. You can see this fruit fly very common model organism to use in biology, and you can see how the fruit fly has been divided into different segments represented by different colors here. And you can see that each segment is associate ID with a particular jean these air all hawks jeans, and they're going to lead to the development of the specific anatomical structures you see present at those segments. So initially, like you saw in the previous page, this embryo just kind of looks like a segmented blob. But through the activation of these hawks genes, specific animal anatomical structures will develop now. Development in general is a highly conserved process, and it's directly linked toe evolution. Which is why, when you look at the embryo formation of different animals, for example, we tend to all look the same in the beginning and then slowly branch out and become different. It's because the developmental process has been passed along through evolution. So even though, for example, fish are not a whole lot like us during development, we actually have gills. Yes, you and I had gills at one point in our life when we were a fetus, we develop gills and then we lost them, and that is because development is so heavily are so highly conserved and is directly links toe evolution. And guess what? There were fish before there were people. So we kind of, you know, carry on some of those traits to this day. But they Onley are present during our development. Now another, uh, another facet of this conservation of the developmental process is the fact that many animals use the same genes and chemical signals to govern body plan development, sea hawks, jeans, very important, highly conserved jeans there. And one last interesting thing to note about development is that many of the same chemical signals are used repeatedly during the course of development. But depending on when and how they're used, they actually will elicit different effects. And this again is just another case in point of how conserved everything is in biology. Biology is not wasteful. Rather, it takes things that already has and repurpose is them to its new needs. So development, highly highly conserved process with a direct link to evolution. And you can see here in this image how these developing embryos all look very similar. Um and this is another reason we use or we or we can use organisms like chickens or sea urchins. For example, toe learn Maura about our own human development because there's so many links. There's so much crossover between the development of a chick or even a sea urchin, believe it or not, and a person. All right, that's all I have for this video. I'll see you guys next time.