Developmental Biology - Online Tutor, Practice Problems & Exam Prep

Development involves processes like cell division and differentiation, where a single zygote transforms into complex multicellular organisms. Key mechanisms include mitotic control and social control, influencing cell behavior. Differential gene expression leads to diverse cell types, regulated by transcriptional and translational factors. Morphogens establish body axes and patterns during development, guiding cells based on concentration gradients. Hox genes, a subset of homeotic genes, play a crucial role in anatomical structure formation, highlighting the evolutionary conservation of developmental processes across species.

Cell Division and Differentiation

Hi. In this video, we're going to talk about development, how organisms start out as that single-celled zygote and progress or develop into these complex multicellular creatures that have specialized tissues, organs, bone structures, and 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 multicellular thing? Well, cell division, obviously, right? And you guys might recall from our discussion of cell division, that mitosis 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 mitotic 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, the systems involved in this are chiefly these mitotic 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 becomes 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. These undifferentiated cells that through differentiation, give rise to specialized 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 cell 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, an epithelial cell? Well, that's determined by its fate and we'll talk more about how cell fate is determined later on. Now, in plants, 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 meristems and actually, plants have multiple meristems. Basically, anywhere you're going to get new growth you have meristems. So, 2 pretty obvious places are the roots, right? The roots need to continue to grow so you have root meristems and also, the shoot. Right? Plants continue to grow upward, they branch out, send out new leaves. You have these shoot meristems and we'll talk more about meristems 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 carte blanche with our stem cells. We can't just continue to produce 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 specialized cells that will function in the circulatory system or the nervous system like neurons that we're talking about or the immune system as we're also talking about.

Alright. Let's flip the page.

Cellular Interactions and Apoptosis

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 are really two main types of interactions that are happening: paracrine, where cells emit chemical messengers that are picked up by the receptors of another nearby cell, and juxtacrine, which occurs when the ligand bound to the surface of one cell interacts with the receptor on another cell. In these ways, cells influence their neighbors to move around, divide, differentiate, and also die, topics which we'll discuss shortly. Differentiating cells have this special way of influencing their neighbors to also differentiate or to behave in a certain way—these are social control mechanisms, which we will talk more about later.

Regarding cells moving, this phenomenon is primarily observed in animals; it is not seen in plants, a topic we'll cover in more detail when we specifically talk about animal and plant development. It's important to note that cells actually have to move around during development to create specialized tissues and form specialized structures. Plant cells, on the other hand, are very good at expanding their size and changing, which results in alterations in the shape and form of the plant. Plant cells are also kind of moving around, just not in the same way that animal cells literally detach from one point and migrate to another part of the embryo. In plant cells, the cells will just expand their size and somewhat alter the shape of the plant as a result. We can see an example of this cell movement 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 form this configuration seen right here.

I previously mentioned dying as an important aspect of development, and indeed, we have discussed apoptosis, a particular type of programmed cell death, before in our discussions on cell signaling. It turns out that programmed cell death is incredibly important to development, and a great example of that is in the development of digits on your fingers. During the course of animal development, cells that make up the webbing between your fingers—similar to the webbed toes of a duck—undergo apoptosis and die out, resulting in separated digits. So, cell death is actually a very important part of life, believe it or not.

Alright, let's flip the page.

Differential Expression, Morphogens, and Pattern Formation

Hello, everyone. In this lesson, we are 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 there are different patterns of gene expression. And this is going to lead to different cell types. So, we're going to have different types of cells. Now, remember, the genetic makeup of every cell in your body is exactly the same. They should all be exactly the same; that causes differential gene expression. What this means is we're going to have signals in our body that tell certain genes in certain cells to turn on and to turn off. So, you're going to have some genes that skin cells will never express and some genes that skin cells will only express. So, there are genes unique to each type of cell in your body in your genetic makeup. And which genes certain cells are 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 transcriptional regulation. 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 mRNA. Now, there's also RNA splicing. So this can create certain mRNAs, which can create certain phenotypes, certain types of proteins, depending on how that RNA is built. Now, you can also have translational regulation. Translational regulation is actually determining which mRNA is allowed to become a protein, or how many of a certain gene are allowed to become a protein. So this is going to regulate the phenotype of the proteins and of the cell 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 are all going to determine differential expression. All of these processes are going to be utilized to give cells unique gene expression and unique phenotypes. Remember, gene expression is the phenotype or the characteristics that are created from the genes that are being expressed. And there are many different ways to modify that expression, including these 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 phenotypes. 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 simplification. So, let's say it only has these three genes. And as you can see, these two different cells in these multicellular organisms are expressing these genes differently. You can see that this first cell is expressing gene A twofold, and it's not expressing gene B whatsoever, and it's expressing gene C onefold. So, obviously, it has this particular phenotype because it expressed these genes this way. Perhaps, this particular gene B was not transcribed at all. Maybe it was silenced and told not to transcribe. So, no mRNA was made whatsoever from this gene. But then let's say that a lot of mRNA was made for gene A, and a lot of it was translated, so there were products were made. This is all going to be dealing with translational and transcriptional regulation. Now, let's look at the other cell. We can see that the other cell did not express gene A whatsoever. So, perhaps, gene A is not transcribed. Maybe it is silenced like the other cell did with gene B. But we can see the gene B in the second cell is expressed threefold. So, this is a really expressed gene, maybe it's transcribed a lot, a lot of mRNA is created, it is enhanced, and it is translated very quickly and very rapidly. Now, we can also see that it does express gene C twofold 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 of transcription, translation, and all these other modulating processes is going to give them their unique phenotypes. 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 your 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? There are 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 of 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 morphogens. Morphogens are molecules used to indicate cell position. They indicate cell position via concentration gradients during pattern formation. What's another word for pattern formation? Another word for formation or pattern formation is morphogenesis. And this is going to be why the name Morphogen comes about because they help with the process of morphogenesis, or the building and developing of the body of the organism, or the patterns in the organism. And these morphogens are very important molecules. Now, I have some examples down here of how these morphogens are going to work because I know that the concept can be a little bit confusing. So morphogens are utilizing a concentration gradient, and that gradient is highest around the cells that emit 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 morphogen, 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 morphogens that these cells get, it's going to tell them the cell's location in the body and what they are supposed to become, what type of tissue and organ they are going to become, and basically what job they are going to do. And, very important, morphogens 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 going to be determined by these morphogen molecules. There are a ton of different Morphogen 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 morphogen, 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 morphogens at work. The first one that we're going to be looking at is bicoid. And the second one we're going to be looking at is nanose. And these are just names of the particular morphogens. Now, this is going to be the egg of a Drosophila, or excuse me, the zygote of a Drosophila that will become a fruit fly. And Bicoid and Nanos are utilized to set up the head and the abdomen. So, Bicoid is at its highest concentration e.g. excuse me, highest concentration in the head region of the zygote. The region of the zygote that will become the head of the Drosophila fruit fly It's gonna have the highest concentration of bicoid, and then bicoid is going to diffuse through the zygote. So, the lowest concentration of the bicoid morphogen is gonna be at the abdomen, Or the tail region of this Fruit Fly. Now, we also have Nanos. Nanos is gonna work in the exact opposite method. It's gonna have its high concentration towards the abdomen region, or the tail region of this particular fruit fly. So, there's going to be really high concentration in the cells that will be the tail, and then it's going to diffuse towards the head. So the head's going to have the really low concentrations. And these 2 morphogens working together tell the cells in the zygote 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 are determined by different morphogens. You have more than just two chemical signals. There are tons of morphogens that are 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 morphogen concentrations and 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. <|vq_23592|>

Body Axes

Previously, I'd 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 b. 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 what's called the anterior-posterior axis. And you can actually see this in our image indicated by this green plane. So here we have our anterior-posterior axis. The anterior end is up here, toward the head, and the posterior end is down here. Oh, that came out ugly, sorry guys. Posterior, alright, good enough.

Now, we also need to be aware of the dorsal-ventral axis which is actually represented by this blue plane, here. Dorsal Ventral Axis. So, the ventral side is toward the belly. So here we have our ventral side and our dorsal side is 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 get the idea. Right? Alright. So, those are 2 of the major and important body axes to be aware of. So when someone says dorsal or dorsally, whatever, they're talking about something toward the back, ventral toward the belly, anterior toward the head, posterior toward your bum. Alright.

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, it takes more like 9 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 a little bit more with gap genes. And then with pair rule genes, we get a little more fine tuned. And then second polarity genes, even a little more fine tuned. And finally, we have the Hox genes, which we'll talk about on the next page, and they lead to effector genes. So, basically, this process of body formation happens in multiple stages over a long course of time. So let's turn the page and talk a little bit more about Hox genes because those are going to be very important.

Hox Genes

Pox genes are a special type of what are called toolkit genes, and these toolkit genes are a small subset of genes that control an organism's development, also known as the genes we've been talking about this whole time. Right? Those genes important to development. So, to get a little more specific, a subset of toolkit genes is these homeotic genes, which are genes that control the development of specific anatomical structures. Hox genes, which come from "homeotic box," hence "Hox" for short, are a type of homeotic gene. They are highly conserved genes, meaning that they've been around for a long time through the course of evolution and they help control development along the anterior to posterior axis. They are activated after segments form. So you might remember those segment genes that we just mentioned on the previous page. After all that, they come towards the end of development and help determine the specific structures that will form in a segment.

Here we have a nice example: you can see this fruit fly, a very common model organism used in biology. You can observe how the fruit fly has been divided into different segments, represented by different colors here. Each segment is associated with a particular gene. These are all Hox genes and they lead to the development of the specific anatomical structures you see at those segments. Initially, like you saw on the previous page, this embryo just looks like a segmented blob, but through the activation of these Hox genes, specific anatomical structures will develop.

Development in general is a highly conserved process and is directly linked to 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 instance, fish are quite different from us, during our development we actually have gills. Yes, you and I had gills at one point in our life when we were a fetus. We developed gills and then we lost them. This is because development is so heavily conserved and is directly linked to evolution, and famously, there were fish before there were people. So we continue to carry some of those traits to this day, but they only appear during our development.

Another facet of the conservation of the developmental process is that many animals use the same genes and chemical signals to govern body plan development. See, Hox genes are very important and highly conserved genes. 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. This is just another case in point of how everything in biology is conserved. Biology is not wasteful; rather, it takes things it already has and repurposes them to meet new needs. So, development is a 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. This is another reason we use organisms like chickens or sea urchins, for example, to learn more about our own human development because there is so much crossover between the development of a chick or even a sea urchin and a person. Alright, that's all I have for this video. I'll see you guys next time.

Here’s what students ask on this topic:

What is the role of cell division in developmental biology?

Cell division is fundamental to developmental biology as it allows a single zygote to transform into a complex multicellular organism. The process is tightly regulated through mitotic control mechanisms, which include various checkpoints in the cell cycle, and social control mechanisms, where neighboring cells influence each other's division. Proper timing and location of cell division are crucial for the correct formation of tissues and organs. This regulation ensures that cells divide at the right time and place, contributing to the organism's overall development and complexity.

How does differential gene expression contribute to cell differentiation?

Differential gene expression is key to cell differentiation, where cells with identical genetic makeup develop into various specialized cell types. This process is regulated by transcriptional and translational factors that determine which genes are expressed in each cell. For example, skin cells and neurons express different sets of genes, leading to their unique functions and structures. Factors like transcriptional regulation, RNA splicing, and post-translational modifications all play roles in modulating gene expression, resulting in the diverse cell types necessary for complex multicellular organisms.

What are morphogens and how do they influence development?

Morphogens are molecules that guide the spatial organization of cells during development by creating concentration gradients. These gradients provide positional information to cells, helping them determine their location and fate within the developing organism. For instance, in Drosophila (fruit flies), morphogens like Bicoid and Nanos establish the head and tail regions, respectively. Cells respond to the concentration of morphogens they receive, which influences their differentiation into specific tissues and organs. Morphogens are crucial for setting up body axes and ensuring proper pattern formation during development.

What are Hox genes and why are they important in development?

Hox genes are a subset of homeotic genes that play a critical role in determining the body plan of an organism along the anterior-posterior axis. These genes are highly conserved across species, indicating their fundamental role in development. Hox genes are activated after the formation of body segments and help specify the identity and function of each segment. For example, in fruit flies, different Hox genes are responsible for the development of specific anatomical structures in each body segment. This precise regulation ensures the correct formation of complex body plans and highlights the evolutionary conservation of developmental processes.

How do cells communicate during development?

During development, cells communicate through two main types of interactions: paracrine and juxtacrine signaling. Paracrine signaling involves cells emitting chemical messengers that are picked up by receptors on nearby cells. Juxtacrine signaling occurs when a ligand bound to the surface of one cell interacts with a receptor on an adjacent cell. These interactions influence cell behaviors such as movement, division, differentiation, and programmed cell death (apoptosis). Effective cell communication ensures coordinated development, allowing cells to form specialized tissues and structures in the correct locations.