Animal Development - Online Tutor, Practice Problems & Exam Prep

Animal development begins with fertilization, forming a zygote through the acrosomal reaction, which prevents polyspermy. The zygote undergoes cleavage, resulting in a morula and then a blastocyst, which implants in the uterus. Gastrulation forms three germ layers: ectoderm, mesoderm, and endoderm, leading to organogenesis and neurulation. The notochord and neural tube develop, with cell determination guiding differentiation into specific cell types. Understanding these processes is crucial for grasping the complexities of embryonic development and the roles of various cellular mechanisms.

Fertilization

Hi. In this video, we'll be talking about the specifics of animal development. Now, development begins with fertilization, which is the proverbial fusion of sperm and egg to form a zygote. This occurs in the fallopian tube, and the egg is actually swept into the tube by the cilia. Fertilization actually has a very short window in which it can occur. It has to occur within 24 hours of ovulation, which is the rupturing of the corpus luteum and the releasing of the egg. Now, when the sperm and egg actually get together, you have a special reaction occur. You see the sperm has this tip seen right here called the acrosome. And the acrosome contains these special enzymes that break down the protective coat around the egg. So the sperm fuses with the egg, releases those acrosomes, which allow it to actually get through the egg and also signal the egg to not let any more sperm in, and this is accomplished by something called the Cortical Reaction, which releases a bunch of calcium which depolarizes the membrane causing a change in membrane potential. And this basically signals the completion of the second meiotic division of the egg. All of that is to say that the sperm and egg fuse, there's a bunch of stuff that happens, and it prevents multiple sperm from fusing with the egg and also allows the sperm and egg to form the zygote and begin development.

Alright, let's flip the page.

Cleavage and Blastulation

After fertilization, the zygote has to travel to the uterus. During this time, cleavage occurs, which are these rapid mitotic divisions and they result in something called a morula. So the zygote turns into the morula as cells divide into smaller cells. This is an important thing to note. These cell divisions do not result in cells the size of the zygote. The zygote is actually dividing, but each time it divides, the individual cells are getting smaller and smaller. Now, there are actually 2 different types of cleavage. There's what we call indeterminate cleavage, where the cells that arise from the cleavage are able to develop into a whole organism. This is how, so-called fraternal twins, can develop. You have indeterminate cleavage, these cells, somehow separate and they, the 2 cells or clumps develop into separate, complete organisms. Now, you can also have determinate cleavage where the cells that arise are committed to differentiation. And differentiation we know, is based on differences in transcription mainly, but gene expression in general.

Now something we briefly mentioned in the lesson on development but didn't really get into was the idea of induction. And that is that differentiated cells can influence the cell fate of nearby cells, of their neighbors. So this is related to those social control mechanisms. Right? Cells can actually induce other cells to differentiate a certain way and this is a very important part of development. And the inducers are chemical signals generally and they are released from one cell, diffuse out, and stimulate a response in some nearby cell that has the proper receptor for that. Now, these cells that are created during cleavage, these individual cells like we see right here, these are called blastomeres. It's simply a cell created during cleavage. The morula, where we talked about the zygote forming, is the early-stage embryo that consists of a solid ball of cells. The key being here, solid. We'll see why because, in fact, during the course of development, that solid ball of cells is going to hollow itself out.

Now, we should also mention cytoplasmic determinants. These are regulatory molecules located in specific regions of the egg so that during the course of these divisions they are not evenly distributed to the resulting cells. So basically, imagine that we have this egg here and it has some cytoplasmic determinants here that I'm representing with blue dots and then, say, down here, it's got these other different cytoplasmic determinants that I'm representing with red dots and then this cell goes through cleavage and divides into 4 unique cells. Well, look at those cytoplasmic determinants. They were not evenly distributed among those cells. This is important because this uneven distribution results in the formation of specialized cells. So, these cells right here, they didn't get the red or the blue cytoplasmic determinants so they're going to go down a different path of development than this cell which got these blue determinants and this cell, they've got these red ones. So, the end game of this is that only certain blastomeres, only certain of these cells that result from the cleavage, will have regulatory cascades triggered by the determinants, meaning they will have certain genes expressed or not expressed as a result and that will differentiate them from say their neighbor who maybe didn't get the same cytoplasmic determinants. Now, once we have the morula, that solid ball of cells, the next phase of development is called blastulation. And this is where the morula develops into a blastocyst.

Now, the blastocyst is basically the mammalian form of the blastula. So blastula is just a hollow ball of cells that forms during development. In mammals, we call this blastula a blastocyst. Just kind of a lot of terminology being thrown around here, trying to keep it simple. So again, blastula is just a hollow ball of cells. In mammals, we specifically call it a blastocyst.
That inner space which we can see, let me take myself out of the shot here. This inner space that you can see in here, that is the blastocele which is a fluid-filled cavity inside the blastocyst. We also have something called the trophoblast that are the cells that surround the blastocele and give rise to the chorion and placenta which are structures that are not part of the developing organism but are support structures. Right? They help keep that developing organism alive. They're very important, the organism couldn't live and develop properly without it, but they don't actually become part of that organism's body. And you can see those trophoblast cells, labeled here in the image and they are on the periphery there, around the outside of that blastocele. The last thing to point out is the inner cell mass which again has been labeled here, and these are cells inside of the blastula that actually give rise to the organism. So those outer cells, the trophoblast, give rise to the support structures, the inner cells give rise to the actual organism itself.

Implantion and Gastrulation

Once the blastocyst is formed, it has to implant in the uterus. This is called implantation. Getting really creative with the names here. Now, hCG is a chemical secreted by the implanted blastocyst, and it lets the body know that pregnancy is underway. In fact, most early pregnancy tests actually test for the presence of hCG. hCG is not the end-all-be-all hormone involved in this, and eventually, it will be replaced by the secretion of estrogen and progesterone, but all of this is a little beyond the scope of what you need to know for Bio 1. So, what you should know is that those trophoblast cells will develop into the placenta, and they will take over the production of estrogen and progesterone and will continue to produce it throughout pregnancy. These hormones are very important to pregnancy and to supporting the developing organism. Now, once we have our blastocyst implanted, it can start to undergo what's called gastrulation, which is the formation of three germ layers, forming a gastrula. As you can see here, we have our blastocyst, right? It has cells around the outside, blastocele, that fluid-filled cavity inside, and what happens is it starts to kind of poke inward like we see happening right here. This is gastrulation, and the result is what we see here, the gastrula. What I'm highlighting in red is the mass of cells. There's a space inside, however, a hollow cavity. Now, let's go over a little terminology real quick. We have our blastopore, that's the opening to this space. We also have our three germ layers. The three germ layers are the ectoderm, which are the cells that give rise to nerves, the adrenal medulla which is a gland in the body, skin, brain, eyes, inner ear, or your vestibular system, if you will. You also have the mesoderm. And the mesoderm, you can see there's actually one little patch right here. This is also mesoderm over here. I'm going to take myself out of the shot so it's easier to see all of this. The mesoderm consists of internal cells that will give rise to organs, the adrenal cortex which is also part of the adrenal gland, a different part though, the blood, bone, gonads, and soft tissues. And then lastly, we have the endoderm. The endoderm is the layer of those innermost cells that you can see in this mustard color. I'm going to circle it in blue right now. This is our endoderm. Those innermost cells form the epithelial linings of the digestive tract, the liver, pancreas, and lungs. In a large part, they form the inner cells of what's called the alimentary canal or, basically, the cavity that runs through our bodies.

Organogenesis and Neurulation

Now after those germ layers have formed, we can begin organogenesis or the process of organ and tissue development. And you might remember that the mesoderm was made up of those 2 pockets. Well, we call those somites, those are pairs of mesodermal tissues. And the cell's position in this, in the somite, determines what it will turn into. So let's imagine that, here is our somite, and it's made up of a bunch of little individual cells. Again, this is just for the purpose of example. So depending on where the cell is located in the somite, it'll turn into different things. It'll actually break away in groups and migrate and form different things. So, this cell might form, let's say, cardiac muscle, like we see here. And maybe these cells, the cell breaks away and forms skeletal muscle. So it's the actual physical position of the cell in the somite that determines what it will develop into. And I'm sure you have some pretty good guesses as to how and why that can happen based on what we've already learned about development, concentration gradients, and chemical signals. Now the mesoderm layer develops into many different things as we can see here, including cardiac muscle, skeletal muscle, tubule cells of the kidney, red blood cells, and smooth muscle cells like those around the intestine. Now in addition to organogenesis, there is another really important process that occurs in animal development, and this is neurulation, which is the actual formation of nervous tissue from primary germ layers. So a couple of points of terminology here. We have the notochord, which you can see here in the picture, this structure. And if I scroll down a little so you can see the other images, The notochord is also present here and here, and it is labeled in this last image here. So, what is the notochord? The notochord is kind of like a primitive backbone that develops in chordates, which is a, a big class of types of animals. And in some animals, it actually develops into the vertebrae of the spine. But in many animals, it is actually just a transient structure. It forms during development, and then it goes away at some point. And that's a totally normal part of the process. And it's because of that link to evolution, that close link that development has to evolution. Now, in addition to the notochord, you should be aware of the neural tube, which is a hollow structure that will eventually form the brain and spinal cord. The neural tube forms in kind of an interesting way. We have this neural plate as it is called, and it basically, as you can see here, it folds in on itself and seals the ectoderm. There is ectoderm on this side. This is also ectoderm. It seals the ectoderm together. So let me get rid of some of these arrows just so this becomes a little more clear, so you can see the ectoderm, or the neural plate folds in on itself and then, eventually, boom, we have the ectoderm fuse. And, again, this neural tube structure that you see here will form the brain and spinal cord. In fact, it swells in certain places to form the embryonic brain. So it'll swell and form little bulges that actually become embryonic brain structures. So the mesoderm cells, that form the notochord are actually the same cells that induce, remember, induction, induce the ectoderm cells to furrow or form this fold and then fuse together to form the, neural tube. The Neural Folds that surround the Neural Group actually give rise to the, central nervous system. Now, the last thing we need to talk about is cell determination, which is that irreversible commitment of a cell to a particular developmental path, which results in a specific cell type. We've talked about this. It happens through the process of differentiation. But what we haven't talked about is how differentiation is actually a gradual process. Cell a can give rise to cell b or cell c. Cell b is capable of giving rise to cell d and cell e, and cell c can give rise to cell e or f. So differentiation occurs in stages. To get from cell a to, let's say, cell f, the cell has to go through that intermediary phase of being cell c. And at that point, its fate as becoming cell f is still not sealed. It could also become cell e. It has options. So differentiation is a gradual process. However, once the cell is committed to a particular path, its fate is sealed. Its destiny is sown in the stars, and it is going to become differentiated to that particular type of cell. Alright. That's all I have for this lesson. See you guys next time.

Here’s what students ask on this topic:

What is the acrosomal reaction and why is it important in fertilization?

The acrosomal reaction is a crucial process during fertilization where enzymes from the sperm's acrosome, a cap-like structure, are released to break down the protective coat surrounding the egg. This allows the sperm to penetrate the egg and fuse with it. The reaction also triggers the cortical reaction in the egg, releasing calcium ions that prevent additional sperm from entering, thus ensuring monospermy. This fusion forms a zygote, marking the beginning of embryonic development. The acrosomal reaction is essential for successful fertilization and the prevention of polyspermy, which can lead to abnormal development.

What are the differences between indeterminate and determinate cleavage?

Indeterminate cleavage results in cells that retain the potential to develop into a complete organism. This type of cleavage allows for the possibility of identical twins if the cells separate. In contrast, determinate cleavage produces cells that are committed to differentiating into specific cell types. These cells have a predetermined fate and cannot develop into a complete organism on their own. The distinction between these two types of cleavage is crucial for understanding how multicellular organisms develop from a single zygote through various stages of cell division and differentiation.

What are the three germ layers formed during gastrulation, and what do they develop into?

During gastrulation, three primary germ layers are formed: the ectoderm, mesoderm, and endoderm. The ectoderm gives rise to the nervous system, skin, brain, eyes, and inner ear. The mesoderm forms structures such as the heart, muscles, bones, blood, and kidneys. The endoderm develops into the epithelial linings of the digestive and respiratory systems, including the liver and pancreas. These germ layers are foundational in organogenesis, where they differentiate into various tissues and organs, establishing the basic body plan of the developing organism.

What is neurulation and what structures are involved in this process?

Neurulation is the process of forming the nervous system from the primary germ layers. Key structures involved include the notochord and the neural tube. The notochord, a rod-like structure, induces the overlying ectoderm to form the neural plate, which then folds to create the neural tube. The neural tube eventually develops into the brain and spinal cord. The notochord may become part of the vertebral column in some animals or remain a transient structure. Neurulation is critical for establishing the central nervous system and is a pivotal step in embryonic development.

How does cell determination and differentiation contribute to animal development?

Cell determination is the irreversible commitment of a cell to a specific developmental path, while differentiation is the process by which a cell becomes specialized. These processes are guided by gene expression and regulatory molecules, such as cytoplasmic determinants and inductive signals. Differentiation occurs in stages, with cells gradually acquiring specific functions. For example, a stem cell may first become a progenitor cell before differentiating into a specific cell type like a neuron or muscle cell. This intricate regulation ensures the proper formation of tissues and organs, essential for the organism's development.