Animal Reproduction - Online Tutor, Practice Problems & Exam Prep

Sexual and Asexual Reproduction

Hi. In this lesson, we'll be talking about animal reproduction, which often is going to be sexual reproduction, which is when two organisms combine their genetic information, usually through the fusion of gametes. Now gametes are formed from meiosis, and these are going to be cells that have just one copy of each chromosome from the parent, making them haploid. Now if this sounds unfamiliar, I recommend you go check out our videos on meiosis and sexual life cycles. Now male gametes are known as sperm, and female gametes are known as eggs. Male gametes usually are smaller and motile, meaning they can move themselves; they can propel themselves. Female gametes tend to be larger and non-motile, meaning they can't propel themselves. Now we call the fusion of sperm and egg fertilization, and this will form a zygote. So we will get a zygote from fertilization, and that zygote is going to be diploid, meaning it will have two copies of each chromosome, one from each parent, thanks to those gametes each being haploid. Now some organisms reproduce through asexual reproduction. And this is when an offspring arises from a single organism, and it only receives genes from that parent. Now, there are a few ways this can occur, one is budding, which is when an organism will kind of grow off of its parent and break off when it becomes mature. You can see that happening in these yeast cells here. Yeast are, you know, eukaryotic organisms, and they reproduce by budding. You can see this parent cell has this bud growing off of it, so does this one. When these buds are mature, they will literally break off from their parent cells. This also occurs in the animal's hydra. They also reproduce through budding. Now fission is when an organism divides into two or more parts, and this is how, for example, bacteria reproduce. This is known as binary fission in bacteria. And as you can see, it's going to lead to two genetically identical organisms. Now, parthenogenesis is a very interesting type of asexual reproduction because it's when growth and development occur without fertilization. You see this in, for example, some species of lizards which are going to be all female, and will just grow and develop offspring in the app without any fertilization, and those offspring will, of course, be female.

So you might be wondering what's the point of sexual reproduction? Because it's more energy-intensive and it's less efficient than asexual reproduction. I mean, look at this little graphic behind me. Asexual reproduction, like we can see here, is going to lead to much greater increases in population size. So what's the advantage of doing it this way? You can't increase the population size nearly as efficiently. It also requires two organisms just to produce one new offspring, or, you know, in the case of some organisms, you'll produce multiple offspring. But you get my point. It requires two parents to get together to generate any offspring. So what's the point? Well, one explanation for this is known as the Red Queen hypothesis, and the idea is essentially genetic diversity. Organisms that reproduce asexually don't have a harder time introducing genetic diversity. Sexual reproduction naturally confers genetic diversity to the population. This is important because the Red Queen hypothesis says that organisms have to constantly adapt and evolve to compete with opposing organisms. This idea comes from a quote from Alice in Wonderland, from the character known as the Red Queen. When Alice is talking to her, she says that, you know, in where I come from, you know, not Wonderland, when you run you get somewhere. You know, when you move, you actually get to someplace. And the Red Queen says, "now here you see, it takes all the running you can do just to stay in the same place." And that is why this is termed the Red Queen hypothesis. It's because organisms have to constantly be running, right, adapting, and evolving, which is why sexual reproduction is favored because it introduces genetic diversity that promotes that adaptation and evolution. And this is necessary because they have to compete with all the other organisms around them that are also constantly adapting and evolving.

With that, let's go ahead and flip the page.

Male Reproductive Anatomy

Hello, everyone. In this lesson, we are going to be learning about reproductive anatomy. But specifically in this lesson, we're talking about male reproductive anatomy. In the next lesson, we'll talk about female reproductive anatomy. Okay. So let's get into our lesson.

Okay. So, genitalia is just the word that means sex organs. This can be anything on the organism that is used for reproduction. Now, the gonads specifically are going to be the organs that produce gametes, like eggs and sperm. And since we're specifically talking about the male reproductive anatomy, we're going to be talking about sperm production, where it happens, and the different parts of the male anatomy. So the testes are going to be the male gonads, and they're going to produce sperm. And they're going to have these many different components. As you guys can see here, it's really complex inside of a testis, and there are a lot of different areas that sperm travel to in their method of production. So in each space, more and more is added to them, or they are stored, or they are matured in a unique location.

Okay. So before we get into the tubules that are used in the testes, we're going to talk about the scrotum, which is going to be a skin pouch that holds the testes. This is going to be a characteristic that's found in almost every single land mammal, I believe. The aquatic mammals, like whales and dolphins, are not going to have this. This is going to be a structure that is outside of the body in land mammals, and you guys know what it's for. It's to maintain the correct temperature of the testes to ensure that the sperm is able to be produced. Now we know that the internal body temperature of most mammals, and humans included, is going to be around 98 degrees. Now, for correct sperm production, it actually can't be 98 degrees. In fact, it has to be 95 degrees Fahrenheit for sperm production. If it is too hot, or if it is too cold, the sperm will not be produced properly. So the scrotum is used to keep the testes outside of the body so they don't get too hot, but still close to the body so they don't get too cold, so they're a perfect 95 degrees. Because if they're not within that temperature, sperm are not going to be produced properly, and then that male won't be able to reproduce.

Okay? Alright. So now we're going to be talking about the seminiferous Tubules and the Epididymis, which are going to be 2 major types of tubes that are used to produce sperm. So the Seminiferous Tubules are coiled tubes in the testes where sperm are produced, so they're very important. So, as you guys can see in this picture, they're going to be these tubes that I'm highlighting in yellow. So, let me label those. Seminiferous tubules are going to be right here. And this is going to be where the production of sperm actually happens. But sperm are not fully ready, and fully ready to go out of the seminiferous tubules. Those are actually multiple steps. And in the epididymis, this is where sperm are going to mature, and they're stored. And the epididymis, I'm going to highlight in blue, is going to be this structure here. The epididymis is going to be where the sperm actually become motile. They're given all of their correct components, and None! \n ``` \n

Female Reproductive Anatomy

Hello, everyone. In this lesson, we are going to be talking about the female reproductive anatomy. Okay. So, let's get into the lesson. And, first off, let's just go over some major anatomy pieces, and then we'll go into more detail. Okay. So first off, the ovaries are incredibly important for female organisms because these are going to be the places where they actually make the eggs, the female gametes. So eggs are going to be made in the ovaries, and they're going to be released periodically. Something interesting to know about female organisms, and human beings included, is that a female, when she is born, already has all of the eggs that she will ever make in her entire lifetime, and it's just that all of those eggs are held and then periodically released throughout her entire life. And whenever she no longer has enough eggs or she's no longer periodically releasing those eggs, that individual will go into menopause. But the ovaries hold all of the follicles, which create all of the eggs, and all of the eggs are already produced before the female's actually born. So she has all of her eggs for her entire life the moment she is born. Okay. So we know that these eggs are periodically released from the ovaries, and that's going to be during the process of ovulation. And whenever an egg is ovulated from the ovaries, it is going to travel down the oviduct. Now, different organisms have different types of oviducts. Organisms that do external fertilization, or actually have their eggs leave their body, are going to have oviducts. We have internal fertilization, so our oviducts don't lead to the external environment. They're going to lead to the interior of the woman's body into the uterus, so these are going to be called the fallopian tubes. These are mammalian oviducts that don't lead to the exterior of the body but lead from the ovaries to the uterus because the fetus develops inside the female. Now, that leads us to talk about the uterus. The uterus is also commonly called, in human beings, the womb. But what I want you guys to know is it's this hollow muscle structure, and its main job, its main focus is to be a place where the fetus actually develops. Okay, everyone? Now, the opening to the uterus is going to be called the cervix, and the cervix during childbirth is actually going to greatly dilate. If you've ever heard of a woman being 6 centimeters dilated or a certain number of centimeters dilated, we utilize that information, the dilation of the cervix, to know how close it is to the time of childbirth. And, in fact, the cervix should dilate to about 10 centimeters at the time of childbirth. So, the cervix, usually quite small, is going to be what opens up and allows the baby to leave the uterus. Now, there's also the lining of the uterus, which is incredibly important. And, this is called the endometrium. And, this is going to be the inner epithelial tissue of the uterus. And this is going to be what is going to change with each menstrual cycle, and it is going to be where the egg, or the zygote, I mean, actually implants into the wall of the uterus. That's going to be on the endometrium tissue. Now, there are many different tissues that make up the uterus. You can see the endometrium here, then there's the myometrium and the perimetrium. The endometrium is going to be the innermost one, and it's probably going to be the most important for this lesson because that's where the zygote actually implants. And this is going to be the tissue that is shed during menstruation. And it can also form the placenta if the woman does become pregnant. Now, you may have heard of endometriosis. This is going to be a condition in some women where the endometrium actually will grow outside of the uterus, and this can be incredibly painful to some women. So endometriosis deals with the endometrium, or inner layer of the uterus, and that's when it's not growing correctly and it can cause pain. So, you guys can see that this is going to be a diagram. Let me scroll down so you can see the other labels here. So this is going to be the basic diagram of the female genitalia, and you guys can see that there are 2 ovaries, one here and one here, and they are actually going to connect to the fallopian tubes, which you guys can see right here. And these fallopian tubes are going to lead into the uterus, and that's where the zygote will implant and the fetus will grow. And then, here we have the cervix, and this little area right here is going to greatly dilate, and it's going to allow the baby to pass from the uterus to the vagina, and then out of the mother, and that is the process of childbirth. Now, let me scroll down. This is going to be the same diagram right here. It's just showing you the position of the female genitalia in the woman, and you guys can see that it's just basically in the pelvic region. And a woman is going to have 2 ovaries on each side of her pelvis, and then she's going to have her uterus and all of that stuff in the middle. Okay? Alright. So, now let's talk about the rest of the anatomy. So, let me scroll down a little bit, so we can see what we are looking at. Okay. So, then we're going to have this is going to be the rest of the anatomy. And the vagina is going to be the muscular opening that comes before the uterus. You guys can see that? That's this right here. And the uterus is going to be what I'm highlighting in pink. You guys can see that the uterus is incredibly muscular. And it's going to have a lot of tissue that is there to support the growing baby. Now, the vulva is going to be composed of 2 different pieces of anatomy. But, basically, what I want you guys to know is these are the external features of the female genitalia, and they're mostly there just to protect the female genitalia from basically the outside world. So you have the labia majora, which encloses and protects the rest of the vulva, so the very outside. And then you have the labia minora, which encloses the vaginal and urethral openings. So, basically, it's just to cover everything up and make sure that it stays safe. And then you're going to have the clitoris, which is going to be the female form of erectile tissue. And you guys can see all of that in this really good image. I really like this picture because it shows you from the side view, while the one up here shows you from the, straight on view. But we always see that view, and I like this one because it shows you where everything is located in relation to everything else. So you guys can see that up here is going to be 1 ovary, and then you guys can actually see that the egg is going to travel down this tube right here into the uterus. So you guys can imagine that if a sperm is going to fertilize an egg, how far does it have to travel? It actually has to travel all the way up into the uterus, and then you guys may or may not know, but fertilization generally happens inside of the fallopian tube, so it has to travel a really long way. So, that is going to be the basis of the female genitalia. Okay, everyone. Let's go on to our next lesson.

Spermatogenesis

Okay, everyone. So in this particular lesson, we are going to be learning about Gametogenesis. But, in particular, we're going to be learning about Gametogenesis, specifically in the male organism, which is the production of sperm. Right? Because Gametogenesis is going to be the production of gametes, and sperm is going to be the male gametes. Now, we have all of these different cell types. And they're going to be the spermatogonia, the primary spermatocytes, secondary spermatocytes, spermatids, and spermatozoa. And all of these are going to be very important for the production of sperm. And the production of sperm is going to be called spermatogenesis. Now, whenever we learned about gametogenesis, we simply learned about meiosis. Right? That one cell divides twice and creates 4 haploid cells. But, unfortunately, it's not that easy, and we're actually going to have more cells doing the division in spermatogenesis. So, first off, you're going to have these spermatogonia cells, which are going to be these undifferentiated cells in the testes of the male organism, and they're going to have to undergo mitosis before they are actually ready to undergo meiosis and form the gametes. And once they undergo mitosis, then they're going to turn into these primary spermatocytes. The primary spermatocytes are diploid cells that are completely ready to go through meiosis. So the primary spermatocytes are going to be found in the seminiferous tubules. It's always hard for me to say. And these are going to be tubules that are found inside of the testes and they are going to be very important for spermatogenesis because this is where it's actually going to take place. Then, once the primary spermatocytes go through one round of meiosis or meiosis 1, then they're going to turn into these secondary spermatocytes, which are then going to be haploid. Because, remember, at the end of meiosis 1, the cells that are produced are haploid. So these secondary spermatocytes are haploid, and they are going to undergo meiosis 2, the second round of meiosis. And they're going to turn into these spermatids, which are undeveloped male gametes. So whenever we think of meiosis, we think the end product is simply these perfect gametes. So, you would assume that they're sperm. But, unfortunately, they're not. They have to go through a couple more steps to make them into mature sperm. So, the immature or undeveloped sperm that are made from meiosis are going to be called these spermatids. And they need to undergo more maturation to become the sperm, which are also called spermatozoa. So, these are the motile sperm that we think about whenever we think about male gametes. Okay. Now, these processes, I have a diagram down here, which I'm going to go through so you guys can see what I actually talked about whenever I talked about these particular divisions. But first, I want you to know that these sperm are going to have these 2 really important structures that compose them. They're going to have something called the acrosome, and they're going to have their flagella. Their flagella's probably pretty self-explanatory. This is going to be their microtubule tail that allows them to swim. And we all know what that looks like. Right? And then, we're going to have the acrosome. And this is going to be the cap of a sperm cell, and it's going to be used to help digest the, outer layer of the egg, and it's really going to help that sperm actually get into the egg, which we talk more about whenever we learn about fertilization in later lessons. Okay, everyone? Alright. So, let's go down and let's have a look at the actual process that takes place. So that, on that side of the lesson is actually going to be the process of all the different cell divisions that actually make these sperm gametes. So, remember, you have these spermatogonium, and they are going to have to undergo mitosis because they're not entirely ready to go through meiosis quite yet. So these spermatogonium cells are going to go under mitosis and create the primary spermatocytes, which you guys can see are diploid. So these diploid cells are ready to undergo meiosis 1, and then they're going to turn into these secondary spermatocytes. And what is going to be the ploidy of these secondary spermatocytes? These secondary spermatocytes are going to be haploid cells. Okay, everyone? So these are haploid cells because at the end of meiosis 1, the cells are haploid. And then they're going to undergo meiosis 2 and create the spermatids. Remember, the spermatids are haploid, but they're underdeveloped gametes. They're not perfectly made yet. They need some alterations, some maturation. And through the process of spermiogenesis, which is that maturation of the sperm, then you're going to get what we think of as actual sperm with their tails and their acrosome. So, you have to actually add some other pieces, like the flagella and the acrosome to these spermatids to create spermatozoa. Okay, everyone? Now, here I have this nice diagram of a sperm, and if you guys were wondering what the acrosome actually looks like, it's right here, and it's depicted in yellow. And it's going to be the cap on the head of the sperm. And like I said, it's going to be used to actually get into the egg, which is actually a difficult process. Okay, guys? Now, the other components of the sperm are, of course, going to be the nucleus because that's where it holds its haploid genetic material that will then fuse with the egg's haploid genetic material to make a zygote. And then, really interesting, right here you can see it says Mitochondria. In this box that I'm drawing, are going to be a whole bunch of mitochondria. I believe it's 75 to 100 mitochondria. And do you guys know what they're going to be used for? They're going to be used to power the movement of that flagella. Remember, mitochondria are utilized to make ATP ATP energy, which you can use to do work. And, the work that needs to be done here is going to be the movement of the tail of the sperm. So, that's why you have the mitochondria right at the base of the tail, so they can power the movement of that flagella tail. Okay? Now, what is this part right here? Do you guys know? Well, this is flagella. Right? And whenever we learned about the cytoskeleton, we learned that flagella are going to be made of microtubules. So, let me spell that right. Microtubules. And, specifically, remember, they have a unique structure. They have a 9+2 arrangement of microtubules. So, of MTs is just how I'm going to write it because it's the shortened way to write it. And do you guys remember what motor protein they use? Remember they're going to use the dynein motor protein. So, they have a 9 plus 2 arrangement of microtubules with the dynein motor protein. And that's going to be this particular structure that you see here. This is a cross section. And it's going to have the 9 exterior microtubules and the 2 interior microtubules and the dynein proteins that move that flagella. Okay? Alright. So that's going to be the basic pathway that you utilize to create sperm. So now let's go down and let's talk about the important hormones that we have that actually help with the creation of gametes. So just so you guys know, these particular hormones are used in males and females, but in this particular section, I'm going to specifically talk about how they're used in males to make sperm. Okay? So spermatogenesis actually begins in puberty of the male, and then it doesn't end until the male's death. So it's going to go from puberty all the way through the rest of his particular life. Okay? And then, we do have some important hormones that are utilized for this process. One of them is going to be called the gonadotropin-releasing hormone or GNRH for short. And, it's going to be a hormone that's going to be released by the hypothalamus. And it can also be called, just so you guys know, it is also called Gonadalobrin, Gonadalibulin. Those are going to be the same two hormones. Just, generally, whenever we're talking about the natural hormone that is made synthetically gonadalibrin. But, they do the exact same thing. Okay? And, these are going to be created by the hypothalamus, and then they're going to travel to the pituitary glands, specifically the anterior pituitary glands. And this gonadotropin hormone is going to be made to actually turn other hormones on. So this is like a checklist. The gonadotropin hormone is made. It travels to the anterior pituitary. And then, it's going to ensure that these LH and FSH hormones are going to be LH hormone is going to be the luteinizing hormone, and I'm going kinda in the way of the definition, so I'm going to go out of the picture for a second. So, this luteinizing hormone is going to stimulate these very interesting cells called Leydig cells. Leydig cells are very important for male development because they're going to be very important because they create testosterone. So, they're going to be found in the seminiferous tubules where the sperm are made. And these cells are going to ensure that testosterone is going to be made in the male organism. So, testosterone is going to be the male sex hormone. And androgen is another word for a sex hormone or a male sex hormone. And it's going to, obviously, help with spermatogenesis. And, it's also going to help with many of the male characteristics that male organisms have that are specific to that particular organism. Now, luteinizing hormone is going to work in conjunction with the follicle-stimulating hormone. They're going to work together. So, the gonadotropin turns both of these hormones on, and together they're going to create the process of spermatogenesis and testosterone production. So, the Follicle Stimulating osterone production. So the Follicle Stimulating Hormone is going to stimulate these really interesting cells, which are really cool, called Sertoli Cells. But, you may have heard of them before. They're commonly also called Nurse Cells. Nurse cells. It says here that they help with spermatogenesis, but they actually do so much. They are going to be cells that are found in the seminiferous tubules that obviously help build the sperm. That's how they go from spermatids to spermatozoa. These sertoli cells help them develop into mature sperm. And these cells are also going to control the amount of nutrients and hormones that these sperm are going to receive. And these cells can even repair DNA damage if they have to to ensure that these gametes, come out perfectly or as perfect as they can get. So that's really interesting. So, now, what we're going to talk about is how these processes control each other with this hormone called Inhibin. Inhibin sounds a lot like the word inhibit, and that is the whole point because Inhibin is going to actually inhibit the release of FSH or follicle-stimulating hormone. So, inhibin is going to be made by our nurse cells. And, inhibin is given out inside of the male testes to turn the follicle-stimulating hormone off. And, basically, what this is is this is a regulation checkpoint. So whenever the follicle-stimulating hormone is stimulating the sertoli cells, we know that spermatogenesis is happening. So what the sertoli cells are going to do is now they're going to create inhibin, which is going to stop the production of FSH. FSH. And, basically, what this is is a control mechanism to ensure that the production of FSH and the production of sperm and testosterone doesn't get out of hand. It's just a check system to ensure that it doesn't continue or go out of control, that sperm, testosterone, and all of these processes are going at a continuous even level. Okay, everyone. So that's all we have for this particular lesson on spermatogenesis. And in our next lesson, we are going to talk about gametogenesis, but specifically, how you make eggs inside of a female. Okay, guys. Let's go into our next Lesson.

Oogenesis

Gametogenesis in females is known as oogenesis, and this is going to result in the production of an ovum. An ovum is a female gamete or an egg cell, and it is going to have particular layers that you should be aware of on the outside: the corona radiata, which is the outer layer that the sperm will actually have to penetrate to fertilize the egg, and the zona pellucida, which is a layer of glycoproteins inside the corona. We'll get to what is going on during fertilization in just a little bit.

Oogenesis actually begins before birth. Remember, spermatogenesis doesn't happen until puberty. Oogenesis begins before birth, but doesn't complete. It actually arrests at a certain phase, and then after puberty, will continue. This process actually stops after what's known as menopause. So, oogenesis starts with oogonia. These are female diploid germ cells that will divide by mitosis, and actually begin meiosis prior to birth. They divide by mitosis and form primary oocytes that will start meiosis and then stop at prophase 1. Each one of these primary oocytes is going to be contained in a structure known as a follicle. This follicle is going to not only contain the primary oocyte but also play a role in the menstrual cycle.

We'll talk about the menstrual cycle in more detail in just a moment, but very briefly, each month, basically, in humans, though the cycle is different in other organisms, each month FSH (Follicle Stimulating Hormone) is going to cause the follicle to mature, and it's going to cause the primary oocyte to complete meiosis 1, and then the resulting secondary oocyte will begin meiosis 2. The follicle will rupture and release the ovum, and the remaining structure will form what's known as the corpus luteum. This is a temporary endocrine structure that will release a lot of progesterone and some estradiol, but it will eventually go away.

The secondary oocytes are going to be cells arrested at metaphase 2 of meiosis 2. If they get fertilized by a sperm, they will actually then complete meiosis 2, and then, form the complete egg cell, and join with the sperm. During the process of meiosis, these oocytes are going to form polar bodies. These are basically small haploid cells that result due to unequal cytokinesis during oogenesis. Basically, each meiotic division during this process is going to result in a polar body, and that polar body doesn't develop into anything. So, unlike spermatogenesis, where these meiotic divisions give rise to multiple cells that all will go on to become sperm, in oogenesis, only one cell of the meiotic divisions is going to go on to form the egg cell. As you can see, the first polar body will form as the primary oocyte completes meiosis 1 and forms the secondary oocyte. And when the secondary oocyte completes meiosis 2,

Menstrual Cycle

The menstrual cycle is a regular cycle in the female reproductive system that can be divided between the ovarian cycle and the uterine cycle. The menstrual cycle will cease after a certain point in females' lives known as menopause. Now the ovarian cycle is going to involve cyclic changes in the follicles of ovaries. Whereas, the uterine cycle is going to be the cyclic growing and shedding of the endometrial lining. Now, the ovarian cycle begins with the follicular phase. This is the part where the follicle matures and gets ready to release the ovum. FSH is going to stimulate follicle maturation and the follicle will secrete estradiol, an important estrogen. Ovulation is the release of the ovum from the follicle and it will travel into the fallopian tube. This occurs in the middle of the ovarian cycle and it's triggered when maximal estradiol concentrations cause an LH spike. And this LH spike causes the follicle to rupture and release the ovum, aka ovulation. Now the luteal phase is the final part of the ovarian cycle. And this is where the follicle transforms into the corpus luteum, that temporary endocrine structure we mentioned. The corpus luteum is going to secrete progesterone, and this is going to maintain the uterine lining, and it's going to provide negative feedback to FSH and LH. Now the corpus luteum will degrade if there is not pregnancy, if fertilization and implantation don't occur. And we'll talk about what happens if they do occur a little later.

Now the uterine cycle, as we said, is going to involve the endometrial lining. The first part of the uterine cycle is actually menstruation. This is where the endometrium is shed. So that's occurring during the follicular phase. Now, the proliferation phase is the second part of the uterine cycle. This is where the endometrium actually grows. And you can see all of this happening in this nice chart right here. Now the final part of the uterine cycle is the secretory phase, where the uterus prepares for implantation. Basically, it gets ready for a fertilized egg to make its way there.

Now, it's worth noting that some organisms actually have what's known as an estrous cycle. And during an estrous cycle, the uterus actually reabsorbs the endometrium in the absence of pregnancy, rather than, secreting or shedding the endometrial lining like we see in humans. And here you can see the various phases, up top we have our, phases of the ovarian cycle, you can see the follicular phase, the luteal phase, and right here in the middle that's ovulation. And notice how on this graph showing hormone levels, we have estradiol reaching its peak rather, right there. That triggers that LH spike that we see and that is what causes ovulation, where the ovum is released from the follicle. And here you can see an example of the shedding and the building up of the endometrial lining. Right, with that let's go ahead and flip the page.

Fertilization and Implantation

Hello, everyone. In this lesson, we are going to be talking about the process of fertilization or the creation of a zygote. Okay. So, let's get into our lesson. And the process of fertilization commonly is also called conception. This is a very common term, especially for human beings. Now, fertilization is going to be the formation of a zygote from the fusion of an egg and a sperm. And, the way this is going to happen is the sperm is going to have to enter the female body, and then it's going to travel through the cervix, which, if you guys remember, is the opening to the uterus. And then, it's going to travel into the oviduct. Now, what are oviducts specifically called in human beings? In human females, they're specifically called Fallopian Tubes. But, they are also going to be called Uterine Tubes. These are both meaning the same thing. I just want you guys to know those terms as well in case you are given those terms on a test. They mean the same thing. Fallopian tube, uterine tube, oviduct, all the same thing in a human being. Okay? And, once the sperm enters the oviduct, there should be an egg there, or an ovum, and that is going to be fertilized by that sperm. Now, upon the sperm entering the ovum, it is going to complete meiosis 2, and then they are going to fuse their nuclei together to create a diploid nucleus or the zygote's nucleus. Okay, everyone? So this is going to be the process of fertilization and it's not as easy as you may think. The sperm has to do a lot of work to actually fertilize this egg and part of it is going to be because of the jelly coat that it has to fight through and all of these reactions are going to have to take place inside of the egg as well. So, the first thing that the sperm is going to do is it's going to do sperm binding. Sperm binding is going to be the very first step of fertilization, and in this step the sperm attaches to the jelly coat. So, the jelly coat, you guys can see, is labeled here. And this is gonna be this orange structure right here, that orange layer of the egg. And this jelly coat is gonna be made of glycoproteins, and it's gonna be this very thick, viscous structure sperm for the egg, and it's going to work to, eliminate a mass number of sperm actually getting to the egg because they have to do so much work to actually get to the egg's membrane. So sperm binding is going to happen, and the sperm is going to attach to the jelly coat. In mammals, the jelly coat is also called the Zona pellucida. Okay, guys? Just so you guys know, let me write that down for you. It's also called the Zona pellucida, in mammals specifically. So, in our eggs, human eggs, it's gonna be called the Zonopelucida. But jelly coat's a little easier to say. So what's going to happen now is once the sperm actually bind the jelly coat, the acrosome or the acrosome is actually going to open up, and the acrosome is that cap on the end of the sperm, or the head of the sperm. And, it's going to open up, and it's going to release these Acrosome enzymes. So, this is going to cause the acrosome reaction to begin. The acrosome reaction is actually going to dissolve that jelly coat, and allow the sperm to actually dig through the jelly coat, and get to the plasma membrane of the sperm. So, the next thing that's going to happen is the sperm membrane fuses with the egg membrane. So now that these membranes actually fuse, which you guys can see in this particular reaction right here. Now that these membranes are actually fused, they're actually going to stimulate another to happen. And that is gonna be the Cortical Reaction. And you guys can see all of these little structures, these little yellow circles here, are gonna be cortical granules. And they're waiting for a sperm to actually bind its plasma membrane with the egg's plasma membrane. And once it does that, it's gonna begin this cortical reaction. As you guys can see, these yellow circles are actually diffusing into the membrane, and they are going to release cortical enzymes, which is actually going to change the chemistry of the membrane of the egg cell. And what this is going to do is it's going to ensure that no other sperm bind to this egg. So what it's gonna do, it's going to block Polyspermy. Polyspermy is going to be, basically, more than 1 sperm fertilizing 1 egg. And in most organisms, you don't want that because you'd have too much genetic material in that particular egg. Some organisms, there are exceptions. But in us, human beings, we only want 1 sperm and one egg to come together because then you have the perfect amount of diploid genetic material. And you guys can see in this diagram here that the genetic material of the sperm is actually entering the egg, and then those two sets of genetic material will fuse to make these zygotes genetic material. So that's a brief overview of how fertilization is going to happen. If you guys want more information about the details of the arcosome reaction or the cortical reaction, and he talks about that whenever we talk about animal development. Okay. So, let's go down and see what happens after the fertilization process has happened. What's gonna happen next? What's gonna happen next is the zygote is going to form and it's gonna start developing into the embryo, into the fetus. Okay? So, what's going to happen is this is the zygote here. The zygote is that first cell that is made by the fusion of the sperm and the egg, and it's gonna have 2 sets of chromosomes. But on the very first day, it's going to start dividing via the process called cleavage. Cleavage is a very specific type of division. It's not like most mitosis. Cleavage is where cells split without getting bigger. So they're gonna stay the same general size. And this is generally going to allow this dividing, zygote, this dividing embryo to move through the fallopian tube without being too big. So you guys can see the very second day cleavage is going to begin and make all of these particular types of cells. Now, do you guys know what this structure is called? Because it's no longer called a zygote once it starts having cleavage. This is going to be called a morula. A morula is going to be a solid ball of cells. Now, the morula is then going to undergo this process called compaction. And that's happening right here. And the cells are actually binding together via desmosomes and via tight junctions, and they're compacting together. And then, they're going to undergo the process of differentiation where they become different cell types in the developing embryo, which will then develop into different tissue types and different organ types. But, all of these steps right here are gonna be happening to the morula. So, on the first day, you're gonna have fertilization and the creation of a zygote. And then, on the 2nd, 3rd, and 4th day, sometimes the 5th day, you're going to have the production of the morula. And it is going to be developing, and compacting, and differentiating. And then, on the 5th or the 6th day, you're going to have the formation of the blastula. So this is gonna be the blastula, and it's different from the morula because it's no longer solid. It's actually going to be a hollow ball of cells. As you guys can see, it's gonna have this cavity. And this cavity is gonna be called the blastocele. And this is gonna be a fluid-filled cavity, and it can also be called the blastocelic cavity, as they call it here in this diagram. Now, this is a blastula. But, whenever you're talking about a blastula that's specifically found in mammals, this is gonna be called a blastocyst. So this is specific to mammals. So our blastulas are called blastocysts. Okay? But all organisms have blastulas. Okay? And this is a hollow ball of cells that forms from the cleavage of the zygote in the fallopian tube. And, it's gonna have 2 types of cells I want you to know about. It's gonna have these inner cell mass, or inner mass cells. And, it's gonna have these trophoblast cells. Now, the inner mass cells or inner cell mass are gonna become the fetus after they begin to divide and begin to form, basically, the baby. This is where the baby's gonna come from. And then, the trophoblast cells are actually going to become the placenta and any other supportive tissue that is needed. So, these trophoblast cells are gonna be the ones that are here in this ring of cells. And then the inner cell mass are gonna be those cells that you see in red. So, there are also different terms for this that I want you guys to know. There are different terms for the inner cell mass. They can also be called the epiblast, or the embryoblast. Hopefully, you guys can see that. The embryoblast, they're all the same thing. They're just different names for the same thing. So, Embryoblast. So these are all also the inner cell mass. They're the same thing. Now, the trophoblast generally only has that one name. Okay? Alright. So that's gonna be the basis of what's gonna happen in the 1st couple of days. So now, what's gonna happen next is the process of implantation. And I really like this diagram here because it does show you where all of the processes that we just talked about actually occur. So you guys can see here, I'm highlighting in yellow. This is the ovary. This is where the egg is going to be made. And this is going to be where this follicle right here reduces or excuse me. The follicle actually produces the egg and releases it. So, the oocyte is gonna be the egg. Now, actually, fertilization doesn't happen in the uterus. Most people think it happens in the uterus. But it doesn't. It's actually going to happen in the fallopian tube right here. We can see those sperm coming to bind with the egg. And then, they are going to form the zygote, which you guys see here. This one right here is showing that the two nuclei of the sperm and the egg are actually fusing together. And then, we're going to form the zygote. Now, we're going to have the cleavage begin around day 2. And this is going to form the morula. And as the morula is dividing, and it's differentiating, and it's compacting, it is going to be traveling through the fallopian tube to the uterus. And then, finally, in Day 5, 6, 7, 8, and 9, we are going to have the blastula actually implant into the wall of the uterus, which you guys can see right here. So, this is the implantation of the blastocyst or the blastula. And then, this particular little mass of cells will begin to grow, begin to differentiate, and form the entire fetus and the placenta as well. Okay, everyone. So that was our lesson on the processes of implantation and fertilization. Next, we're going to talk about childbirth and gestation.

Pregnancy and Birth

Pregnancy or gestation, as it's known, is when there's one or more embryos in the uterus. Now, pregnancy in humans is divided into trimesters. And during the first trimester, there's a lot of interesting stuff happening. Following implantation, this hormone, human chorionic gonadotropin hormone, or HCG as it's often referred to, will be secreted and will actually prevent the degradation of the corpus luteum and halt the menstrual cycle. Interestingly, this hormone is actually what a lot of pregnancy tests test for. And you can see an example of that right behind me there. Now, the outer layer of the blastocyst is known as the trophoblast, and this is what's going to grow into the endometrium to form the placenta. Now, the placenta is an incredibly important organ. It's going to allow for the exchange of materials, nutrients, and wastes between the mother and fetus. This is how the developing embryo is going to eat, breathe, and get rid of its waste products. So, very very important. The developing fetus is connected to the placenta by an umbilical cord, and this actually contains two arteries and a vein. Now, the fetus is going to or technically the embryo is considered a fetus once it has its adult structures in their rudimentary form. And this is going to happen through organogenesis or the development of organs. And you can see here how the fetus is curled up inside the womb, and how it has this bundle of two arteries and a vein that connects to the placenta. Now, in humans, we give birth to live offspring. This is known as, or we are known as viviparous organisms for this reason. But some organisms lay eggs, and this is known as oviparous. Whereas, some reproductive strategies are ovoviviparous, and this is when the egg will remain inside the parent until it's ready to hatch. Now with humans, we undergo labor. These are uterine contractions that will expel the fetus from the uterus. And this is stimulated by the hormone, oxytocin. And this is actually involved in a positive feedback loop, because pressure from the head of the fetus on the cervix stimulates oxytocin release. Oxytocin stimulates uterine contractions, which are going to increase the pressure on the cervix leading to more oxytocin release. So you can see how this is a positive feedback loop. Now, we, humans, are considered eutherians because we give birth to live developed offspring. There are other types of mammals though, known as monotremes, which actually lay eggs, and marsupials that give birth to underdeveloped offspring that have to remain in this pouch that actually contains the mammary gland nipple. Now, mammary glands are what define mammals and are one of the things that define mammals, and these are going to be glands that secrete milk in a process known as lactation. The hormone prolactin stimulates this milk production, and oxytocin actually stimulates the secretion of milk in response to suckling by the infant. That's all I have for this lesson. I'll see you guys next time.