Spermatogenesis

- [Voiceover] We're going to talk about a pair of really important structures in the male reproductive system called the testes. They sit inside the scrotum, and have two really primary functions. First, they produce the male's contribution to a baby, which is his sperm. Second, they make the majority of the major male hormone, testosterone, but we'll really only discuss the sperm production role for now. So let's look inside the testes and see what we find. So inside we find this really convoluted set of tubes in light blue here. These are called seminiferous tubules. The sperm are actually made inside these tubules, and the testosterone is made by cells called leydig cells that hang out on the outside of these tubules. Anyway, the sperm are made in the seminiferous tubules, and they travel out of the tubules and into the epididymis to mature and get ready to head off, via ejaculation, to try and find an egg to fertilize. So to appreciate the process of sperm production and how it all happens, we need to take a look inside the seminiferous tubules. So let's take a look inside. This is a cross section of the tubule, sort of magnified so we can see the components of it better. So this light blue layer along the top here is a muscle like layer that helps to propel sperm through the tubules and into the epididymis, so it does this via, sort of, coordinated muscular contractions that move in a wave like fashion down the tubes The coordinated movement pattern is called peristalsis, so if you think about squeezing a tube of toothpaste from the bottom to the top to get a little bit of toothpaste out, peristalsis is pretty similar to that. So, anyway, after leaving the seminiferous tubules the sperm, sort of, drain out into this network of tubes here called the rate testis, then after the rate testis they drain to the epididymis where they hang out to mature and be stored for a while. So that's just a little bit on peristalsis and the movement of the sperm through the tubes, but back to the cross section here. These radially arranged cells in a bit darker blue, they're called sertoli cells, and just so you're aware, sertoli cells are packed into these tubules in a way more crowded fashion. This is just an easy, sort of, schematic way of looking at them and seeing how they do what they do, which you'll soon see. So the general idea is that sperm develop between two sertoli cells, and they sort of develop as the shuffle down between the two cells toward the lumen here. By the way, a lumen is a hole down the center of a hollowed tube, so, for example, the lumen of a garden hose is the part where the water travels through. So let's get to the details of how this all happens. We'll zoom in here on, say, this part here, but we really could pick anywhere along these tubes because it's all the same process, and let's say this here is a sertoli cell, and there's a sertoli cell on the other side, but I'll just put S to designate sertoli cell, and that light blue bit up top is that smooth muscle layer that does peristalsis, so this purple cell here, what is that? That's called a spermatiogonium, and you have these spermatogonium between each set of neighboring sertoli cells. They're sort of the precursor to the mature form of sperm. They're the actual germ cell where all our sperm comes from, so they go through different developmental stages in a process called differentiation until they form what we know as sperm. So, immediately, you might think, "Well, what if these spermatogonium are differentiating "down the pathway to become mature sperm, "what happens when they all do that? "Won't we run out of spermatogonium?" And that's a great thought, so how that problem is solved is that when spermatogonium under go mitosis and split into two spermatogonium, one will differentiate into the next precursor sperm cell down the pathway of making mature sperm, and the other one will just keep being a spermatogonium, so it'll give rise to another two cells, and one will differentiate, and one will keep being a spermatogonium, and so on. So let's officially start here. Our spermatogonium will divide via a mitosis, and one of the daughter cells will differentiate into a primary spermatocyte. We'll just draw that one. Remember, the other is going to revert back to being a germ cell, a spermatogonium, so this primary spermatocyte here has to cross over this linkage between the two sertoli cells, that's called a tight junction, and the tight junction effectively creates two compartments. One up here, and that's called the basal compartment. Basal because it's closest to the base or the basal region of the sertoli cells, and one compartment down here called the lumenal compartment because it includes that lumen we mentioned earlier. So because they're really tightly separated by the tight junction here, these two different compartments have really different chemical environments. They have different signaling molecules and proteins floating around in them, and that helps each compartment to bring on a different stage of development for our developing sperm. Anyway, back to the tight junction. It sort of senses the primary spermatocyte coming close and it opens up, and the primary spermatocyte moves through and starts to enlarge by increasing it's cytoplasm because it's actually getting ready to divide and differentiate into two secondary spermatocytes, and then that tight junction actually reforms super quickly behind it, like before the primary spermatocyte is even fully through, and the idea behind that quick reformation of the tight junction is so that you don't get much leakage from one compartment into the other, so that their environments can stay pretty different to each other. So back to our primary spermatocyte. It's passed through the tight junction now, and it hasn't really changed except enlarging a little bit by gaining more cytoplasm, so now it divides and differentiates into two secondary spermatocytes, but there's actually a pretty big difference between the division that the spermatogonium did to produce the primary spermatocyte and the new spermatogonium, that division was by mitosis, and this division where the primary spermatocyte divides to create two secondary spermatocytes. This is called meiosis. So they sound similar, mitosis, meiosis, but in mitosis you enlarge and split into two identical daughter cells that are genetically identical to the original cell, but in meiosis you give each of your daughter cells half of your chromosomes. So each primary spermatocyte has 23 pairs of chromosomes, and each chromosome is a pair of sister chromotids, and you probably notice that these chromosomes have all undergone crossing over. They're a mixture of pink and blue from homologous chromosomes from mom and dad, so just a reminder, that yes primary spermatocytes were created from spermatogonium by mitosis, but at a certain point, the primary spermatoctyes decide to undergo meiosis. So prophase one starts in these primary spermatocytes and crossing over happens in these primary spermatocytes, and then metaphase one, and anaphase one, and telophase one, and cytokinesis happen to split our primary into two secondary spermatocytes. So when the primary spermatocytes differentiate into secondary spermatocytes, they give each of their daughter cells a half of their chromosomes, so now each secondary has 23 chromosomes, still with a sister chromatid each. So now what happens? Well, we have our secondary spermatocytes. Each having 23 chromosomes in sister chromotid configuration, and now they need to differentiate. So they do, they differentaite into spermatids, which are are starting to look something like sperm, and two spermatids per secondary spermatocyte are created. So there would be four here, but I've only drawn in the spermatids from one of the secondaries. I've only drawn two in, and notice that these spermatids, they're a little bit more embedded into the sertoli cells. They get a lot of nutrients that way. Importantly, though, when they differentiate from secondary spermatocytes to spermatids, the second half of meiosis happens, what's called meiosis two. So meiosis one was completed earlier when we went from primary spermatocytes to secondary spermatocytes, and by undergoing the second step of meiosis here, we further reduce the chromosome copy number by half. So instead of 23 chromosomes each with a sister chromotid, these newly made spermatids each have 23 single copies of each chromosome. And we need sperm to have only one copy of each chromosome because after a sperm fertilizes a female's egg, the eggs end up with also only copy of each chromosome, so when their nuclei fuse, they create a set of twenty three pairs of chromosomes. One set from the father's sperm and one set from the mother's egg, and that's what we want. So now for the last step that happens in between the sertoli cells. The spermatids differentiate into spermatozoa. One spermatozoa per spermatid in a process called sperspermiogenesis, and each spermatozoa has a single copy of each chromosome. So notice that one primary spermatocyte ends up giving rise to four sperm. Remember, what you see here should actually be doubled. So you should see two more spermatozoa because I've only shown the products of one of the secondary spermatocytes. So down here at the newly minted sperm stage, we're not exactly done yet. The immature sperm still has to travel to the epididymitis to mature into sperm that are fully capable of carrying out fertilization, so in the epididymitis they gain more mitochondria, and they gain longer flagella, and at that point they're ready to start their journey in hopes of fertilizing an egg.