精子发生

we're gonna 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 laid egg cells that hang out on the outside of these tubules anyway the sperm are made in the seminarist tubules and then they travel out of the tubules and into the epididymis to mature and get ready to head off via ejaculation to try to 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 seminary's tubules so let's take a look inside this is a cross section of the tube you'll sort of magnified so we could see the components a bit better so this light blue layer along the top here is a mussel like layer that 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 seventy-first tubules the sperm sort of drained out into this network of tubes here called the r88 Estus then after the r88 Estus 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 Parker blue they're called sertoli cells and just so you're aware in reality 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 and 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 they shuffle down between the two cells toward the lumen here by the way a lumen is a hole down the center of a hollow 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 then there's a certain cell on the other side but I'll just put s to designate certainly cell and that light blue bit up top is that smooth muscle layer that that does peristalsis so this purple cell here what is that that's called a spermatogonia 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 in a process called differentiation until they form what we know is 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 when we run out of spermatogonia and that's a great thought so how that problem is solved is that when spermatogonium undergo 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 spermatogonia so it'll give rise to another two cells and one will differentiate and one will keep being a spermatogonia and so on so let's officially start here our spermatogonium will divide via mitosis and one of the daughter cells will differentiate into a primary site and we'll just draw that one remember the other is going to revert back to being a germ cell a spermatogonia 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 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 luminal compartment because it includes that lumen we mentioned earlier so because they're really tightly separated by the 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 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 and it opens up and the primary spermatocyte moves through and starts to enlarge by increasing its cytoplasm because it's actually getting ready to divide and differentiate into 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 news format ago Neum 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 Jeanette eclis 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 chromatids 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 spermatogonia by mitosis but at a certain point the primary spermatocytes decide to undergo meiosis so prophase 1 starts in these primary spermatocytes and crossing over happens in these primary spermatocytes and then metaphase 1 and anaphase 1 and telophase 1 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 chromatid configuration and now they need to differentiate so they do they differentiate into spermatids which are starting to look something like sperm and two spermatids per secondary spermatocytes are created so there would be four here but I've only drawn in the sperm at 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 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 so instead of 23 chromosomes each with a sister chromatid 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 one copy of each chromosome so when their nuclei fuse they create a set of 23 pairs of chromosomes one set from the father 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 sperm neo Genesis 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 were not exactly done yet the immature sperms still have to travel to the epididymis to mature into sperm that are fully capable of carrying out fertilization so in the epididymis they gain more mitochondria and they gain longer flagella and and at that point they're ready to start their journey in the hopes of fertilizing an egg