Intercourse is a giant gamble. I imply, the psychological dangers alone that we take with {our relationships} are taxing sufficient: When is he going to textual content me again? Am I adequate? What if she doesn’t like the true me? However the emotional value we pay for intimacy can virtually appear paltry in comparison with the bodily investments that our our bodies make with a view to reproduce. And the sexes every make this wager in another way. The feminine gamete is, biologically talking, a giant wager. It’s like placing all of your chips on black. An ovum and follicle take over a yr to develop, and, as human cells go, it’s enormous — about a tenth of a millimeter, simply giant sufficient to be seen with the bare eye. Plus, it requires an entire community of supporting tissues within the uterus whereas it waits for fertilization, which — if that doesn’t occur — then must be shed and re-built a month later. After which there’s the dedication. As soon as an egg has been ovulated, all the pieces within the feminine reproductive system is devoted to coping with that one cell for nearly a month. And if it will get fertilized, you’re 10 months. However in the case of enjoying the reproductive roulette wheel, the male reproductive system takes the alternative technique. If feminine gametes quantity to a high-stakes wager, the males’ are principally the penny slots. Human sperm are solely about one-hundred-thousandth the mass of an egg. They usually’re virtually as simple to supply as pores and skin cells — really, the method by which they mature is fairly related. That’s as a result of sperm are small and stripped down — not invested in a lot past a nucleus, and a tail, and a few additional mitochondria to gasoline their journey. All they must do is drop off their DNA, and crumble. The chance of failure remains to be there. In reality, with male gametes, the percentages are overwhelmingly in opposition to any single sperm really carrying out its mission. However whereas the feminine reproductive system meets this danger head-on, with a single, huge play, the male’s tries to beat the percentages with the sheer drive of numbers. Right here’s a query for you: why are an important elements of the male’s reproductive system actually left hanging? The testes — the gonads of the anatomical male — are tasked with making male gametes, sperm, and the androgen hormone testosterone. However not like the ovaries, that are protected contained in the physique, testicles dangle exterior the belly cavity, within the scrotum. As a result of — regardless that sperm will not be terribly huge or advanced — they’re sensitive in the case of temperature. The speed at which they divide, copy DNA, and even really feel the results of some hormones, are all diminished on the core physique temperature of 37 levels. So the testes must be exterior the physique cavity to succeed in the decrease temperature essential for correct spermatogenesis, or sperm manufacturing. It’s a uncommon occasion of sensitivity amongst what are in any other case fairly hardy and expendable cells. However that’s to not say that the testes aren’t advanced. Every one is split into about 250 sections, referred to as lobules, that are loaded with tightly-coiled seminiferous tubules. These tubules are the sperm factories, made from a stratified epithelium surrounding a central fluid-filled lumen. And this sperm-making work is supported by cells all through and across the tubules. Among the many most essential are the sertoli cells contained in the tubes, which nourish growing sperm cells, type of like how feminine follicle cells assist oocytes. There are additionally Leydig cells that secrete testosterone, very like the corpus luteum that secretes estrogen. However in comparison with egg-making, spermatogenesis is — as I discussed — a fairly low cost and straightforward course of, biologically. It’s really type of like how your pores and skin cells are produced — with stem cells on the basal layer that produce immature cells, which in flip get compelled towards the floor as they mature. However on this case, the sperm develop on the outer fringe of the tubule, and progress inward towards the lumen. And all of it begins with a set off that — for those who’ve seen our lesson on feminine replica — ought to look fairly acquainted: a hormonal cascade. When puberty comes a-knocking, the hypothalamus begins releasing gonadotropin-releasing hormone, or GnRH. And this tells the anterior pituitary to secrete follicle-stimulating hormone and luteinizing hormone into the blood, identical to in females. However in females, LH results in the discharge of estrogen within the follicles, whereas in males, it spurs the Leydig cells to launch testosterone. In the meantime, the FSH triggers the sertoli cells to launch androgen-binding protein, or ABP. This binds to the testosterone, creating giant, native concentrations of the stuff, which is in the end what triggers the manufacturing of sperm. The targets of the testosterone are the outermost cells within the tubules, referred to as spermatogonia. They’re the stem cells that set the sperm-making course of in movement, by dividing. And truly, they’ve been doing that each one alongside — even earlier than puberty. All through childhood, a spermatogonia cell makes use of mitosis to repeatedly divide into two an identical daughter cells. However when puberty begins, the testosterone causes them to divide in another way — as a substitute of splitting into two an identical spermatagonia cells, they start producing two distinct spermatogonia. One sort of daughter cell — often called a kind A cell — stays up close to the basal lamina and simply retains dividing, so that you by no means run out of spermatogonia. However the different form — the kind B’s — get pushed down the tubule towards the lumen, and flip into major spermatocytes. These major spermatocytes then go into meiosis I, and kind two smaller haploid cells referred to as secondary spermatocytes. They then rush via meiosis II, and their ensuing daughter cells whole 4 spherical spermatids. These spermatids now have the entire 23 chromosomes they want for fertilization. However they aren’t precisely cell. In the event that they’re ever gonna discover themselves an egg, they’ll want a technique to get round — they want a tail. The method by which a spermatid elongates, grows a tail, or flagellum, and formally turns into a cell sperm is named spermiogenesis. And the entire course of takes about 5 weeks. But it surely’s not prefer it holds something up, as a result of there are lots extra the place these spermatids got here from. In the long run, every major spermatocyte provides rise to 4 precise sperm. And contemplating what number of spermatogonia are repeatedly dividing into spermatocytes, it’s simple to see how a mature male can crank out 1500 sperm a second. That’s a far cry from all of the funding that goes into making a single ovum. Now, clearly, if sperm are going to get something achieved, they’ve to depart the seminiferous tubules that made them. So, even as soon as they’ve tails, they nonetheless want somewhat assist getting going. That’s why every tubule is surrounded by a number of layers of myoid cells, which, like clean muscle, rhythmically contract, utilizing peristalsis to squeeze sperm — and a few fluid secreted by the sertoli cells — via all of the twists and turns, towards their subsequent vacation spot, the rete testis within the posterior testis. From there, the sperm — though nonetheless motionless — go away the testes, by means of the epididymis, an extended, tangled set of tubes behind the testes the place they’ll spend the following few weeks gaining their mobility. The majority of the epididymis consists of the large duct of the epididymis, which, imagine it or not, may very well be uncoiled to stretch about six meters — although I don’t counsel doing that. This duct is stuffed with tiny microvilli referred to as stereocilia, which give an enormous floor space to assist reabsorb some additional fluid, and assist cross alongside vitamins to feed the idling sperm. It takes sperm almost 20 days to work their method via this labyrinth, throughout which era they proceed to mature. As soon as via the duct they enter the inferior epididymis, the place they achieve mitochondria so they’ll have power to swim laborious at a second’s discover. At this level they’ve the {hardware} to swim, however nonetheless lack the flexibility to really transfer on their very own, a ability stored below wraps till they go away the epididymis and get activated by a sequence of glandular secretions. When that point comes, throughout ejaculation, the sperm circulation from the epididymis via the vas deferens, a tube that travels up behind the bladder and joins with the duct from the seminal gland to create the ejaculatory duct. The left and proper ejaculatory ducts cross into the prostate gland the place they empty into the urethra, which runs from the bladder, via the penis, and into the surface world. This technique of tubes feeding into tubes permits the entire essential glands to make their contributions to a shifting wave of seminal fluid that helps sperm mature and carry out their final perform — fertilization. The ensuing mixture of sperm, testicular fluid, and gland secretions — which we name semen — present sperm with transportation, dietary power, chemical safety, and at last prompts their motility. So let’s check out these glands. The seminal vesicles are a pair of small, hole glands behind the bladder that secrete a yellowish, barely alkaline fluid that incorporates coagulating enzymes, fructose, and different issues to assist nourish and transport sperm. Apparently, the vesicles additionally secrete particular prostaglandins that assist improve sperm success exterior of the physique, by reducing the viscosity of a feminine’s cervical mucous, and really triggering a reverse peristalsis of the uterus that helps draw sperm up the feminine reproductive tract. Now, the prostate gland encircles the urethra simply inferior to the bladder. Throughout ejaculation it contracts to squeeze its personal particular secretion into the urethra. These things incorporates primarily citric acid and an enzyme cocktail that assist retains semen liquified — and thus simpler to transfer and swim via. And eventually we have now the pea-sized bulbo-urethral glands beneath the prostate, that secrete a clear mucus that drains into the urethra to filter out any acidic urine previous to ejeculation. Talking of, by now you might be questioning if we’re ever going to get to what’s in all probability the very first thing you consider in the case of the male anatomy. The penis hangs from the perineum, between the pubic bone and coccyx. It consists of a shaft that ends in an enlarged tip referred to as the glans penis, surrounded by a fleshy cuff of foreskin. Internally, the penis incorporates three layers of erectile tissues, every wrapped in a layer of dense fibrous connective tissue. This spongy community is made from connective tissue and clean muscle that’s filled with tiny vascular areas that fill with blood throughout sexual arousal, and make the penis broaden and turn out to be inflexible. The ensuing erection is what helps the penis penetrate the vagina. And, the truth that we’re solely now getting round to speaking about it right here on the finish ought to inform you that — as with the feminine exterior genitalia — the penis really isn’t all that essential by way of general reproductive perform. Evolutionarily talking it’s only a supply system, designed to get male gametes as shut to feminine gametes as potential, and reap some payoff for his or her investments. Now! What that payoff seems to be like in time period of fertilization is the place we’re headed subsequent week. However for now, you discovered all you’d ever care to about testicular anatomy, the steps of sperm manufacturing, and the way it’s influenced by gonadotropin and testosterone. We additionally checked out how sperm mature, and the way they go away the physique on a tide of glandular secretions throughout ejaculation. Thanks to our Headmaster of Studying, Linnea Boyev, and thanks to all of our Patreon patrons whose month-to-month contributions assist make Crash Course potential, not just for themselves, however for everybody, all over the place. For those who like Crash Course and need to assist us preserve making movies like this one, please take a look at patreon.com/crashcourse. This episode was filmed within the Physician Cheryl C. Kinney Crash Course Studio, it was written by Kathleen Yale, edited by Blake de Pastino, and our guide is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics staff is Thought Cafe.