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Biological Reproduction Gamble · Acedra
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Biological Reproduction Gamble
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0:00
Sex is a big gamble. I mean, the mental risks alone that we take with our relationships are taxing enough: When is he going to text me back? Am I good enough? What if she doesn’t like the real me? But the emotional price we pay for intimacy can almost seem paltry compared to the physical investments that our bodies make in order to reproduce. And the sexes each make this wager differently. The female gamete is, biologically speaking, a big bet. It’s like putting all your chips on black.
0:24
An ovum and follicle take over a year to develop, and, as human cells go, it’s huge -- about a tenth of a millimeter, just large enough to be seen with the naked eye. Plus, it requires a whole network of supporting tissues in the uterus while it waits for fertilization, which -- if that doesn’t happen -- then have to be shed and re-built a month later. And then there's the commitment. Once an egg has been ovulated, everything in the female
0:44
reproductive system is dedicated to dealing with that one cell for almost a month. And if it gets fertilized, you’re looking at 10 months. But when it comes to playing the reproductive roulette wheel, the male reproductive system takes the opposite strategy. If female gametes amount to a high-stakes bet, the males’ are basically the penny slots. Human sperm are only about one-hundred-thousandth the mass of an egg. And they’re almost as
1:06
easy to produce as skin cells -- actually, the process by which they mature is pretty similar. That’s because sperm are small and stripped down -- not invested in much beyond a nucleus, and a tail, and some extra mitochondria to fuel their journey. All they have to do is drop off their DNA, and fall apart. The risk of failure is still there. In fact, with male gametes, the odds are overwhelmingly against any single sperm actually accomplishing its mission.
1:29
But while the female reproductive system meets this risk head-on, with a single, big play, the male’s tries to beat the odds with the sheer force of numbers.
1:47
Here’s a question for you: why are the most important parts of the male’s reproductive system literally left hanging? The testes -- the gonads of the anatomical male -- are tasked with making male gametes, sperm, and the androgen hormone testosterone. But unlike the ovaries, which are protected inside the body, testicles dangle outside the abdominal cavity, in the scrotum. Because -- even though sperm may not be terribly big or complex -- they’re touchy when it comes to temperature.
2:13
The rate at which they divide, copy DNA, and even feel the effects of some hormones, are all diminished at the core body temperature of 37 degrees. So the testes have to be outside the body cavity to reach the lower temperature necessary for proper spermatogenesis, or sperm production. It’s a rare instance of sensitivity among what are otherwise pretty hardy and expendable cells. But that’s not to say that the testes aren’t complex.
2:36
Each one is divided into about 250 sections, called lobules, which are loaded with tightly-coiled seminiferous tubules. These tubules are the sperm factories, made of a stratified epithelium surrounding a central fluid-filled lumen. And this sperm-making work is supported by cells throughout and around the tubules. Among the most important are the sertoli cells inside the tubes, which nourish developing sperm cells, sort of like how female follicle cells help oocytes.
3:01
There are also Leydig cells that secrete testosterone, much like the corpus luteum that secretes estrogen. But compared to egg-making, spermatogenesis is -- as I mentioned -- a pretty cheap and easy process, biologically. It’s actually kind of like how your skin cells are produced -- with stem cells at the basal layer that produce immature cells, which in turn get forced toward the surface as they mature.
3:20
But in this case, the sperm develop on the outer edge of the tubule, and progress inward toward the lumen. And it all begins with a trigger that -- if you’ve seen our lesson on female reproduction -- should look pretty familiar: a hormonal cascade. When puberty comes a-knocking, the hypothalamus starts releasing gonadotropin-releasing hormone, or GnRH. And this tells the anterior pituitary to secrete follicle-stimulating hormone and luteinizing hormone into the blood, just like in females.
3:44
But in females, LH leads to the release of estrogen in the follicles, whereas in males, it spurs the Leydig cells to release testosterone. Meanwhile, the FSH triggers the sertoli cells to release androgen-binding protein, or ABP. This binds to the testosterone, creating large, local concentrations of the stuff, which is ultimately what triggers the production of sperm. The targets of the testosterone are the outermost cells in the tubules, called spermatogonia.
4:07
They’re the stem cells that set the sperm-making process in motion, by dividing. And actually, they’ve been doing that all along -- even before puberty. Throughout childhood, a spermatogonia cell uses mitosis to continuously divide into two identical daughter cells. But when puberty starts, the testosterone causes them to divide differently -- instead of splitting into two identical spermatagonia cells, they begin producing two distinct spermatogonia.
4:31
One type of daughter cell -- known as a type A cell -- stays up near the basal lamina and just keeps dividing, so you never run out of spermatogonia. But the other kind -- the type B’s -- get pushed down the tubule toward the lumen, and turn into primary spermatocytes. These primary spermatocytes then go into meiosis I, and form two smaller haploid cells called secondary spermatocytes. They then rush through meiosis II, and their resulting daughter cells total four round spermatids.
4:55
These spermatids now have all of the 23 chromosomes they need for fertilization. But they aren’t exactly mobile. If they’re ever gonna find themselves an egg, they’ll need a way to get around -- they need a tail. The process by which a spermatid elongates, grows a tail, or flagellum, and officially becomes a mobile sperm is called spermiogenesis. And the whole process takes about five weeks. But it’s not like it holds anything up, because there are plenty more where those spermatids came from.
5:18
In the end, each primary spermatocyte gives rise to four actual sperm. And considering how many spermatogonia are continuously dividing into spermatocytes, it’s easy to see how a mature male can crank out 1500 sperm a second. That’s a far cry from all the investment that goes into making a single ovum. Now, obviously, if sperm are going to get anything accomplished, they have to leave the seminiferous tubules that made them.
5:41
So, even once they have tails, they still need a little help getting going. That’s why each tubule is surrounded by several layers of myoid cells, which, like smooth muscle, rhythmically contract, using peristalsis to squeeze sperm -- and some fluid secreted by the sertoli cells -- through all the twists and turns, toward their next destination, the rete testis in the posterior testis. From there, the sperm -- although still immobile -- leave the testes, by way of the epididymis,
6:06
a long, tangled set of tubes behind the testes where they’ll spend the next few weeks gaining their mobility. The bulk of the epididymis consists of the enormous duct of the epididymis, which, believe it or not, could be uncoiled to stretch about six meters -- though I don’t suggest doing that. This duct is full of tiny microvilli called stereocilia, which provide a huge surface area to help reabsorb some extra fluid, and help pass along nutrients to feed the idling sperm.
6:33
It takes sperm nearly 20 days to work their way through this labyrinth, during which time they continue to mature. Once through the duct they enter the inferior epididymis, where they gain mitochondria so they’ll have energy to swim hard at a moment’s notice. At this point they have the hardware to swim, but still lack the ability to actually move on their own, a skill kept under wraps until they leave the epididymis and get activated by a series of glandular secretions.
6:56
When that time comes, during ejaculation, the sperm flow from the epididymis through 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 right ejaculatory ducts pass into the prostate gland where they empty into the urethra, which runs from the bladder, through the penis, and into the outside world. This system of tubes feeding into tubes allows all of the necessary glands to make their
7:21
contributions to a moving wave of seminal fluid that helps sperm mature and perform their ultimate function -- fertilization. The resulting mix of sperm, testicular fluid, and gland secretions -- which we call semen -- provide sperm with transportation, nutritional energy, chemical protection, and finally activates their motility. So let’s take a look at these glands. The seminal vesicles are a pair of small, hollow glands behind the bladder that secrete
7:44
a yellowish, slightly alkaline fluid that contains coagulating enzymes, fructose, and other things to help nourish and transport sperm. Interestingly, the vesicles also secrete special prostaglandins that help increase sperm success outside of the body, by decreasing the viscosity of a female’s cervical mucous, and actually triggering a reverse peristalsis of the uterus that helps draw sperm up the female reproductive tract.
8:08
Now, the prostate gland encircles the urethra just inferior to the bladder. During ejaculation it contracts to squeeze its own special secretion into the urethra. This stuff contains mainly citric acid and an enzyme cocktail that help keeps semen liquified -- and thus easier to move and swim through. And finally we have the pea-sized bulbo-urethral glands below the prostate, that secrete a clear mucus that drains into the urethra to clear out any acidic urine prior to ejeculation.
8:33
Speaking of, by now you may be wondering if we’re ever going to get to what’s probably the first thing you think of when it comes to 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 called the glans penis, surrounded by a fleshy cuff of foreskin. Internally, the penis contains three layers of erectile tissues, each wrapped in a layer
8:54
of dense fibrous connective tissue. This spongy network is made of connective tissue and smooth muscle that’s full of tiny vascular spaces that fill with blood during sexual arousal, and make the penis expand and become rigid. The resulting erection is what helps the penis penetrate the vagina. And, the fact that we’re only now getting around to talking about it here at the end should tell you that -- as with the female external genitalia -- the penis actually isn’t
9:18
all that important in terms of overall reproductive function. Evolutionarily speaking it’s just a delivery system, designed to get male gametes as close to female gametes as possible, and reap some payoff for their investments. Now! What that payoff looks like in term of fertilization is where we’re headed next week. But for now, you learned all you’d ever care to about testicular anatomy, the steps of sperm production, and how it’s influenced by gonadotropin and testosterone. We also
9:44
looked at how sperm mature, and how they leave the body on a tide of glandular secretions during ejaculation. Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon patrons whose monthly contributions help make Crash Course possible, not only for themselves, but for everyone, everywhere. If you like Crash Course and want to help us keep making videos like this one, please check out patreon.com/crashcourse.
10:06
This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.
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