Reproductive System, Part 2 - Male Reproductive System: Crash Course Anatomy & Physiology #41

Summary

This video explores the biological intricacies of human reproduction, contrasting the high-stakes investment of female gametes with the abundant production of male sperm. It delves into the anatomy and function of the male reproductive system, highlighting the complexities of sperm production, maturation, and ejaculation. Key elements include the hormonal regulation of spermatogenesis, the role of various reproductive glands, and the physiological processes ensuring sperm viability and mobility. The video underscores the evolutionary strategies of both sexes in their reproductive roles, providing a comprehensive understanding of the male reproductive anatomy and function.

Takeaways

• 🎰 The concept of sex as a gamble is introduced, highlighting the mental and emotional risks involved in relationships and the biological investments in reproduction.
• 🥚 The female reproductive system's investment in a single ovum is described as a 'big bet', requiring significant development time and resources, with the potential for monthly shedding if fertilization doesn't occur.
• 🧬 The male reproductive strategy is contrasted as a 'penny slot' approach, producing sperm that are numerous, small, and relatively easy to create, with the aim to overcome the odds through sheer numbers.
• 🌡 The testes' sensitivity to temperature is explained, with the necessity for them to be outside the body cavity to maintain an optimal temperature for sperm production.
• 🏭 The structure of the testes is detailed, with lobules and seminiferous tubules functioning as 'sperm factories', supported by various cell types including Sertoli and Leydig cells.
• 🔬 The process of spermatogenesis is likened to skin cell production, starting from stem cells and progressing through stages of development to become mature sperm.
• 🔄 The continuous division of spermatogonia is described, with the onset of puberty triggering a change in division that leads to the production of spermatocytes and eventually sperm.
• 🚀 The transformation of spermatids into mobile sperm through spermiogenesis is outlined, detailing the development of a tail and the maturation process that takes about five weeks.
• 🌊 The journey of sperm from the testes through the epididymis is described, where they gain mobility over a period of about 20 days, facilitated by the duct's structure and secretions.
• 💧 The role of various glands in ejaculation is explained, with the seminal vesicles, prostate gland, and bulbo-urethral glands each contributing to the composition and function of semen.
• 🐍 The anatomy and function of the penis are discussed, emphasizing its role as a delivery system for sperm, with erectile tissues facilitating penetration during sexual arousal.

Q & A

What is the main theme of the video script?

The main theme of the video script is the biological and physiological differences between the male and female reproductive systems, focusing on the processes of gamete production and the journey of sperm from production to ejaculation.

Why is the female gamete considered a 'big bet' in terms of biological investment?

The female gamete, or ovum, is considered a 'big bet' because it takes over a year to develop, is much larger than a sperm, and requires a significant amount of supporting tissue in the uterus. If fertilization does not occur, the resources invested in that ovum must be shed and rebuilt each month.

How does the male reproductive system's strategy differ from the female's in terms of gamete production?

The male reproductive system's strategy is to produce a large number of sperm, which are much smaller and less resource-intensive than eggs. This approach is likened to 'penny slots' in a gamble, as it relies on the sheer number of sperm to increase the chances of fertilization.

Why are the testes located outside the body cavity?

The testes are located outside the body cavity in the scrotum because sperm production requires a lower temperature than the core body temperature of 37 degrees Celsius. This external location allows for the necessary temperature regulation for proper spermatogenesis.

What role do Sertoli cells play in the male reproductive system?

Sertoli cells nourish developing sperm cells within the seminiferous tubules, similar to how female follicle cells support oocytes. They also play a role in the maturation process of sperm.

How does the process of spermatogenesis compare to skin cell production?

Spermatogenesis is similar to skin cell production in that both processes begin with stem cells that divide and mature. In the case of sperm, this happens in the seminiferous tubules, with cells moving from the outer edge toward the lumen.

What triggers the start of sperm production in males?

The start of sperm production in males is triggered by a hormonal cascade that begins with the hypothalamus releasing gonadotropin-releasing hormone (GnRH), which in turn signals the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

How long does the process of spermiogenesis take?

The process of spermiogenesis, which is the development of spermatids into mature sperm, takes about five weeks.

What is the purpose of the epididymis in the male reproductive system?

The epididymis serves as a storage and maturation site for sperm. It is a long, tangled set of tubes where sperm gain their mobility over the course of about 20 days before they are ready for ejaculation.

What are the roles of the seminal vesicles, prostate gland, and bulbo-urethral glands in the production of semen?

The seminal vesicles secrete a fluid rich in enzymes and nutrients to support sperm, the prostate gland contributes a fluid that helps keep semen liquified, and the bulbo-urethral glands secrete mucus to clear the urethra of acidic urine before ejaculation.

What is the primary function of the penis in the context of reproduction?

The primary function of the penis in reproduction is to deliver sperm to the female reproductive system. It contains erectile tissue that fills with blood during arousal, leading to an erection that facilitates penetration during sexual intercourse.

Outlines

00:00

🎲 The Gamble of Reproduction

The risks of reproduction are examined, focusing on the mental and emotional investments involved in relationships, such as waiting for communication and concerns about self-worth. The physical investments are contrasted between sexes, with the female reproductive system making significant commitments for egg development and the male system producing sperm in large quantities with minimal investment. The difference in reproductive strategies is highlighted, emphasizing the high-stakes nature of female gametes versus the low-stakes, high-volume approach of male gametes.

05:00

🔬 Inside the Testes: Sperm Production

This section delves into the anatomical and physiological details of sperm production. It explains the necessity for testes to be outside the body due to temperature sensitivity. The structure of the testes, including lobules and seminiferous tubules, is described, along with the roles of Sertoli and Leydig cells. The process of spermatogenesis is compared to skin cell production, outlining how spermatogonia divide and mature into sperm through meiosis, ultimately producing numerous sperm cells continuously.

10:04

🚀 From Spermatid to Mobile Sperm

The journey of a spermatid transforming into a mobile sperm is detailed. The process of spermiogenesis, where spermatids grow tails and become mobile sperm, takes about five weeks. Sperm travel through the epididymis, gaining mobility and energy. The path sperm take during ejaculation involves the vas deferens, ejaculatory ducts, and urethra. The role of glandular secretions in facilitating sperm movement and maturation is explained, highlighting the contributions of seminal vesicles, the prostate gland, and bulbo-urethral glands.

🧬 The Penis and Reproductive Delivery System

The penis's structure and function as a reproductive delivery system are explored. It consists of erectile tissues that fill with blood during arousal, aiding in vaginal penetration. Despite its prominent role in reproduction, the penis is considered less critical compared to internal reproductive organs. The episode concludes with a teaser for the next week's topic on fertilization, and a note of gratitude to contributors and supporters of the Crash Course project.

🎥 Production Credits

This segment lists the production team and contributors behind the episode. It acknowledges the filming location at the Doctor Cheryl C. Kinney Crash Course Studio, writers, editors, consultants, directors, sound designers, and graphic team members. The support from Patreon patrons is also recognized, emphasizing their role in enabling the creation of educational content for a broad audience.

Keywords

💡Gamete

Gametes are reproductive cells that unite during fertilization to form a new organism. In the context of the video, female gametes (eggs) are described as a significant biological investment, while male gametes (sperm) are produced in large quantities with minimal investment. This highlights the different reproductive strategies of the sexes.

💡Spermatogenesis

Spermatogenesis is the process of sperm production in males. It occurs in the seminiferous tubules of the testes and involves the transformation of spermatogonia into mature sperm cells. The video describes this process as relatively simple and efficient compared to the production of female gametes, emphasizing the large number of sperm produced.

💡Ovulation

Ovulation is the release of an egg from the ovary. It marks a crucial point in the female reproductive cycle and is a significant biological investment, as the female body dedicates substantial resources to prepare for potential fertilization. The video contrasts this with the continuous and abundant production of sperm in males.

💡Testes

The testes are the male reproductive organs responsible for producing sperm and the hormone testosterone. They are located outside the body in the scrotum to maintain a lower temperature necessary for spermatogenesis. The video explains their structure and function, highlighting their role in the male reproductive system.

💡Follicle-stimulating hormone (FSH)

FSH is a hormone involved in the regulation of the reproductive processes. In males, it stimulates the Sertoli cells in the testes to support sperm production. The video mentions its role in both male and female reproductive systems, noting its importance in initiating spermatogenesis and ovulation.

💡Luteinizing hormone (LH)

LH is another hormone crucial for reproductive function. In males, it stimulates the Leydig cells in the testes to produce testosterone. The video discusses its role in triggering testosterone production, which is essential for the development of male secondary sexual characteristics and the continuation of spermatogenesis.

💡Sertoli cells

Sertoli cells are found within the seminiferous tubules of the testes and provide support and nourishment to developing sperm cells. The video describes their function in nurturing spermatocytes and their role in the overall process of spermatogenesis, drawing a parallel with the role of follicle cells in female reproduction.

💡Leydig cells

Leydig cells are located in the testes and are responsible for producing testosterone. The video explains how these cells, stimulated by LH, secrete testosterone, which is crucial for the development of male characteristics and the regulation of sperm production.

💡Semen

Semen is the fluid that contains sperm and is ejaculated from the male reproductive system. It includes secretions from various glands, such as the seminal vesicles and prostate gland, which provide nutrients and support for the sperm. The video details the composition of semen and its role in transporting and protecting sperm during fertilization.

💡Erection

An erection is the process by which the penis becomes engorged with blood, becoming firm and erect. This physiological response is necessary for sexual intercourse, allowing for the delivery of sperm to the female reproductive tract. The video discusses the anatomical and physiological aspects of erection, including the role of erectile tissue.

Highlights

Sex involves significant mental risks and emotional investments.

Female gametes, or ova, are biologically high-stakes bets, taking over a year to develop.

Human sperm are much smaller and easier to produce than eggs, likened to penny slots.

Testes are located outside the body to maintain the lower temperature necessary for sperm production.

The sperm-making process, spermatogenesis, is supported by various cells like sertoli and Leydig cells.

Spermatogenesis involves a series of cell divisions starting from spermatogonia to mature sperm.

The hormonal cascade triggered by puberty starts the production of sperm.

Mature males can produce about 1500 sperm per second.

Sperm gain motility in the epididymis, a long set of tubes behind the testes.

Various glands, including the seminal vesicles, prostate gland, and bulbo-urethral glands, contribute to semen.

The penis is a delivery system designed to get male gametes close to female gametes.

An erection helps the penis penetrate the vagina during sexual arousal.

Semen provides sperm with transportation, nutritional energy, and chemical protection.

The testes have a complex internal structure with about 250 sections called lobules.

Each lobule contains tightly-coiled seminiferous tubules, the sperm factories.

Transcripts

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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,

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.

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.

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

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

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.

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.

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

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

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

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.

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.