The Reproductive System: How Gonads Go – CrashCourse Biology #34


The number one question on the
mind of every organism on earth, if that organism
happens to have a mind, is: how do I make more of myself? It’s bigger than all
the other questions combined, including how am I
going to feed myself? And what’s the meaning of life? Because, from a
biological perspective, we know what the meaning of life is. Biology has answered that question.
It’s reproduction. Different organisms go about
reproducing in different ways: You can make more of
yourself by yourself, a strategy called
asexual reproduction, OR you can team up with somebody
else and make a baby that’s genetically different than both
of you through sexual reproduction. From liver flukes to pine trees,
99% of the eukaryotic organisms on earth use sex to reproduce,
at least some of the time, because by creating offspring
with a slightly different genome, helps the new generation
stay one step ahead of pathogens or competitors. Or if you’re the pathogen,
it helps you stay ahead of that pesky host that’s
always trying to kick you out. But still, sex is inconvenient
and it’s a lot of work: First you have to find somebody
to mate with, which means you have to get out of bed
and brush your teeth and stuff. Then, if you’re an animal, you have
to find somebody who’s willing to mate with you, and then
figure out whether he or she is going to provide higher or
lower quality genes than yours. Thankfully, and unsurprisingly,
animals’ reproductive systems have evolved to streamline all of
those inconveniences to address one, and only one, aim: to get your
sex cells where they need to be. So, sex. How does it work? I thought you’d never ask. Reproductive systems, like all the
other systems we’ve discussed, take on an incredible diversity
within the kingdom Animalia. For instance, some female spiders
mate with a bunch of different males and stash their sperm
in different storage units. When she’s ready to fertilize
her eggs, the female spider will choose which male
spider she liked the best and let his sperm out the
storage unit to fertilize her eggs! Hyenas, meanwhile, have
female-dominated social system, and it’s the alpha female who
chooses who she mates with. And she has sex using an enlarged
sensitive sex organ, a clitoris, that looks exactly like a
penis, called a pseudopenis. And a duck’s penis can be a
quarter of the length of its body, and shaped like a corkscrew. Want to know why? Look it up!
Actually don’t! Google that with care!
Just don’t press play on the video. The point here is that while the
delivery systems may be somewhat different from animal to animal,
the fundamentals are the same. In order to do the sex, an organism
needs to find another of its species that has a
different type of gamete, or sex cell, than their own. Gametes, you’ll recall, are haploid
cells, meaning they have only one set of chromosomes, and they’re
formed by the process of meiosis. And there are only two kinds of
gametes: One is the ovum, or egg. In plants it’s called an ovule. The egg is always a large cell that
takes a lot of time and energy investment to make, and
it’s usually not very mobile. The other type of gamete, sperm,
are smaller, a lot more plentiful, easy to make, and always
more mobile than eggs. Most animals have either one
or the other type of gamete, though hermaphroditic species,
like garden snails and some flowering plants,
can produce both. In the magical moment that
one of these sperm finds one of those eggs, the two
fuse together to create a single diploid cell that
has all of the instructions to make a new seahorse or
secretary bird or whatever it is. But let me get your mind right
about what we really mean when we talk about sex. Because we humans have external
sex organs, called genitals, we tend to think of them as key indicators
of who’s male and who’s female. But the fact is, genitals
are only one byproduct of a much, much more important
and fundamental distinction: From a biological perspective,
the only thing that makes sexes different is that females produce
big, not very mobile gametes, and the males make smaller,
much more mobile gametes. Across the spectrum of all things
that reproduce sexually, that’s pretty much the only consistent
difference between boys and girls. Therefore, all reproductive
systems and reproductive behavior are designed entirely around
the production, storage, and delivery of these gametes. For instance, because sperm are
really mobile, males within a species are generally the more mobile ones
who go out to find a mate. This is even true for plants:
female gametes of a flowering plant generally stay in one place while
the pollen, which ends up producing the sperm, gets picked up by a
pollinator, or sometimes just sprays out every which way in the wind, hoping to bump into the
right kind of ovule. In animals, we see all
kinds of crazy behaviors where mating is concerned. And of course not every animal
goes about courtship in the same way, but one thing is pretty consistent:
Females tend to be pickier about the quality of their mates,
because while a male animal could conceivably fertilize thousands
of eggs every year, a female has only a
limited number of eggs, and she’s spent a lot
of energy developing them, so she wants them to be fertilized
with high quality genes. Plus, in cases where both parents
stay together after fertilization, she also wants those genes to be
attached to a high-quality provider. This often results in males
having to do a lot of showing off in order to get a lady’s attention. Males of a species are generally
louder, larger, brighter, more combative than the females. Basically, they’re
putting on a big show so the females can size up how
awesome that guy’s genes are. But for all those differences,
during the development of the embryo, there are actually very
few physical differences between males and females,
at least at first. You and I, we didn’t start out
being a male or a female. While you were hanging
out in our mom’s uteri, you didn’t have a sex at
all until about two months. Before that, we had all the pieces
to become either male or female, but our genes hadn’t gotten together
to determine whether or not our gonads, the glands
that make the gametes, were going to become
ovaries or testes. In mammals, that decision is made
by the sex-determining chromosome: If an offspring has two of the
same kind of sex-determining chromosome, called XX,
it will be female. And if it has two different
chromosomes, XY, it will be male. The same is true for some other
animals like fruit flies, and even some plants
like Gingko trees. However, the opposite
is true for birds: boy birds have XX and
girl birds have XY. Go figure. In mammals, the default setting
for sex is always female. Absent a signal from
the Y chromosome, ovaries form and begin working
on developing female structures. If there is a Y, the ovaries
instead form into testes, and parts that would be female
turn into male structures, for instance, the clitoris I
mentioned, which is sensitive and has spongy tissue in it,
actually becomes part of a penis. But it’s worth pointing
out that by this time, some features are already in place
before the sex is determined. Nipples, for instance,
form before this point, so that’s why men have them,
even though they don’t do anything. Now, once the sex is determined,
the ovaries and testes pump out estrogen and testosterone. Meanwhile, the brain is
growing and creating receptors, organized differently in males
and females, that will later determine how both estrogen and
testosterone are used in the body. Soon after a baby girl is born,
she’ll have half-formed versions of all the eggs she’s ever
going to have for her whole life, then at puberty, once a month, one of those eggs will finish
forming and be released. But for baby boys, the
sperm-making does not begin until around puberty. Most of the time when a young
animal starts getting close to sexual maturity, secondary
sex characteristics crop up: In humans more body hair appears,
boys all of a sudden develop facial hair, while both
sexes get more pubic hair. Also muscle and fat get
redistributed around the body, the most obvious
example being breasts. In other animals, secondary sex
characteristics include things like manes on male lions,
a big old funky rack of feathers on male peacocks,
antlers on male deer. Males really have
the market cornered on fancy, showy
secondary sex characteristics. So, by the time an animal
has reached sexual maturity, the males and females of a species
often look pretty dissimilar. Not just of each other,
but of their previous non-sexually-developed forms. Basically showing the world that
their different reproductive structures that they were born with
are now in full gear, and they’ve got some really different jobs to
do, based on what sex they are. So let’s go over how this
all works with human people. And of course, ladies first. As you know, the gonads of a female
embryo turn into two ovaries, one on either side of the
uterus, with its oviducts, or fallopian tubes,
reaching out toward them. The ovaries are where those
precious eggs are kept. Maybe the biggest difference
between women’s and men’s reproductive set-up is that
women have a menstrual cycle, typically a four-week process in
which one egg matures in an ovary and is released to be
drawn into the fallopian tubes, a process called ovulation. If, while the egg makes
its way down the fallopian tube to the uterus, a sperm finds it and
fertilizes it, there’s a chance that the fertilized egg
will implant on the endometrium, a tissue layer inside the wall
of the uterus and a baby will grow. However, it’s estimated that
up to 70% of fertilized eggs don’t take hold in the endometrium. This could be because women’s
bodies have sort of a built-in genetic testing: If something’s
suspected to be wrong with the growing embryo, the lining of her
uterus that she’s built up over the past month will shed, and the
woman will menstruate as usual. This material leaves the female
reproductive system through the narrow lower end of the
uterus, the cervix, and then out into the muscle-lined
tract of the vagina, and those are of course the same
structures through which a newborn baby passes and
through which the sperm enter. While a woman’s body is busy all
month developing the next egg, getting it ready for fertilization,
and shedding her uterine lining if it’s not fertilized, males are
undergoing a completely different process that calls on a lot of other highly specialized
reproductive structures. We start of course, with the testes,
which are made up largely of a bunch of coiled tubes
called seminiferous tubules, which are where the sperm form. Unlike a woman’s ovaries,
the testes are outside of the body, because in order to make sperm,
they have to be kept at a specific temperature,
usually about 2 degrees celsius cooler than inside
of the body cavity. For that reason, the testes are
kept in a pouch called a scrotum that’s in charge of keeping
the testes at the perfect sperm-making temperature. After being produced in the testes,
human sperm spend about 3 weeks coiled in tubes in the
scrotum called the epididymis, and that’s where they
mature and grow flagella, the little whip-like tails
sperm are so famous for and which make them able
to move around and swim. Now the sperm stay here until
they’re ready to leave the body, so before we, or they,
can go any further, we have to set
the stage for that. As you know, in humans
and some other animals, the penis usually sits around,
not doing much except for letting urine out of
the bladder from time to time. But every so often a male
realizes that he’s totally going to get the chance to mate! At this point,
spongy tissue in the penis fills with blood, and BAM, erection! Some animals like raccoons,
whales and walruses actually have a literal bone in their
penis to help the erection along. But either way, the point is to
allow the penis to enter the vagina, which scientists call coitus, and deposit the sperm
he’s put so much into making. These sperm travel in
a special fluid, semen, whose ingredients aren’t combined
until they’re ready to be released by a series of muscular
contractions that cause emission, more commonly known as ejaculation. At this point, the contractions
carry the mature sperm from the epididymis,
through two muscular ducts called the vas deferens,
which carry them up from the testes, up and over the bladder,
and down past the seminal vesicles. Here, with contributions from
the nearby prostate gland, they pick up a bunch of
fluid that contains mucus, coagulating enzyme, ascorbic acid
and sugars that the sperm are going to need for their trip. Now the semen is complete,
and it travels down the short ejaculatory ducts to
the urethra to be released at the end of the penis,
where if the timing is right, one among the hundreds of millions
of sperm in that emission can find and fertilize an egg. That, my friends,
is how we all get our start. To find out, or to remind yourself,
what happens after fertilization, you can always check out this
video on embryonic development. But, fittingly enough, this
wonderful beginning marks the end of our treatment of
the Animal Kingdom. Please join us next
week when we go deeper into the other kingdoms
that we share this planet with the bacteria, archaea and protists. Thank you as always for
watching Crash Course Biology. If there’s any sex stuff
that you want to go over again, there’s a table of contents. Thanks to everyone who
helped make this video, especially to Amber for
illustrating all of those gonads. If you have any questions,
ideas, or comments for us, Facebook, Twitter,
or the comments below. We’ll see you next time.

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