Hey y'all. Browns back. This time we're doing some more histology, and we're
getting into reproductive histology. So today we are gonna be investigating, as
you might guess from the cover of this video, the testes. The testes are the
primary reproductive organ in men, and when we say primary reproductive organ
what we mean is the organ where gametes or sex cells are made. These are the
cells that unite to form a new person, and in men these form in the organ
called the testes. Testes being plural the singular being testis.
T-E-S-T-I-S will be the singular form. Like so. And the testes contain these little
compartments full of tightly coiled tubules called the seminiferous tubules,
which is what we're seeing over here on the left-hand side of the screen in
cross-section. So what I want to do now is kind of zoom in on...on a section of
testes, and look at kind of the general histology of the testes. As I mentioned
previously, germ cell production, that is in the case of men sperm cell production,
takes place inside these tightly coiled tubules called the seminiferous tubules.
And as you know, if you cut a tube in cross-section you get a circle. So as I
go in here I can draw a circle around a single seminiferous tubule. Very nice.
You can see if you look closely that the seminiferous tubules
is surrounded by a thin layer of regular connective tissue, and since this is
regular connective tissue you're gonna see numerous fibroblasts. There's a
couple out here in the edge about one o'clock or so if we're looking at this as a
clock face, but I want to draw your attention to the more numerous cells
with their very highly compressed nuclei. So here we can see this very, very,
flattened nucleus. There's one down here at the bottom as well. These cells over
here are called myoid cells or myoepithelial cells, and these myoid
cells are weakly contractile out they're gonna act kind of like smooth muscle, and
they're gonna gently compress the seminiferous tubule
to move the fluid and the cells that are contained in the fluid out here in the
lumen, along into the genital ducts. Inside this connective tissue capsule we
find the layer of tissue that is going to become the sperm cells, and this is
called the germinal epithelium. Germinal, because we're making germ cells, and
epithelium, because duh. We're gonna look at the germinal epithelium in a little
bit more detail here very shortly...in the...actually in the next image we're
gonna look more carefully at how we make those germ cells, but before we do that I
want to talk about one other part of what we're looking at here, and that is
the interstitial, the area in between the seminiferous tubules. If you look in the
bottom left-hand corner of your screen you'll see a little pack of cells,
it's nice broad round spherical nuclei cells, these are called inter...oh goodness.
That was cool. I don't know how I did that, but that was neat. These are called
interstitial cells. Very good.
When I was actually a young man, and was learning this stuff for the first time,
they were called the cells of Leydig, but now we simply call them interstitial
cells, and these interstitial cells function to secrete testosterone. These
are the cells that are responsible...test...I misspelled testosterone. Let's fix this. (audiable sound effects). That's what erasers are for. Testosterone.
Very good. These are the cells that are responsible for the reason that men
don't look like boys. It's the testosterone these interstitial
cells are stimulated by the hormone called interstitial cell-stimulating
hormone or ICSH, which is the exact same molecule as luteinizing hormone in women.
It's gonna stimulate these cells, they're gonna produce testosterone. These cells
are initially active during the fetal stage of life, and it is in fact these
cells in the testosterone that they produce that are responsible for the
developing fetus taking on kind of the male body plan.
The default body plan for humans is female. The male genital development will
only occur if testosterone is present. And so, if testosterone is not present
even if somebody has a Y chromosome, or if somebody doesn't have the
testosterone receptor, then they won't develop male genitalia. So, it's really
fascinating if you ever want to learn more about androgen insensitivity
syndrome, those are people who are XY but don't see testosterone, so they developed
as developmentally as females, it's really neat stuff. But in vast majority
of folks that have a Y chromosome these cells are active up through about the
fourth month of development, and then a shut off and they stay off until the
little boy goes into puberty, sometime around nine ten years old. So this look
more closely at the germinal epithelia, and some of the cells there that are
actually going to go into making germ cells or sperm cells. So we've zoomed in
a little bit here we can see a little better the cells of the germinal
epithelium. We can see are myoid cell out here.
This is very nice. You can see a weekly standing fibroblast nucleus there, but
we're interested in the cells of the germinal epithelium. In the first cell I
want to draw your attention to, are these guys right out here. Here's an example of
one. This is called a spermatogonium, and the spermatogonia are kind of like
little spermy stem cells. A man of healthy reproductive age is producing
sperm to the tune of hundreds of millions a day, and that's a lot of cells
to be making. So if we're gonna develop cells into sperm cells we need to
replace the ones that got...that got turned into sperm. So these spermatogonia
are constantly undergoing mitosis in order to replenish the cells that become
sperm cells. Now there are actually two different kinds of spermatogonia. You
can't tell the difference between them histologically.
So we're not gonna learn...to learn how to tell them apart or anything cos you can't.
But there there's one called the Type-A spermatogonia that's a little bit less
differentiated, it's much more stem celly, and it's just doing mitosis and then
some of those can become type-B spermatogonia, which are cells that have
now terminally differentiated, and are on the track to becoming sperm cells. That's
all we're gonna get into about that we don't need to do any more. As we move
further in towards the lumen of the seminiferous tubules we see some much larger cells like
this guy right here, like this guy right here. These cells are called
spermatocytes, and these are the cells that are in the process of meiosis.
♫ I can't spell today ♫
♫ You're welcome for the entertainment ♫
Meiosis. Very good, and there's actually two different kinds of spermatocytes
we have primary spermatocytes, which are in the process of meiosis 1, and we have
secondary spermatocytes, which are in the process of meiosis 2.
It's really hard to tell them apart, histologically, especially kind of at
this introductory level, but if you guess primary spermatocyte nine times out of
ten you're gonna be right, because meiosis one freezes in prophase one for
about twenty-one about 21 days or 22 days. I think. Yeah, we'll say 22 days. It's
about three weeks. So there's a good chance that any spermatocyte you see is
probably a primary spermatocyte, because they stay frozen in prophase one for
three weeks, and then very rapidly move through the rest of meiosis one and all
of meiosis two until they become one of these little guys and that's a spermatid.
So it's spermatid is all done with meiosis. It's a haploid cell, so it's
gonna have 23 chromosomes, each with just one chromatid, it's all done with mitosis...
..I mean meiosis, pardon me, but it still has some morphological development to
undergo. A spermatid is not capable of fertilizing an egg. It has to be a
spermatozoon. Before we get to the full development of the spermatid in the
spermatozoon; however, we need to look at one more cell that's present in the
germinal epithelium. These cells are called the spermatogenic line, and most
of the cells you look at when you look at a seminiferous tubule in section are
cells of the spermatogenic line, but there's still one other cell that's very
important and necessary for the production of the...of the germinal line
here, and that is the sustentacular or sertoli cells. On the right we have the
image we were just looking at, and what I'm gonna do is draw kind of an outline
around a single sustentacular cell. I'll do my best to.
Very good. Come around this way, and like so. And like so. So as best as I can tell everything
inside this black line is a single sustentacular cell. The nucleus is here.
You can tell this is a sustentacular or sertoli, that's kind of the old name for
it, so, because it's got a lot of euchromatin in there and a pretty
prominent nucleolus. Whereas these cells that are in the process of meiosis have
a lot more condensed chromatin in them. Sertoli cells are broadly pyramidal.
Generally tend to be broader towards the outside edge of the seminiferous tubule
and more narrow as you get towards the lumen, and their primary function as
the named sustentacular would suggest is to support, to sustain, the
developing spermatocytes and spermatids. As those sperma...spermatocytes and
spermatids are developing, especially as the spermatids are developing from early
spermatids too late spermatids, which I'll get into more in just a second,
they're gonna be shedding cytoplasm, they're gonna be getting rid of anything
that's not needed to fertilize an egg cell. So as those developing
spermatocytes, and especially the spermatids are shedding cytoplasm, it is
the sustentacular or sertoli cells that will endocytosis that shed
cytoplasm. These guys are also secreting testicular fluid so they are secreting
the fluid that fills the lumen and provides a transport medium for the
fully mature spermatozoa. As you can see in the image on the left they're also
carrying out a very important function if you look at that number 8 and follow
the arrow down you'll notice that these two sertoli cells number 7 and number 7
here are attached to one another via a tight Junction. Right there. There's a
tight Junction. Which means that everything on the other side of this
dashed line, everything above the dashed line that I just drew, is separated from
external environment here. So this sertoli cell is connected to...the nucleus
is down there...but we'll just say there's another one right over here, and these
guys are gonna be connected to one another by these tight junctions, and
that's gonna form what's known as the blood testis barrier. This is probably
something you've never thought about before, but let's look at it just a
little more closely. This cell down here with the number 2, so 1 is the basement
membrane number 2 is the cell, this is a spermatogonia. These are stem cells. These
are genetically identical to every other cell in this man's body, which means that
his immune system sees these as self. However, once the type B spermat...
..spermatogonia enter into prophase 1 of meiosis and become spermatocytes,
primary spermatocytes, they're gonna engage in genetic recombination. Crossing
over is going to be occurring during that 3 weeks it's frozen in prophase 1,
which means that every cell on the other side of this dash line is now non-self
and subject to immune attack. For this reason the sustentacular cells and
these...specifically these tight junctions are going to shield the developing
spermatocytes and spermatic...spermatids and eventually the spermatozoa, which are
genetically different from the man who created them, shield them from immune
attack. So that's pretty cool. The last thing we need to talk about before we
end our video, we're already 15 minutes in, is the development of early
spermatids, so this little guy right here is an early spermatid, into one of these
little guys right here which is a late spermatid, and then from one of these
little guys right here the late spermatid into a fully mature sperm cell.
This process is called spermiogenesis. Meiosis going from spermatogonium to
spermatocytes to spermatid is called
spermatogenesis, but going from early spermatid to late spermatid to
spermatozoon is a purely morphological change and is called spermiogenesis.
Some of the major features of spermatogenesis that I want to...want you
to pay attention to are one, the nucleus is going to compress...bad handwriting...
..two we're gonna lose lots of cytoplasm. So cytoplasm be gone. It's gonna grow a tail.
See it right there. So it's gonna grow this long tail, and it's also gonna
develop a little pouch at the front called an acrosome. So all of these
things occur during the process of spermiogenesis. If we look at this lovely
cartoon of a mature sperm cell by Wikimedia Commons user lady of hats. This
is a really nice work that she did this totally for free she put this thing into
the public domain, which is really fantastic. We're gonna see that the sperm
probably can be constructed of a head and a tail. The head is gonna contain the
nucleus. I often use the analogy the spermatozoon is kind of like a little
guided missile, it's a little guided missile of DNA that is seeking out its
target the egg cell. The nucleus is the payload; that's the thing it has to
deliver inside the cytoplasm of the egg cell so that we can get (inaudible) and
eventually a zygote. But, it's not easy. That egg cell is well protected by a
thick outer covering called the zona pellucida, that the sperm cell is gonna
have to burrow through in order to fertilize the egg. Fortunately the
acrosome, which we saw down here, contains enzymes that will help break down that
zona pellucida allowing the sperm cell to penetrate the egg cell. We also notice
that the mid-piece here, that kind of connects the head to the rest of the
tail, contains numerous mitochondria. These are the smallest cells that a
human can make. They are tiny, and they have to swim a really long way.
The mitochondria are gonna break down sugars turn it into ATP to power the
flagellum that makes up most of the tail. This flagellum will beat much like a
propeller, moving the sperm cell through the female reproductive tract and
allowing it to eventually find an egg cell, fertilize it, and make a new person.
That's it for now. Sorry the video went a little bit long. This is a really
fascinating topic, but I hope you enjoyed it. Have fun. Study hard. We'll see you later.