Notes

- [Voiceover] We're going to talk about
00:04
a pair of really important structures
00:06
in the male reproductive system called the testes.
00:09
They sit inside the scrotum,
00:11
and have two really primary functions.
00:14
First, they produce the male's contribution to a baby,
00:18
which is his sperm.
00:20
Second, they make the majority
00:22
of the major male hormone, testosterone,
00:25
but we'll really only discuss
00:27
the sperm production role for now.
00:29
So let's look inside the testes and see what we find.
00:34
So inside we find this really convoluted
00:37
set of tubes in light blue here.
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These are called seminiferous tubules.
00:42
The sperm are actually made inside these tubules,
00:45
and the testosterone is made by cells called leydig cells
00:49
that hang out on the outside of these tubules.
00:52
Anyway, the sperm are made in the seminiferous tubules,
00:55
and they travel out of the tubules
00:57
and into the epididymis to mature
01:00
and get ready to head off, via ejaculation,
01:02
to try and find an egg to fertilize.
01:05
So to appreciate the process of sperm production
01:08
and how it all happens,
01:09
we need to take a look inside the seminiferous tubules.
01:12
So let's take a look inside.
01:14
This is a cross section of the tubule,
01:17
sort of magnified so we can see the components of it better.
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So this light blue layer along the top here
01:23
is a muscle like layer that helps to propel sperm
01:26
through the tubules and into the epididymis,
01:30
so it does this via, sort of, coordinated muscular
01:33
contractions that move in a wave like fashion down the tubes
01:37
The coordinated movement pattern is called peristalsis,
01:41
so if you think about squeezing a tube of toothpaste
01:44
from the bottom to the top to get
01:46
a little bit of toothpaste out,
01:47
peristalsis is pretty similar to that.
01:51
So, anyway, after leaving the seminiferous tubules
01:54
the sperm, sort of, drain out into this network
01:57
of tubes here called the rate testis,
02:00
then after the rate testis they drain to the epididymis
02:04
where they hang out to mature and be stored for a while.
02:07
So that's just a little bit on peristalsis
02:09
and the movement of the sperm through the tubes,
02:11
but back to the cross section here.
02:13
These radially arranged cells in a bit darker blue,
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they're called sertoli cells,
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and just so you're aware,
02:20
sertoli cells are packed into these tubules
02:23
in a way more crowded fashion.
02:26
This is just an easy, sort of, schematic way
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of looking at them and seeing how they do what they do,
02:31
which you'll soon see.
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So the general idea is that sperm develop
02:36
between two sertoli cells,
02:38
and they sort of develop as the shuffle down
02:40
between the two cells toward the lumen here.
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By the way, a lumen is a hole down
02:46
the center of a hollowed tube,
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so, for example, the lumen of a garden hose
02:50
is the part where the water travels through.
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So let's get to the details of how this all happens.
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We'll zoom in here on, say, this part here,
03:00
but we really could pick anywhere
03:02
along these tubes because it's all the same process,
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and let's say this here is a sertoli cell,
03:07
and there's a sertoli cell on the other side,
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but I'll just put S to designate sertoli cell,
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and that light blue bit up top is that smooth muscle layer
03:15
that does peristalsis,
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so this purple cell here, what is that?
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That's called a spermatiogonium,
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and you have these spermatogonium
03:23
between each set of neighboring sertoli cells.
03:27
They're sort of the precursor to the mature form of sperm.
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They're the actual germ cell where all our sperm comes from,
03:35
so they go through different developmental stages
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in a process called differentiation
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until they form what we know as sperm.
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So, immediately, you might think,
03:46
"Well, what if these spermatogonium are differentiating
03:48
"down the pathway to become mature sperm,
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"what happens when they all do that?
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"Won't we run out of spermatogonium?"
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And that's a great thought,
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so how that problem is solved is that when spermatogonium
04:00
under go mitosis and split into two spermatogonium,
04:03
one will differentiate into the next precursor sperm cell
04:07
down the pathway of making mature sperm,
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and the other one will just keep being a spermatogonium,
04:13
so it'll give rise to another two cells,
04:15
and one will differentiate,
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and one will keep being a spermatogonium, and so on.
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So let's officially start here.
04:22
Our spermatogonium will divide via a mitosis,
04:25
and one of the daughter cells will differentiate
04:28
into a primary spermatocyte.
04:30
We'll just draw that one.
04:32
Remember, the other is going to revert back
04:34
to being a germ cell, a spermatogonium,
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so this primary spermatocyte here
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has to cross over this linkage
04:41
between the two sertoli cells,
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that's called a tight junction,
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and the tight junction effectively creates two compartments.
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One up here, and that's called the basal compartment.
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Basal because it's closest to the base
04:54
or the basal region of the sertoli cells,
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and one compartment down here called the lumenal compartment
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because it includes that lumen we mentioned earlier.
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So because they're really tightly separated
05:06
by the tight junction here,
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these two different compartments
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have really different chemical environments.
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They have different signaling molecules
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and proteins floating around in them,
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and that helps each compartment
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to bring on a different stage of development
05:21
for our developing sperm.
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Anyway, back to the tight junction.
05:25
It sort of senses the primary spermatocyte
05:28
coming close and it opens up,
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and the primary spermatocyte moves through
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and starts to enlarge by increasing it's cytoplasm
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because it's actually getting ready to divide
05:38
and differentiate into two secondary spermatocytes,
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and then that tight junction actually
05:45
reforms super quickly behind it,
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like before the primary spermatocyte is even fully through,
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and the idea behind that quick reformation
05:53
of the tight junction is so that you don't get much leakage
05:56
from one compartment into the other,
05:58
so that their environments can stay
06:00
pretty different to each other.
06:02
So back to our primary spermatocyte.
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It's passed through the tight junction now,
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and it hasn't really changed except
06:07
enlarging a little bit by gaining more cytoplasm,
06:10
so now it divides and differentiates
06:13
into two secondary spermatocytes,
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but there's actually a pretty big difference
06:17
between the division that the spermatogonium did
06:19
to produce the primary spermatocyte
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and the new spermatogonium,
06:23
that division was by mitosis,
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and this division where the primary spermatocyte
06:28
divides to create two secondary spermatocytes.
06:32
This is called meiosis.
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So they sound similar, mitosis, meiosis,
06:37
but in mitosis you enlarge and split into
06:40
two identical daughter cells that are
06:43
genetically identical to the original cell,
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but in meiosis you give each of your daughter cells
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half of your chromosomes.
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So each primary spermatocyte has 23 pairs of chromosomes,
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and each chromosome is a pair of sister chromotids,
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and you probably notice that these chromosomes
07:05
have all undergone crossing over.
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They're a mixture of pink and blue
07:08
from homologous chromosomes from mom and dad,
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so just a reminder, that yes primary spermatocytes
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were created from spermatogonium by mitosis,
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but at a certain point, the primary spermatoctyes
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decide to undergo meiosis.
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So prophase one starts in these primary spermatocytes
07:27
and crossing over happens in these primary spermatocytes,
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and then metaphase one, and anaphase one, and telophase one,
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and cytokinesis happen to split our primary
07:38
into two secondary spermatocytes.
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So when the primary spermatocytes differentiate
07:44
into secondary spermatocytes,
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they give each of their daughter cells
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a half of their chromosomes,
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so now each secondary has 23 chromosomes,
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still with a sister chromatid each.
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So now what happens?
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Well, we have our secondary spermatocytes.
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Each having 23 chromosomes
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in sister chromotid configuration,
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and now they need to differentiate.
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So they do, they differentaite
08:10
into spermatids, which are are starting
08:12
to look something like sperm,
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and two spermatids per secondary spermatocyte are created.
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So there would be four here,
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but I've only drawn in the spermatids
08:24
from one of the secondaries.
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I've only drawn two in,
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and notice that these spermatids,
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they're a little bit more embedded into the sertoli cells.
08:32
They get a lot of nutrients that way.
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Importantly, though, when they differentiate
08:37
from secondary spermatocytes to spermatids,
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the second half of meiosis happens,
08:43
what's called meiosis two.
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So meiosis one was completed earlier
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when we went from primary spermatocytes
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to secondary spermatocytes,
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and by undergoing the second step of meiosis here,
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we further reduce the chromosome copy number by half.
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So instead of 23 chromosomes each with a sister chromotid,
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these newly made spermatids each have
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23 single copies of each chromosome.
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And we need sperm to have only one copy of each chromosome
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because after a sperm fertilizes a female's egg,
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the eggs end up with also only copy of each chromosome,
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so when their nuclei fuse,
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they create a set of twenty three pairs of chromosomes.
09:25
One set from the father's sperm
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and one set from the mother's egg,
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and that's what we want.
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So now for the last step that happens
09:32
in between the sertoli cells.
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The spermatids differentiate into spermatozoa.
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One spermatozoa per spermatid
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in a process called sperspermiogenesis,
09:44
and each spermatozoa has a single copy of each chromosome.
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So notice that one primary spermatocyte
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ends up giving rise to four sperm.
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Remember, what you see here should actually be doubled.
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So you should see two more spermatozoa
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because I've only shown the products
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of one of the secondary spermatocytes.
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So down here at the newly minted sperm stage,
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we're not exactly done yet.
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The immature sperm still has to travel to the epididymitis
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to mature into sperm that are fully capable
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of carrying out fertilization,
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so in the epididymitis they gain more mitochondria,
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and they gain longer flagella,
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and at that point they're ready to start their journey
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in hopes of fertilizing an egg.