Biology: Spermatogenesis: Meiosis in Males

very vague

04/15/2012

~ Daina

I am a little disappointed in the detail of the video. I was hoping for more than just a general overview.

Good lesson

12/16/2009

~ jfilip

This really clarified what makes spermatogensis different than other forms of cellular reproduction. Lets hope it helps on my final.

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Cell Reproduction
Understanding Meiosis
Spermatogenesis: Meiosis in Males

I would be completely remiss if I did not take you guys into the human body, or any organism for that matter and talk
to you about the way they actually do meiosis.
See mitosis is fairly easy to visualize, you can visualize the lower layers of your skin cells replicating, to eventually
form the upper layer of your skin. You can picture organ repair. You can even picture cancer, but how does one
picture meiosis and forming the products of meiosis?
And let’s talk about spermatogenesis. Spermatogenesis, obviously, has to do with the genesis or the generation of
sperm. It happens in that organism, which we have just decided to call the male. In fact, the definition of a male is the
creature with the active gamete, generally speaking. So, let’s take a look, first of all, of at just an overview of meiosis
and then we will take a look at how this pertains to the male organ, particularly the human male organ.
Remember meiosis, what do we have? We are going to start out with a cell and that cell is going to have the diploid
number of chromosomes and that cell is going to go through its doubling, just like it does in mitosis. And then in the
doubling of the chromosomes, we are going to split our homologs. And in splitting our homologs, we have performed
our first division. So, we are out of meiosis one, okay?
Now, we’ve separated our homologs that is the good news, the bad news, if bad is what you want to call it, is that the
homologous chromosomes are still doubled. So, we have to do one more division. And so, literally, this is a
sequential process. Well, sequential is nice, but how do you do sequential in an organ, you know, and so therefore,
we have to consider the fact that there must be layers. And, this must be one layer, this must the next layer, either
working inward or outward, this must the next one, and this must be the next one.
Well, that is exactly the way the male testes works. Let’s take a look at a diagram of the testes, okay. “Umm, umm”,
you are not seeing cells here, but you’re seeing some important ultra structure to this thing. And, what we’re seeing
is, we’re seeing what are called seminiferous tubules. And, it is within the seminiferous tubules that I want to
concentrate, right now. These sperm forming tubules, if you will, are where meiosis is going to occur. So, we are
going to zoom in microscopically and take a look at one of these seminiferous tubules.
And, so now we are looking inside one of the tubules, and you see what I referred to a second ago, layers of cells,
okay. And, there seems to be some space between these cells, and there seem to be cells of different morphologies,
of different shapes. And, eventually it looks like you got, right in the center of this tubule, in that portion called the
lumen, remember the inside of a tubule is called a lumen of the tubule. And the lumen, that looks like you have these
things, which resemble sperm.
Well, let’s zoom still closer and take a look at the real inside, the microscopic view, of a seminiferous tubule. And,
what’s kind of cool here, is we can actually now see the layering that occurs. All righty. Let’s take a look. What I am
going to do is I am going to line this up along side of this and we will take a look, and I will overlap them just a wee bit
and we can see what is going on here.
Okay, literally, these are lining up cell for cell. Now, here is the point, as we move through, we are eventually going to
produce sperm. And so, looking at the bottom, again where I was down in the lumen, of that tube, we are going to
see that the eventual product are these things called sperm or spermatozoa.
Now, we have some names for all of these things, and as it works through, you can see that it’s a primary, secondary,
final product story. As we get, and I am going to work backwards, just to give you an idea, spermatozoa, zoa meaning
animal like, okay, are the final product, and we shorten that to sperm. The product just before the sperm, or the
spermatozoa, is this, literally, haploid cell, and that haploid cell is called the spermatid. And, the spermatid
corresponds right across here. And then, we are going to get to what is called the secondary – we have secondary
and we have the primary. Now, the primary spermatocyte and the secondary spermatocyte, and I am going to
abbreviate that, lest I run out of room, primary spermatocyte and secondary spermatocyte. And, once again, we can
see what we have. We have the original cell, okay, and then from the original cell we go to the primary spermatocyte,
which will be located in this layer right across from it, and then we move to the secondary spermatocyte, which will be
just above the spermatids, and then we move to the spermatids and the spermatozoa.

So, you are looking at this cell, and you are saying, “Well what’s that”? Well, this is not one of the spermatocyte; this
is actually called a sertoli cell. And, a sertoli cell is a nutrient cell, to provide nutrients for these developing cells.
Well, all of that being said, what happens next, and what are some of the limitations and what exactly does a sperm
look like? Well you know the most important thing you have to realize about sperm is that they are built for their
function. And, what’s their function? There function is to penetrate an egg. So, if we take a look at just a diagram of
a sperm, and you know in a development unit of this course we go into a little more detail on this, but the bottom line
is this, that a sperm has a long flagellum, and that flagellum is absolutely surrounded by mitochondria, and you guys
probably now the function of mitochondria, is to burn energy, okay. At the tip of the sperm, is a structure called the
acrosome, this is an enzyme acrosome, and that is where the enzymes are carried to penetrate the egg. And here, of
course, is going to be the 1N nucleus. The haploid nucleus that is going to target, that once the acrosome allows
penetration of the egg membrane, the nucleus will get in there.
Just a couple of other things, in terms of structure and function. Interestingly enough, sperm production has to occur
several degrees below body temperature, bet you didn’t know that? And, therefore, especially in humans, we find that
the testes actually are encased in a sack that hangs down below the body, called the scrotum. And, in essence, it is
air conditioning. In essence, it is a way to expose the testes to air circulation, so that they can be several degrees
below the body temperature. And so, that’s kind of, a lot of my students always say to me, you know any guy will tell
you this, “why are such sensitive organs”, I mean think about, those of you who are girls, know this story. What does
you mother do when you are little, she says, “if that boy bothers you, you just kick him hard”, and mom tells you where
to kick to him. And, whether that is nice or not, the bottom line is, what a strange evolutionary device, to have such a
sensitive organ descended below the body, but structure follows function, doesn’t it? And if the testes are going to be
made several degrees below body temperature, the testes has to be descended below the body for, that so called, air
conditioning.
But, one more thing that I have to ask you, structure follows function right? You have all these cells turning into
sperm; the sperm are constantly leaving the male’s body, from the age of puberty, practically till death. Why doesn’t a
guy run out of testes? Why doesn’t the testes just kind of disappear after twenty years? You never thought of that did
you? Now you are saying, “please tell me the answer.” Are you going to be scared if I say, “well they do”? No, they
don’t, and there is a reason for that.
And I have one more thing to show you. See this cell right up here, this germ cell, the one that is going to give rise to
this spermatozoa and the spermatids? Well guess what? This cell can also do mitosis, and if it can do mitosis, it can
replenish tissue in the testes, and therefore some of the cells will go into meiosis, some will replenish the testes.
So, again structure follows function. Meiosis happens in the testes, the testes were organs built to do meiosis.