00:00
In this slide, I’m going to explain briefly that
there are three components to spermatogenesis.
00:09
Firstly, there is a spermatogonial phase,
and I’ll explain that in a moment. But this
is basically where the stem cell, the spermatogonia
undergo a series of divisions to replace itself,
but also to give rise to a population that
they’re going to go forward and differentiate
into spermatozoa. Then there is the spermatocyte
stage. This phase is where the spermatogonia,
having gone through a series of mitotic divisions,
enter into divisions of meiosis. Whereby,
they’re going to have their chromosome number
reduced to a haploid complement, and therefore,
half the DNA content as well. And then finally,
there is this stage called the spermatid phase.
01:02
That’s the stage where the spermatids undergo
final structural changes, rather dramatic
structural changes as you’ll see, to then form
the sperm, all be it at this stage, very
motile. But they have the structural characteristics
and appearance as of spermatozoa. And again, on
the left-hand side, you can see the diagram,
it’s going to persist. And I’m going to
refer to that as we go through. Let’s, first
of all, look at the spermatogonial phase.
01:40
There are a series of spermatogonia up against
the wall of the tubule, which is rather thin.
01:46
And those spermatogonia, very hard to distinguish
really, in histological section stained with
hematoxylin and eosin. So I’m only really
going to point out the principle of what’s
happening with these cells here, and try and
illustrate an example of them. But again,
let me emphasize, they are really hard to
distinguish in sections. Well, this type A
dark spermatogonia undergoes a mitotic division
on a regular basis to replace itself and to
give rise to a population of cells that are
going to go forward, as I mentioned earlier,
to become the spermatogenic population of
cells and form the spermatozoa. Now sometimes,
a person may have chemotherapy treatment.
The young male or even adult male or even
old male may have chemotherapy because of cancers.
And that can wipe out all these spermatogenic
cells because they’re undergoing division.
However, they will lose the ability to produce
sperm during that period. But because these
spermatogonia, sitting up against the basement
membrane, are also at a resting phase during
their lifespan, they can still later on give
rise to more spermatogonia, and therefore,
the person can acquire again a population
of spermatogenic cells. I said earlier young
adult and old adults. It’s because in males
after puberty, males produce spermatozoa all
their life until they die. Unlike in the female
when menopause comes about and no longer is an
oocyte release an ovulation. These spermatogonia,
as I said, mitosis and changes into two other
cells, but you can see also up against the
wall of the seminiferous tubule. And again,
I stress they’re very hard to identity.
04:02
But the pale spermatogonia, they go through a
series of mitotic divisions. And then finally, those
cells are initiated to go into the meiotic
process. They get to the stage where they
are termed Type B spermatozoa, or I should
say Type B spermatogonia. These are the ones
destined then to go forward. So really in summary
here, there is this division of spermatogonia
forming another population of spermatogonia, and
then a further division to create a population
of cells that will go forward to produce the
spermatogenic population of cells you
see in the epithelium in front of you in these
slides. Let’s now look at the spermatocyte
stage, the cells undergoing meiosis. The very
first meiotic division is easy to see, or
at least evidence of the first stage of meiosis
is easy to see because the cells accumulate
an enormous clumping of their chromosomes,
all the chromatin clumps in these cells.
05:20
And they are the first easiest cell to identify. And
you can see many of them in this section through
the testis. They are called primary spermatocytes.
They’re in the primary, first process of
meiotic division.
05:35
The green labelled cell on the base
of the seminiferous tubule in the diagram
represents a spermatogonia. It’s a flattened
cell, whereas, the two above it and the one
to the left represent the primary spermatocytes
that have moved away from the wall of the
seminiferous tubule. Those cells then move into
a second meiotic division. And that division
is very quick. It’s often very difficult
to find these secondary spermatocytes.
06:14
Secondary spermatocytes are going through the second
meiotic division. They’re much smaller cells,
but you can see here in this section through
the testis, eosinophilic cells with again
clumped chromosomal content in the nucleus.
So, look at the slide and you should be able
to identify the difference between primary
spermatocytes and secondary spermatocytes.
06:42
And these are represented by the cells in
blue in this particular diagram, the ones
away almost next to the green colored slides
that were the primary spermatocytes.
06:58
These cells then quickly go through a stage where
they develop into spermatid structures, the
spermatid phase. And these cells are identified
because they have a very rounded, small rounded
rather prominently and dark stained nucleus, but the cells
are quite small. And again, they’re represented
in the diagram by the blue cells on the top,
smaller cells, and some are even sort of shaped
in a funny way because they’re going through a
morphogenesis to form a very early spermatozoa.
07:37
They then go into a later stage where they
then form these little tiny structures you
see at the very luminal aspect of the seminiferous
tubule. By now, they have a shape that you’re
more familiar with, with identifying spermatozoa
as shown in the diagram. On the top of the
diagram, these long pale nuclei, blue stained nuclei,
and the long tail has developed. Well, those
late spermatids that you see here are developed
from the more smaller circular early spermatid
shown in the diagram, a total transformation
in structure. And finally, these cells are released
into the luminal space, these late spermatids.
And then they move through the fluid all the
way along the length of the seminiferous tubule
into the mediastinum into the rete testis
where they then moving on further into
some of the ducts that I’ll talk about in
a later lecture.