Hi, there. Welcome to this lecture in which
we’ll be looking at the development of the
egg, that’s oogenesis, and development of
the sperm, that’s spermatogenesis.
So we'll start off with the primordial germ cells,
and we’ll look at oogenesis and spermatogenesis,
and I will touch on fertilization and what
happens after fertilization as the fertilized
egg is transported down the oviduct towards
the uterus. Finally, we’ll briefly indicate
some of the key things about in vitro fertilization.
In the process of gametogenesis, which is
a combined term for oogenesis and spermatogenesis,
there are four major phases. The first phase
is the origin of the primordial germ cells
themselves and the reason which they migrate
to the gonads. The second phase is the increase
in the number of germ cells by mitosis.
And the third is the reduction in chromosomal
number by the process of meiosis.
Finally, each egg and sperm will have to mature
and develop into its final form.
Now, of these four phases, the first is essentially
identical in male and female embryos.
The remaining three are very different between
males and females. And when we talk about
these processes, we’ll always put the female
first because the baseline condition of humankind
is female, and males are diverted away from
female sex by complex developmental signals.
So, let’s begin with the origin of the primordial
germ cells. Now, perhaps, unexpectedly, they
differentiate quite a long way from the body
of the embryo. They first appear in the yolk
sac at very early stages in the embryonic
period. In this diagram, we can see the amniotic
cavity at the top and the yolk sac below and
the developing body of the embryo in between
the two of them and primordial germ cells
are marked by little red dots. At a slightly
later stage, we have a cutaway view of the
embryo, and we can see the gut in the midline,
and the red dots show the path that the primordial
germ cells are taking as they make their way
towards the gonad. So they’re actually migrating
as individual cells following signals within
the tissues through which they’re moving
to find the gonad itself. There, they go and
populate the gonad. In this view, we can also
see a section, and
we can see how the primordial germ cells are
migrating up through the dorsal mesentery
of the gut, and then around to populate the
gonad. In this diagram, we can also see kidney
tubules, and these are part of the mesonephric
system. If we follow that tubule
around, it terminates into a mesonephric duct.
Here’s a stereoscopic view of the same process
taking place. So you can see that, of course,
it’s identical in both sides of the embryo
and also, as when the primordial germ cells
reached the developing gonadal primordium,
at that time, there are also cords that have
grown in from the surface of the gonadal ridge
itself. Now, of course, the gonadal ridge
is a mesodermal structure, so the coelom is
covered in what’s called a “mesothelium”.
It’s not a true epithelium. It’s merely
a condensed form of mesoderm on the outside.
But nonetheless, the cords are still growing
into the underlying mesoderm. And the primordial
germ cells will take up associations with
those primary sex cords.
Now, the number of germ cells varies considerably
during the course of development. If we look
at females, first of all, we can see that
the number increases very rapidly up to about
seven million or so, and then it falls equally
rapidly. Most of the primordial germ cells
will, in fact, die in females before birth.
Thereafter, the numbers steadily diminish,
and at the time of the menopause, there’ll
be effectively no normal primordial germ cells
left. Here’s a diagram which outlines the process
of oogenesis. We can see that the original
oogonium will give rise to the primary oocyte,
and then it will enter these two meiotic divisions.
But it will not complete these divisions until
long after birth. Indeed, the second meiotic
division is not completed until fertilization.
So we can see that studying this in the abstract,
looking at this diagram on its own, it’s
actually kind of missing the important information.
So, first of all, we have to add the background
of the developing follicle. In this diagram,
this is what we do. We’re looking at the
developing primordial germ cell, but we now
add it in the cells of the follicle.
We can see that it goes through stages such as the
primordial follicle, and then after birth,
we have the primary follicle. After puberty,
we’ll move on to the secondary follicle,
and in all of this as it’s not completed
the first meiotic division.
Following through to later stages, we can
see that the tertiary follicle, the third
class of follicle, will go on to give rise to
the ovulated egg itself, and then fertilization
may take place. So the second meiotic division
is not complete until this point. But just
as we looked at in the context of the follicles,
so we also have to look at in the context
of the ovary. In this diagram, moving around
the blood vessels, we can see primary follicles
steadily growing, becoming secondary follicles,
tertiary follicles becoming mature, becoming
ripe, and then the ovum actually hatching
from that follicle. If we’re to follow the
follicle, in time, they would now become the
corpus luteum, which is involved in the signaling
about pregnancy. And if we are to look at
the fertilized egg which is released, then
it may be fertilized up towards the ampulla
of the oviduct, and they’d have to make
its way down the oviduct subsequently.
This is a picture of a mature follicle in scanning
electron micrograph. So it’s rather nicely,
the arrangement of cells within it. And you
can see that the egg cell itself is relatively
large and is surrounded by cells derived from
the follicle, and these are corona radiata
cells. Let us now look at the spermatogenesis, and again,
it will form from the spermatogonium, primary
spermatocyte, and then enter the two meiotic
divisions, which will finally give rise to
spermatids, and these would differentiate
into spermatozoa. This process is not confined
to one particular time in males, and will,
in fact, continue through adult life.
This illustration shows that process of maturation
as it moves to become the definitive spermatozoon.
You can see a number of different changes
that will take place. So a flagellum will develop.
An acrosome will appear at the tip,
and the acrosome will be involved in actually
mediating the fertilization process until
finally, the adult free-swimming sperm is fromed.