Hello. Welcome to Byte Size Med. In this
video, we're going to go over the steps of meiosis and apply them to understand oogenesis. During development, the embryo,
it has primordial germ cells. Now these primordial germ cells, they are going
to migrate from the endoderm of the yolk sac to the gonadal ridge, which is the developing ovary.
So they're ultimately going to form gametes, which in this case is the mature oocyte or the ovum. The
process by which the germ cell forms a gamete, that is gametogenesis, and that process is by meiosis.
Here it's from the oogonium to the mature oocyte or the ovum, and the process is called oogenesis.
To understand the steps of oogenesis, we'll need to know meiosis first. So we're going to go back to
a bit of cell biology. There's mitosis and there's meiosis. Mitosis is how somatic cells divide.
Somatic cells are diploid cells, they have 46 chromosomes in 23 pairs, that's 2n. So 2n means
diploid and they divide into two diploid cells. Now the germ cell, like somatic cells, it's diploid
as well. It has 46 chromosomes and 23 pairs. This is 2n, diploid. By meiosis, that number is
going to come down to half, that's n or haploid. This is versus mitosis where the number didn't
change. In mitosis, one diploid cell is divided into two diploid cells. See there's no change
in the chromosome number. In meiosis, after two series of divisions, one diploid cell divides into
four haploid cells. So its number reduces by half. If this germ cell were to form more
germ cells, that would be by mitosis. But for the germ cell to form
gametes, that is by meiosis. First let's go over the stages of mitosis. So
mitosis has prophase, prometaphase, metaphase, anaphase and telophase. And in between mitotic
divisions, there is an interphase. That has a G1, S and a G2 phase in it. DNA replication happens
in the S phase, where the chromosomes duplicate. During prophase, they condense. In prometaphase,
the spindles, they destroy the nuclear membrane and attach to the centromeres of the
now condensed and visible chromatids. During metaphase, they line up along
the equatorial plate. They split at the centromere and move towards opposite poles
during anaphase, and they reach the opposite poles during telophase. The nuclear membrane reforms,
cytoplasm division by cytokinesis gets completed, and there's formation of two cells with the
same number of chromosomes as the parent cell. Meiosis is the same, but it's
very different. Now putting down 46 chromosomes is difficult, so we're going to
use four just to make it simpler to understand. Unlike mitosis, meiosis has two divisions, a
first and a second meiotic division. Before the first meiotic division, again the cell does
go through an interface just like mitosis. The DNA replicates during this phase, there's
double the number of chromosomes, which are now called sister chromatids held together
at the centromere. Once the cell enters meiosis, it has the same stages as mitosis, with
prophase, metaphase, anaphase and telophase. Now we're going to put one next to each phase
because this is the first meiotic division. So we start off with prophase one, and now is
when it starts to get different. Prophase one in meiosis is longer and it's got sub-stages. Leptotene,
Zygotene, Pachytene, Diplotene and Diakinesis. Leptotene is like the prophase of mitosis, where
the chromatids, they start to condense. But they are still not clearly visible. So they go from
being fine and thread-like to becoming shorter and thicker. Zygotene. Homologous chromosomes, they
find each other and they pair up. Now this does not happen in mitosis. So they actually lie parallel
to each other with the synaptonemal complex in between, but we're going to put them side by side
so that we can understand the rest of the steps. This process of pairing it's called synapsis
and the two pairs together will now have four chromatids. So it's called a tetrad. Now we move
into pachytene, where chromatids from different pairs they overlap at different points. So this
is how exchange of genetic material happens and the process is called crossing over. During
diplotene, that synaptonemal complex, it disintegrates and the points of overlap, they
become adherent and those are called chiasmata. During the last stage, that's diakinesis, they
separate. So now you can see that there has been an exchange of genetic material
that happened during crossing over. Now these chromatids, they completely condense. The
nuclear membrane disappears by the spindles that are forming from the centrioles. So this phase
would be similar to prometaphase of mitosis, and now we're done with prophase one. There is
chromatid condensation and crossing over. Now these chromatids are going to line up along the
equatorial plate. Sound familiar? That's because this is metaphase. During anaphase, in mitosis
remember there was a split at the centromere. That doesn't happen here. So randomly one set of
chromatids is going to move to either pole of the cell. During telophase, they reach the poles, then
the spindles disappear. The nuclear membrane comes back. Cytokinesis has divided the cytoplasm.
So now we have two cells, which have half the number of chromosomes as the parent cell. So the
first meiotic division is a reduction division. Now we're going to go into the second meiotic
division, which is easier than the first because the steps are like mitosis. Again there's an
interphase before this. But there's no DNA replication that happens, because they are already
doubled. So we'll pick up one of these cells now. In prophase, the chromatids will condense and in
late prophase, spindles forming from the centrioles, they disintegrate the nuclear membrane and attach
to the centromere of the chromatids. They will line up along the equatorial plate during metaphase.
They will split at the centromere and move towards the opposite poles during anaphase. They will reach
the poles, the spindles will disappear, the nuclear membrane will reform during telophase. And with
cytokinesis, the cells have now divided into two. The chromosomes, they uncoil and they become
thread-like again. So all of this was in telophase. So one cell divided into two cells and each of
those two cells divided into two cells again. From one diploid cell, there are four haploid
cells. Now why did they become haploid? Because after fertilization, that number will
go back up to being diploid again. The diploid cell was a germ cell,
the haploid cell is a gamete. And now we're going to apply this to
oogenesis. The primordial germ cell, it divides multiple times as it migrates forming
more germ cells. So what kind of division is that? That is mitosis. It's not forming gametes, it's just
forming more germ cells. So it's mitotic divisions. They migrate to the gonadal ridge, which is
the developing ovary. The ovary has an outer germinal epithelium which has cuboidal cells.
Below that is dense connective tissue called the tunica albuginea, then there's the ovarian
cortex, which is continuous with the medulla. These cells, they're going to migrate into the
cortex and then they differentiate to form oogonia, or the primordial ova. These are diploid cells.
The oogonia undergo mitosis to form more oogonia, until around five months of gestation. After that,
some of them can degenerate, but what remain, they undergo the first meiotic division. These are the
primary oocytes, but they get arrested in prophase one of meiosis, during the diplotene stage. So the
primary oocyte gets stuck in prophase of the first meiotic division. This is meiotic arrest. This is
the situation at birth. So everything till now was until birth and they stay in this stage until
puberty. At puberty, just before ovulation, there's a surge of the luteinising hormone. Under that
influence, one primary oocyte completes the first meiotic division and it forms a secondary oocyte,
which gets most of the cytoplasm. The other cell is called a polar body. It's the first polar
body. Now each of these have half the number of chromosomes, because remember the first
meiotic division is a reduction division. The secondary oocyte, it quickly enters the second
meiotic division, where it gets stuck again in metaphase. So this is the second meiotic arrest,
and it gets stuck here until fertilization. Only if fertilization happens, can the secondary oocyte
continue the second meiotic division and become a larger mature oocyte or an ovum, which again gets
most of the cytoplasm and the other cell is a second polar body. The first polar body, that could
either degenerate or it could also go through a second meiotic division to form two more polar
bodies. But ultimately, all three polar bodies are going to degenerate and what remains is just one
functional mature oocyte in the end, which is the gamete, and this has half the number of chromosomes
than the germ cell that started the process. So all of this that was oogenesis and the oocyte
that is inside cells. So there's a coat of cells around it forming a follicle and the follicle
has different developmental stages. All of that is folliculogenesis and together under the
influence of hormones they form an ovarian cycle. So that is meiosis and oogenesis. If
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