Oogenesis

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Oogenesis is the process of developing ova
or egg cellsThe basic structural and functional units of life. within the ovariesThe female gonads that produce eggs and hormones. of females.

Folliculogenesis is the growthAn increase in size and number of cells. of all of that
granulosa and those other structures around the

oocyte. So this process folliculogenesis is covered
in another mini lecture of mine called ovaries and

ovarian folliclesStructures in the ovaries that contain developing oocytes.. This mini lecture covers only
oogenesisThe process of egg cell formation. which is the cellular divisions that the

ova experience. Neither of these processes are
the processes that take place in the uterusThe muscular organ where a fertilized egg implants and develops..

Both folliculogenesis and oogenesis take place in
ovaries. However, the cells that are involved in

oogenesis and specifically in folliculogenesis, they
generate the hormones that dictate the uterine

cycle. Let’s clear up some other terminology
quickly. Um, oogonia are stem cells. Oocytes are

those stem cells going through the stages of
meiosis and that’s happening in the ovary. An ovumA mature egg cell

or plural ova is technically what’s ovulated out
of the ovary to meet up with spermatozoa. However,

I have I’ve taught from non-science majors
textbooks that refer to oocytes as ova,

but technically it’s an ovulated egg. Oh my god,
don’t use the word egg. That’s terrible word. It

refers to all of these structures here. A zygote
is a fertilized ovum that has already met up with

sperm and is ready for implantation into the
uterus. Unlike sperm production in the male,

the process of oogenesis begins during fetal
developmentThe process of growth and differentiation. instead of at puberty. The oogoniumAn immature female germ cell that develops into an oocyte.

is a stem cell and there’s many of them and they
contain a full set of chromosomes called a diploidA cell with two sets of chromosomes, one from each parent (2n).

number. Right? So you you get 46 chromosomes in
every cell. Um these are found in the outer region

or the cortex of the ovaries and they undergo
cell divisions during fetal development and they

produce millions of cloned or copied cells.
Most of these are degenerated before birth,

right? Like why make so many just to get rid of
them? But some of them are stimulated to grow

into a slightly larger cell which is called a
primary oocyte. These are cells that have begun

their first cell division of meiosis in fetal
development. They don’t complete it and they

remain suspended in this like partial state
of division until a date much later in the

individual’s life. At birth, only about 400,000 to
4 million oocytes remain in both ovaries combined.

Oogonia are not present meaning that this
supply of eggs is the finite number for

life. At puberty about 40,000 ova remain and
the meiosis process is now strictly regulated by

estrogen and progesteroneA hormone that supports pregnancy and regulates the menstrual cycle.. Every month there are a
bunch of oocytes that want to be picked to ovulate.

about 25 to 30 oocytes will restart and fully
complete meiosis one. However, unlike the

equal divisions of cells in spermatogenesis
in oogenesis, the divisions create one big oocyte

and one little tiny polar bodyA small, non-functional cell produced during oogenesis.. A polar body is
not like an official cell. It’s kind of like an

inactive cell and it has no purpose. It holds 23
chromosomes just like the primary oocyte but during

cytokinesis when the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. is divided among
resulting cells the polar body received

little to none of the organellestructures within a cell that perform specialized functions. that would allow it
to get started with making the proteinsLarge molecules made of amino acids with various functions in the body. essential

to life. So now we have 25 to 30 primary oocytesImmature egg cells halted in development until puberty.
that are responding to rising levels of estrogen

at this point and they all want to be ovulated
and potentially fertilized. As a side note,

they are all contained in tertiary or graafian
follicles at this time. All of these they start

to enter into that second round of cell
division in meiosis 2. This round of cell

division might or might not create a second polar
body, but it definitely creates a secondary oocyteThe egg cell released during ovulation..

This secondary oocyte is again arrested before
completing meiosis 2. Hopefully, it’s ovulated

and it’ll complete meiosis 2 upon joining its
chromosomes with those from a spermatozoa. Sorry,

spermatozoon. If the secondary oocyte doesn’t get
ovulated, it’s going to try again next month

and then the next and the next. At some point, it
might give up and it might be lost to atresia or

breakdown. Menopause is it’s got a couple
different definitions, but in terms of hormones,

it’s defined as a drop in estrogen, which is
usually the result of the end of oocytes to ovulate.

The ovary at this point usually has numerous
corpus albicansA scar-like structure that forms when the corpus luteum degenerates., which look like scars upon

the ovary. Menopause sometimes can be specifically
defined in terms of time as the first year without

a uterine cycle which would indicate no estrogen.
That doesn’t mean that there are absolutely no

oocytes left just because you’re not having a cycle.
There may be some. They may refuse to ovulate each

month. They might be like, “Hey, I’m not ready.
Can I get back to you next month?” And this can

create skipped months of the uterine cycle leading
up to menopauseThe natural cessation of menstruation and ovarian function.. There can also be a bunch of

oocytes one month that are like, “Hey, this could
be our last chance. Let’s all ovulate together,

which can sometimes cause uh multiple pregnancies
in years leading up to menopause. This period of

time leading up to that first year without a
uterine cycle is also called or referred to

as perimenopause, which in my opinion has a lot of
symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). that were way worse than menopause. I’m

sorry for that personal, but I am 50 years old.
I mean, you get an idea of where I am in life.

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