We will now explore how the ovarian cycle responds to hormonal changes in the body
and of those changes are mirrored with changes in the uterus.
So ovulation is timed very, very tightly not only to developments in the egg,
but also to developments in the uterus.
Initially, these changes are driven by follicle stimulating hormone, FSH,
and luteinizing hormone, LH, coming from the pituitary gland.
Follicle stimulating hormone does exactly what we think it?s named what imply it would do.
It makes the follicle cells surrounding the egg a little more active
and one thing that they?re going to do is secrete estrogen.
So follicle stimulating hormone, as we see it making a little bump on the screen,
is going to cause an increase in the level of estrogen
being released by the granulosa cells surrounding the egg.
Surely thereafter, a surge in luteinizing hormone from the pituitary gland
is gonna have several important effects.
First, it?s gonna cause the oocyte to complete meiosis one
and creating a secondary oocyte and the first polar body.
Thereafter, the secondary oocyte will begin meiosis two,
but arrest in metaphase and it will not complete in metaphase
or any further steps in meiosis two unless it is fertilized.
The luteinizing hormone surge
is also gonna create an increased production of a new hormone
called progesterone, from the granulosa cells.
And if you look at the chart, you can see that the levels of progesterone
rise fairly sharply after the surge of luteinizing hormone.
The luteinizing hormone spike also causes the Graafian follicle
with its large antrum to rupture,
releasing the egg from the ovary, hopefully into the oviduct.
The cells that are going to be surrounding that expelled egg reorganize themselves
and instead of being called the cumulus oophorus
we?ve decided to give them yet another name,
the corona radiata, and they?re surrounding the glycoproteins deep to them
the zona pellucida and then the oocyte.
Granulosa cells that remain in the ovary
are going to form a structure called the corpus luteum
or the yellow body and it will continue producing progesterone
and that?s what maintains the high levels of progesterone
for the next few days in the menstrual cycle.
If fertilization does not occur, the corpus luteum will start to dwindle, rescind
and eventually go away becoming a small little corpus albicans
or whitish body within the ovary and at that point
the levels of progesterone are going to drop very precipitously.
The hormonal cycle is not only affecting development of the egg
but also development of the uterus itself to allow implantation to occur,
should fertilization occur.
As levels of follicle stimulating hormone rise,
the granulosa cells create estrogens such as estrone and 17 beta-estradiol.
These are gonna influence the basal level of the uterus
to extend spiral arteries further up,
extend uterine glands further up and proliferate so the basal level
will then create a follicular level
and the lining of the uterus will thicken appreciably.
Under the influence of luteinizing hormone and progesterone,
we?re going to have continued development of the uterine lining
entering what?s called the progestational phase
or the phase that?s ready to accept the fertilized egg.
If fertilization does not occur,
then the levels of progesterone produced by the corpus luteum will drop.
At that point, the spiral arteries that supply the progestational uterus will spasm, close
and then the follicular layer will become ischemic,
die and then be shed during the menstrual phase.
Thank you very much,
and we?ll come back and see what happens when fertilization actually does occur.