Last week we discussed how an oocyte goes from primordial germ cell to a mature (Graafian) follicle ready to be released into the world! We’re going to pick up at ovulation.
The oocyte activates itself with Maturation Promoting Factor to unfreeze its meiotic division that its been stuck in. What stimulates that? We don’t really know, but so far it seems to be linked to progesterone. So, after ovulation, the oocyte will start dividing again, but instead of making two daughter cells, it makes one daughter cell and a polar body. Basically, it does all the work to make two viable cells but gives all the good stuff (cytoplasm, organelles, etc.) to only one of them. The other is called the polar body and it basically shrivels up and dies. This time, though, we finish up the first round of meiosis, go into the second round and stop at metaphase. The oocyte freezes there unless it is fertilized, when it starts up again.
And then a bunch of other fun stuff happens, but what about the ovary? What’s going on back there? Especially with all those extra follicular cells and all the other stuff that was made? Basically, it makes a sort of shriveled body that is called the corpus luteum and they start working as an endocrine organ, which is kind of really cool. It’s new job is to start secreting progesterone and estrogens, which prevents new the ovulation cycle from restarting too soon. And this is why progesterone being linked to Maturation Promoting Factor doesn’t seem to make sense.
Without getting too chemical about it, ovulation is stimulated by a surge of LH. This stimulates proteases to degrade the collagen fibers of the tunica albuginea (ovary layer) and the theca externa (of the Graafian follicle). This thins out the walls and follicle essentially bulges out of the weakened area and bursts out. You know the finger looking structures of the fallopian tube? Those fimbriae sweep up the ovum and draw it in then.
Now back to the corpus luteum. If the ovum that was released doesn’t get fertilized, then the corpus luteum stops secreting progesterone. Instead it starts shrinking (involution). It pretty much degenerates until it becomes a little white scar called the corpus albicans.
If fertilization does occur, the human chorionic gonadotrophin, which is the substance we test for in pregnancy tests, will instead stimulate the corpus luteum to stick around and secrete progesterone for another 6 months. After that, it sort of gradually declines until it starts secreting relaxin. Relaxin is the hormone responsible for…wait for it…relaxing the connective tissue of the pubic symphysis, a necessary step in childbirth.
Next week, we’ll look at other parts of the female reproductive system!