Extra points to you all who will read the menstrual cycle post on your iPad.
Welcome to part two of Female Reproduction, the hormones and menstrual cycle. Part one covered the basic anatomy, and today we're doing...the menstrual cycle. Here is the picture you're going to need:
It's a lot, Sci will admit. But the glorious thing is how is all entertwines and fits together so beautifully! The whole thing just comes together in perfect synchronicity. Brilliant.
Here are the hormones you're going to need:
Gonadotropin-releasing hormone (GnRH): This hormone actually starts being secreted long before you enter puberty. It is secreted from an area of the brain called the hypothalamus (a truly fascinating brain area, btw). What GnRH actually does is pulse on a monthly cycle, controlling the release of OTHER hormones, specifically follicle stimulating hormone and leutinizing hormone.
Follicle-stimulating hormone (FSH): Follicle stimulating hormone stimulates your follicles, specifically the ovarian follicles, which we will get to in a minute.
Luteinizing hormone (LH): This hormones stimulates the development of the ovarian follicles, and is very necessary for ovulation, which we'll get to in a minute.
Estrogens:There are a bunch of these, but beta-estradiol, estrone, and estriol are the main three. By their powers combined, estrogens contribute to the growth and fat deposits of the breasts, deposit the fat around your hips and butt to make you all womanly (also to make you loathe shopping for jeans), and even promote the closing of the epiphyseal bone plates to stop growth. During puberty, they also cause growth of the fallopian tubes, the uterus, and the vagina. Busy hormones, these.
Progestins: The big ones in the body are Progesterone and 17-alpha-hydroxyprogesterone, but we're just going to talk about progestins. These are going to increase the endometrial lining, creating a nice cushy place for an egg to implant. These also cause the growth of the alveoli of the breasts to allow for milk production, and even cause the secretion in nutients in the fallopian tubes as the egg travels down. Progestins are all about make life good for the egg, fetus, and baby.
So this is all really complicated:
So we're going to simple it down, starting with the hormone cycling.
Ahhhh. That's better.
Contrary to popular belief, scientists and physicians place menstruation at the BEGINNING of the cycle, rather than at the end. Starting with a clean slate, as it were.
So at the beginning of the cycle, estrogens and progestins are low. Estrogens climb to a peak at about 12 days post menstruation and then drop off, only to bulge again in the last 12 days. Progestins stay low until after ovulation, at which point they balloon up to keep that egg comfy. FSH and LH also start out pretty low, and then peak JUST before ovulation, and then back down again.
What you can see in this graph that makes Sci happy is the interactions going on. You can see that the estrogen increase, but the sudden DROP in estrogens is what TRIGGERS the spike in LH and FSH, which in turn trigger ovulation!! Wild, right?!
So as for the ovulation itself, what's going on there?
Here you may observe the ovary, in all its glory. Starting at the top left, you can see the "primordial follicle". This is your basic ovum (already been through meiosis), which is surrounded by granulosa cells. Every month, in response to the spike in FSH and LH (which in turn is a response to a spike in estrogen) about 20 ova will start to mature. The ovum grows, the granulosa cells around it multiply, and the ovum starts to secrete a follicular fluid and form a space inside the granulosa cells called the antrum.
As LH and FSH increase, one follicle is going to out-compete the others. The other follicles give up in defeat and decide to retire and never make it to the Olympics. LH and FSH continue to increase.
Round about day 14 following the onset of menstruation (which usually lasts 5 days), the chosen follicle swells. Fluid starts to ooze out. And then, the ovum comes bustin' out.
You notice that the ovum there has a little halo around it. This is called the corona radiata, and will protect the ovum on its way through the perilous world of the female genitalia. Also notice the yellow bits at the bottom of the ovary. These are the leftover bits of the ovulated follicle, and are called the corpus luteum (meaning "yellow body", yes, it's really yellow) and is stimulated by LH when ovulation occurs. This corpus luteum will persist in the ovary for about 14 days, and will produce the progestins that keep the uterus a nice, soft place for the ovum to be happy.
So now the ovum travels down the fallopian tubes, coming to rest in the uterus. If it is not fertilized, the ovum will be shed along with the endometrial lining about about 14 days. If it IS fertilized, the real crazy starts happening. That's next time. Or maybe the time after that...Sci's gotta cover some contraceptives first.
So anyway, we've got one more thing left to cover before this whole cycle is complete
The endometrial lining
This is the lining inside the uterus, which is shed every month along with the ovum. There are three major phases, the proliferative, the secretory, and the menstrual.
1) The proliferative phase: During the first 11 days following your last menstruation, the gradually increasing estrogens cause the cells of the endometrial lining to muliply, plumping it up (estrogen increasing, causing endometrial lining to increase AND LH and FSH to surge, do you see how it's all coming together?!)
2) The secretory phase: this is after ovulation, when the corpus luteum produces progestins, and the estrogens surge again, both hormones causing the endometrial lining to thicken more, and giving it lots of blood flow. Your final endometrial lining will be around 5-6mm thick.
...and then we have...
And it all comes together.
Two days before you begin menstruation, the corpus luteum (the yellow thing up in the ovary) dissolves. This means the progestins coming from it stop (you can see the dive in progestins in the graph). The estrogens also decrease. With the decreased progestins and estrogens, your endometiral lining can't hold it together, and the entire thing involutes. Necrosis occurs, and the outer layers of the endometrium are shed. The net result is 5 days of bleeding (on average) and 40mL of blood lost, along with 35mL of serum. Somewhere in there is the ovum, which by that point is past its expiration date anyway.
And we begin again.
Isn't it cool how this all comes together!!! Observe:
Estrogens increase, the endometrial lining increases. The estrogens drop, which causes a surge in LH and FSH, which lead to ovulation. The LH causes a corpus luteum to form, which released large amounts of progestin. The progestin and the estrogen keep up the endometrial lining until the corpus luteum dissolves. IT ALL COMES TOGETHER!!! It's like an awesome dance of hormones. This is the sort of thing that gets Sci really excited about science.
Stay tuned! Next time we'll cover what happens to this cycle if you put some hormones on top of it: the story of oral contraceptives.