Endocrinology of Reproduction part 3

As promised, I'm finally going to talk about the female!

The female seems complicated because there's the menstrual cycle (the cycle of the functional layer of the uterus being built up and shed) and the ovarian cycle (the cycle of the follicles developing and so forth). These cycles are actually very closely intertwined, however.

The Ovarian Cycle

First, we're going to look at the changes in the ovaries. There are two main phases: the follicular phase, in which a follicle is being developed. At the end of this stage, the follicle ruptures, releasing an ovum into the fallopian tubes. Following this is the luteal phase, in which the ruptured follicle becomes the corpus luteum ("yellow body") which secretes progesterone to prepare the uterus for pregnancy.

A follicle starts out as a single ovum surrounded by some granulosa cells, which are surrounded by thecal cells (which are essentially just differentiated ovarian connective tissue cells). As the follicle grows, a fluid-filled space called the antrum forms within the granulosa cells. While several follicles begin developing each cycle, only one is "chosen" to develop fully: the Graafian follicle. (If multiple follicles fully develop, you can get twins.) This continues to grow until eventually the follicle ruptures and the ovum inside is released, still surrounded by a few granulosa cells.

The ruptured follicle then degenerates to become the corpus luteum, which as I said secretes hormones to build up and maintain the functional layer of the uterus.

Hormones in the Follicular Phase

At the beginning of the follicular phase, FSH is slightly elevated to help the follicle to grow. As the follicle grows, it secretes ever-growing amounts of oestrogen, which exerts negative feedback on the hypothalamus and anterior pituitary, and so levels of FSH decline around midway through the follicular phase. Oestrogen, however, continues to grow, as oestrogens stimulate proliferation of granulosa cells, which then go on to produce more oestrogen.

As an aside, remember how I mentioned the ability of androgens and oestrogens to interconvert in my last post? Well, this is important here. You see, LH stimulates the thecal cells to turn cholesterol into androgens. These androgens then diffuse into the granulosa cells. FSH then stimulates the granulosa cells to convert androgens to oestrogens with the help of an enzyme called aromatase (which for some reason reminds me of Roserade). And that's how the follicle produces oestrogen!

Anyway, back to the story. Eventually oestrogens climb so high that they actually start producing positive feedback instead of negative feedback. This produces the LH (and FSH) surge, where these levels spike. (Normally this is simply referred to as the LH surge as LH spikes much more than FSH.) Following this surge, the hypothalamus and anterior pituitary become desensitised, allowing LH and FSH levels to rapidly fall again. The most important part about the LH surge is that it's what causes the follicle to rupture and ovulation to occur.

Hormones in the Luteal Phase

As I've mentioned several times now, following ovulation, the follicle degrades to become the hormone-secreting corpus luteum. Progesterone (and oestrogen) from the corpus luteum prepares the uterus for pregnancy. Eventually degradation of the corpus luteum causes progesterone levels to fall, which causes the functional layer of the uterus to degrade, which causes menstruation. This also reduces negative feedback to the hypothalamus and anterior pituitary, however, so FSH/LH levels can rise again.

If a pregnancy does occur, this doesn't happen, as the embryo secretes hCG (human chorionic gonadotropin), which maintains the corpus luteum. The corpus luteum is then maintained until the placenta can secrete progesterone by itself.

And I think that's pretty much it for the reproductive system! Just one more body system (gastrointestinal) to go for this unit!