Endocrinology of Reproduction part 2

Now we're going to talk more about LH and FSH and what they do! We'll start off by talking a lot about the male, though there is some overlap with the female.

The Male

In a nutshell, endocrine control of the male reproductive system starts with release of GnRH by the hypothalamus, which triggers release of LH and FSH from the anterior pituitary. LH then stimulates Leydig cells in the testes to produce androgens (which then go on to stimulate sperm development), whereas FSH stimulates the Sertoli cells to aid in the survival of sperm. Of these two, LH is more important: without FSH, there may be sperm malformations, but without LH there is complete infertility.

To prevent LH and FSH levels from climbing up too high, there are negative feedback loops in place. As a reminder, negative feedback is where production of a product inhibits production of more products. In the case of the male reproductive system, there are two main inhibitory hormones. The first, inhibin, is secreted by Sertoli cells. It inhibits release of FSH from the anterior pituitary. The second, testosterone, is from the Leydig cells, as mentioned above. Some testosterone goes back to the hypothalamus and anterior pituitary, where it is converted into oestradiol, which inhibits GnRH and LH release.

As an aside, the reason why some guys get "man boobs" on steroids is because the testosterone is being converted to oestradiol and inhibiting FSH and LH. Be careful with yo' steroids, kids! Additionally, high exposure to other chemicals that look like oestrogens, such as xeno-oestrogens and phyto-oestrogens, are hypothesised to be responsible for dropping sperm counts etc.

But oestrogen isn't all bad for guys. Oestrogens, including oestradiol, plays roles in the growth spurt, as well as the closing of epiphyseal growth plates near the ends of long bones. The reason why girls stop growing before guys is because girls have more oestrogens, so the epiphyseal growth plates close up sooner.

High alcohol consumption is also bad for fertility, in both men and women. Alcohol increases levels of β-endorphins, which make you feel good after drinking alcohol. At high levels, this can injure virtually all cells of the reproductive system, from GnRH-producing cells in the hypothalamus, to the LH- and FSH-secreting cells of the anterior pituitary, to the Sertoli and Leydig cells of the testes themselves.

The Female

Women are a bit more complicated, because monthly cycles and all that. Still, the general principle is the same: GnRH stimulates release of LH/FSH, which stimulate the gonads (except now we're talking about ovaries). Also, instead of androgens, we're now dealing mainly with oestrogens and progesterone.

Just as an aside, the female hormones and male hormones can interconvert. In fact the oestrogens pretty much all come from androstenedione (a weak androgen), which in turn comes from DHEA (dehydroepiandrosterone), which comes from the zona reticularis adrenal cortex. All of these eventually trace back to cholesterol, the mother of all steroid hormones. There's not much more to say on the female from this lecture, but not to worry- the next post will focus almost entirely on the female.

Puberty

So what causes us to hit puberty in the first place? An increase in GnRH causes us to go through puberty, but what causes the change in GnRH to start with? In some cases, people may have GnRH-secreting tumours which cause them to go through puberty very young (i.e. younger than age 8-9), but that's not the case for everyone.

One hypothesis is that leptins, hormones secreted by adipose (fat) cells, cause us to go through puberty. The idea is that when we have enough fat, the leptin levels become high enough to stimulate the hypothalamus and anterior pituitary. If someone isn't nourished enough, or is still small, they may not go through puberty.

Stress and strenuous exercise can delay puberty. It's hypothesised that stress and strenuous exercise increase β-endorphins (just like alcohol), so these factors can have similarly destructive effects on our reproductive systems. Strenuous exercise may also decrease our fat reserves, decreasing leptins and thus delaying puberty.