What causes menopause?

This started as an argument conversation with my wife. I wondered aloud what causes menopause. She marveled at my stupidity; had I never heard that menopause happens when a woman runs out of eggs? I guess I had heard that, but it sounded like the sort of thing that........is........well, wrong.

I instinctively turned to Wikipedia, which only muddied the waters:
A natural or physiological menopause is that which occurs as a part of a woman's normal aging process. It is the result of the eventual atresia of almost all oocytes in the ovaries.
...but then (of course) there's no article about atresia. Hmmm, so lack of eggs causes menopause, but what causes the lack of eggs? Menstruation, right?

Wrong. A quick Google search turns up this tidbit:
Menopause occurs when a woman's ovaries run out of functioning eggs. At the time of birth, most females have about 1-3 million eggs, which are gradually lost throughout a woman's life. By the time of a girl's first menstrual period, she has an average of about 400,000 eggs. By the time of menopause, a woman may have fewer than 10,000. A small percentage of these eggs is lost through normal ovulation (the monthly cycle). Most die off through a process called atresia, which is not well understood.
So, menstruation is responsible for only a tiny percentage of egg loss (or, more properly, oocyte loss). And again with the atresia, only this time we have the helpful (not) addition that it's "not well understood."

So, we're back to square one: what causes menopause? By this time, I don't really care, because I've convinced myself that what I first thought is right: menopause is like puberty, it's triggered by one's age (with some exceptions). Yeah, sure, the proximate cause is that you run out of eggs, but that's like saying puberty is caused by all the new hormones rushing through your body. True, but not very useful because it doesn't tell you what creates all those new hormones.

One bright side to this boring story is that I got to read about the "grandmother hypothesis." I recommend it highly (if you like "just-so stories" about evolution).

No comments: