Men can father children at an older age than women can have a baby. So far, but maybe women will some day be able to last a long as a man in fertility years.
Men can do fatherhood longer in life because testicles' regenerate ' or more accurately continually are able to produce more sperm no matter how old a male gets. In contrast, ovaries have a catch of eggs sitting there that can be accessed until they are gone.
Women have their peak numbers of eggs in fetal life: now we think there are about 7 million at 20 weeks of pregnancy (I have told patients 4 million, studies vary in what they say), and by puberty a women will only have 300,000 potentially fertile eggs, and women only ovulate about 400 eggs in our lifetime. We have been saying that 99.9% of the eggs of the ovary die, and of those we are born with, this is true. As long as women have eggs in their ovaries they are not menopausal and not infertile. When a woman runs out of eggs she becomes both menopausal and infertile.
Tilly and his co-workers in a study a few years ago wanted to figure out more about why this happens, and began to study mice ovaries. And to their surprise, they found evidence to challenge the theory of only eggs that we are born with. These researchers have found, in mice, that there are stem cells in ovaries, and not only do they exist, but they are important to maintaining our fertility and renewing some of these lost eggs. These stem cells, if found and put back in the menopausal ovary they can make new eggs. This would help women who became sterile from cancer treatments. Or perhaps the stem cells from the ovary could be cultured in a dish and be a source of eggs for women undergoing IVF. A gene called Stra8 controls the process of these stem cells. How they have proved that these cells exist in mice and these researchers now feel that all mammals will have these capabilities, even humans! His goal is to be able to stimulate this stem cell maintenance and thus allow women to have ongoing ovarian function for years past the normal age! Testicles do this: they regenerate new sperm about every 120 days, if they can make human egg stem cells stay away during menopause the ovaries will now behave as testicles! He’s already finding these egg stem cells in young and menopausal human females!