One of the fascinating things about science is that its conclusions are constantly changing. Because of new experimental techniques and closer investigation, many “scientific facts” that I was taught at university are now known to be false. This makes science interesting, to say the least! As someone who has published original research that has drawn conclusions regarding the nature of the atomic nucleus, I often wonder how long it will take for some of those conclusions to be demonstrated false!
In a previous post, I discussed Bateman’s Principle, which some evolutionists considered to be a scientific law. However, we now know that not only is Bateman’s Principle not true in many, many species, it was also based on faulty experiments. In the course of the discussion that followed, a commenter mentioned another scientific “law” that is probably wrong – the idea that a woman is born with all the egg cells she will ever have. I decided to look into this topic, and I was amazed at what I had missed in the course of my normal scientific reading. Thank you, Shevrae, for alerting me to the new advances in this area.
In case you didn’t have a detailed course on human anatomy and physiology, you might not know that it has been considered a scientific fact since the 1950s that when a woman is born, she has all the egg cells she will ever have. This “fact” is based on some really good research. During the fourth month of development in the womb, it has been shown that female babies start producing oogonia, which are cells that start to develop into egg cells. However, they are stopped early in their development and are surrounded by a protective layer of cells. This structure (the cell that is “frozen” in development and its protective case) is called a primordial follicle.
When a woman is born, she has hundreds of thousands (sometimes millions) of primordial follicles in each ovary, but some degenerate during childhood. When she reaches puberty, she generally has about 400,000 primordial follicles in her ovaries. After puberty, hormone cycles regularly cause some of the primoridal follicles to continue development. The nature of the protective cell layer changes, and the cell inside the protective layer continues its development into an egg cell. Interestingly enough, however, the process will not fully complete unless the developing egg cell is fertilized by a sperm. If no fertilization takes place, the cell that has been developing dies without ever forming a true egg cell.
All of the above statements are (as far as we can tell) true. Based on these facts, it has been taught for more than 60 years that since a woman starts out with hundreds of thousands of primordial follicles when she is born, she never makes any new ones. Thus, a woman is born with all the potential egg cells she will ever have. New research indicates that this conclusion is false.
The story starts in 2004, when Jonathan Tilly and his colleagues were looking at the process of egg production and the death of developing eggs in mice. What they found was that the number of dying “eggs-in-development” greatly outnumbered the primordial follicles that the mice started out with. As a result, they concluded that something must be making new primordial follicles. In the end, they found some stem cells in the mice ovaries that were doing the job.1 Since then, they have found that the primordial follicles produced by the stems cells end up developing normally. They can even be fertilized and develop into normal mice.2 Thus, there seems to be no question that female mice produce lots of egg cells even after they are born.
Since mouse reproduction had not been studied nearly as closely as human reproduction, it was thought that perhaps this was a special case for mice and maybe other specific kinds of mammals. However, Tilly and his colleagues have been busy. In their most recent study, they looked at ovaries that were removed from women who were undergoing “sex reassignment” surgery. They studied the cells in those ovaries and found that about 1.7 percent of them fit the molecular profile of a stem cell. They also showed that when transplanted into another ovary, those stem cells produced new primordial follicles.3 Now they have not yet shown that these primoridial follicles will follow the normal development path and can be fertilized, as is the case in mice. However, I expect that is only a matter of time.
So let’s suppose that this research holds up over time and we can conclusively state that ovary stem cells do produce new egg cells over the course of a woman’s life. This leads to an obvious question: “If a woman can make egg cells over the course of her life, why start out with so many when she is born?” Here’s my thought: Any good engineer will design redundancies in an important system. The reproductive system is obviously important, so the “starter batch” of primordial follicles with which a woman begins life is simply a redundancy designed to ensure reproduction, even if the process that produces new egg cells is halted for some reason.
Regardless of whether or not my thought has any merit, we now know that something taught as “scientific fact” for more than 60 years has been demonstrated to be false. That’s the nature of science, and it certainly makes my life as a scientist very interesting!
1. Johnson J, Canning J, Kaneko T, Pru JK, and Tilly JL., “Germline stem cells and follicular renewal in the postnatal mammalian ovary,” Nature 428:145-150, 2004.
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2. Tina Hesman Saey, “Adult Women Might Replenish Eggs,” Science News, April 7, 2012, p. 8
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3. Yvonne A R White, Dori C Woods, Yasushi Takai, Osamu Ishihara, Hiroyuki Seki, and Jonathan L Tilly, “Oocyte formation by mitotically active germ cells purified from ovaries of reproductive-age women,” Nature Medicine 18:413–421, 2012.
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