Note: This program first aired on Deecmber 29, 2012.
I was asked by a listener recently, if female chickens are born with all of the eggs they will lay over the course of their lives, like human females are. This made me wonder if all females have this characteristic, if that is a distinguishing feature of being female—that the cells that become the gametes form in the embryo, and are not replenished as the individual matures. Conventional wisdom states that we females have a set amount of eggs (more than likely more than we will release in the reproductively active period of our lives), we do not make more as we go along. This is in contrast to males, who are able to continually replenish their supply of gametes.
We need to pause for a moment and return to high school biology, and revisit the concepts of meiosis and mitosis. Our cells have two sets of DNA in them, one from our mother and one from our father. That DNA is in the form of chromosomes. Mitosis is simple cell division, we get two cells when we started with one. The chromosomes in the cell all line up, split in half, and each half then replicates itself in the new cell; each new cell ends up with two sets of chromosomes, just like the parent cell had. Meiosis is cell division that results in the formation of gametes. What makes gametes special is that they have only one set of DNA in them, a mash up of DNA from the mother and the father. In meiosis, the chromosome pairs line up, swap DNA and then get separated from each other into two separate cells. Then the chromosomes themselves split in half, and the halves end up in two more cells, yielding a total of four cells from the original one, each with a single set of genetic information. That is what makes gametes gametes, they have only one set of DNA and they need to join with another gamete cell to get a full set of genetic information, enough to make a new individual. The traditional thinking has been that , at least in mammals, meiosis occurs in the ovaries of the embryo. As a female’s reproductive organs are forming, meiosis is going on, and all the eggs the female is going to have are formed then.
Before we get to the chicken issue, I have to break it to you that the conventional wisdom here has been shown to be wrong. As early as 2004, researchers discovered that in mice, the females seemed to be able to maintain a supply of healthy living eggs, even when fed toxins that would kill their follicles. This meant that the mice could generate new eggs, a discovery that flies in the face of the conventional understanding that mammals are born with the only eggs they will ever have. Since then researchers have identified the stem cells in human ovaries, called oogonial stem cells, that do the same thing for us. These oogonial stem cells are able to undergo meiosis at any point and produce new oocytes or eggs. What isn’t clear is if these oogonial stem cells are just a back up system, or if they share responsibility for egg production with the original meiotic cells in the ovary.
So, on to chickens. It turns out this is a very difficult question to answer definitively. Most of the literature refers to the “all the eggs you’ll ever make” phenomena as occurring in either mammals, or “higher animals ” (which by the way are apparently all vertebrates except fish). There is, thus far, no evidence of oogonial stem cells in chickens, but that doesn’t mean they aren’t there, it just means no one has looked, which is a different thing altogether. Likewise, this conventional wisdom principle has likely been laid like a blanket on top of our knowledge of all “higher animals”, meaning, it is unlikely that scientists have studied the embryos of every species close enough to declare that all higher animal females are always born with all their eggs. To be fair, closely related things usually behave in closely related ways, so this blanket technique may not be as imprecise as I am making it sound. We do know that certain worms continually make new oocytes, so it is not unheard of in that natural world, but chickens are closer to the higher animals than to round worms.
The final word? I suspect that chickens, like humans, make most of their eggs during the development of the embryo, and may, like humans, potentially retain the ability to generate new eggs as needed. This question illustrates nicely, the limits of our knowledge, and way we construct what we know, or think we know about the natural world. Thanks Greg, and keep your questions coming.
References:
From Science Daily, a digest of the original study from Massachusetts General Hospital and the University of Edinburgh, demonstrating that female mice can grow new eggs: http://www.sciencedaily.com/releases/2012/07/120726180259.htm
National Geographic’s take on the same study: http://news.nationalgeographic.com/news/2012/02/120229-women-health-ovaries-eggs-reproduction-science/
Need a review of mitosis and meiosis? Check out this nice slide show from the good folks at PBS: http://www.pbs.org/wgbh/nova/miracle/divide.html
A bit here on the traditional take on female oocyte development: http://anthro.palomar.edu/biobasis/bio_2.htm
Whoa. Here’s a scientific article about what can regulate meiosis in chickens!! http://www.biomedcentral.com/1471-213X/8/85
The truth? Its complicated… http://rstb.royalsocietypublishing.org/content/277/955/201.short
