In which contestants – egg and sperm – must come together to get to the next round
Germ cells (eggs and sperm) are special. Uniquely, each has the potential to contribute to an entire new organism and is specially adapted for that job. For example, sperm cells have tails (flagella) that propel them towards the egg, while eggs contain granules that are released after fertilisation to form a barrier against other sperm.
It was August Weismann who proposed, in 1893, that the cells producing eggs and sperm were different from the cells in the rest of the body. In his view the body was just a housing for the germ cells. In other words, a hen is only an egg’s way of making another egg.
The germ cells are indeed different. Early in development, they are set apart from the cells that will form the rest of the body, move through the embryo to the gonads, and mature into germline stem cells that can produce eggs or sperm; during a lifetime a woman may ovulate several hundred eggs, while a man may produce more than a trillion sperm. Sex cells have only half as many chromosomes as other cells. But that is not their only difference. When eggs and sperm are being made, their DNA is chemically modified at specific locations, a process known as imprinting (see ‘Male and female genomes’).
Researchers are investigating ways to make artificial germ cells. Nobody has yet managed to grow spermatozoa outside the body, partly because these cells seem to need the complex environment found in the tissues where they grow.
Sperm from sperm cells?
An alternative approach is to use stem cell technologies. Stem cells have the potential to develop into many different types of cell. In mice early versions of egg and sperm cells have been made from skin cells. The pre-sperm cells can be encouraged to grow into fully fledged sperm in the testes of live mice and can then be injected into eggs to make mouse pups.
Although this approach is still experimental, and has not been tried in humans, it raises some fascinating medical and ethical issues. For example, it could allow infertile women – or even men – to generate ‘artificial’ eggs carrying their own genes. (A woman would not be able to make sperm, as this requires genes on the Y chromosome. See our ‘Hormones at work’ article.) Theoretically, a gay couple could create a baby in which they had each contributed equally to its genetic make-up. How would we react to that?Lead image:
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