Like a Virgin, Prasad, Aarathi [free children's ebooks pdf .TXT] 📗

cells were kept apart from all our other cells, possibly as a way to protect the integrity of their essential genetic material, in case nutrients became scarce and reproduction had to be delayed. Germ cells, of course, have a special place in our life cycle because they are essentially immortal – they provide the fundamental link from one generation to another. They must also be able to re-create the entire organism. At the same time, as we have seen, they have a pernicious shelf life, made all the more frustrating to many people by the fact that each of us has a limited supply. The first question to tackle, then, is whether a woman’s body might somehow be triggered to make eggs on its own, without resorting to getting follicles from an outside source – in essence, creating a new supply of eggs. In fact, there is a debate around whether a woman is really born with all the eggs that she will ever have in her life, a debate that has been raging for nearly a hundred years, since the German anatomist Heinrich von Waldeyer-Hartz first proposed, in 1870, that all female mammals stop producing eggs around the time they are born.

‘Less evolved’ species, such as flies, birds, and fish, can and do generate new eggs throughout their adult life, which means that the ability to produce new eggs was lost somewhere during evolution, before the emergence of mammals. Yet the ability of males to produce sperm throughout adulthood was conserved in species from flies through to man. Why would the process of evolutionary selection deem it an advantage to endow women with only a fixed number of eggs that sit stagnant and are subjected to years, if not decades, of ageing before they erupt from a follicle during ovulation? The more logical, more robust approach would be to keep generating fresh eggs and fresh sperm. At least in theory. But theory aside, new evidence points to signs that mammals also have the potential to generate new eggs.

Stem cells have received a great deal of attention in the press for their ability to renew on their own, unlike all of our other cells, which only age and die off. Every type of cell, tissue, and organ in our bodies must be created from the genetic material contained in the egg and sperm, the precursors of the embryo. This power to generate the totality of components required to build a human is called totipotency. Stem cells derived from embryos are better suited to fulfil this role than those taken from an adult body, because in early embryos every cell has the potential to generate into any number of different cell types. As development proceeds, and cells become more fixed and decided as to their ultimate fate – that is to say, after a certain point in development, one cell will only be able to become a brain cell, another will only become a muscle cell, and so on – the stem cells lose their totipotent potential and become pluripotent, able to generate several different cell types but not all. But because extracting embryonic stem cells currently requires destroying an embryo, the technique is besieged with controversy, particularly in the US. Whether it will become possible to coerce adult stem cells into acting in a similar manner has still not been proven, though researchers are trying to revert skin stem cells to an embryonic state.

Regardless, egg-making stem cells have been found in the ovaries of adult mice, monkeys, and humans that retain stem cells with the capacity to renew the egg pool. At the moment, there’s still no evidence that these cells form new eggs naturally inside a woman’s body, but experiments are being conducted to see if they could be coaxed in a dish to make eggs. And if they could be coaxed to do the same inside a woman’s body, these stem cells could provide an unlimited supply of eggs, and also be used to postpone age-related ovarian failure and perhaps the menopause. Instead of receiving donor eggs or undergoing a tricky ovary transplant, a woman could receive a transplant of stem cells and let nature take its course.

For now, research into the precursors of eggs has yielded more auspicious results, including the closest anyone has come to creating a virgin birth in mammals. In the 1980s, scientists had made the first attempts to create mice with two fathers or two mothers and, as we have seen, these experiments failed because of the way in which genes are imprinted – turned on or off by chemicals within the DNA. Trying to create a baby from two sets of DNA regardless of their origin went nowhere; instead of getting one ‘dose’ of a gene, the offspring usually ended up with double the amount from one parent and none from the other. Then, in 2004, came the breakthrough: Kaguya. Created by a group of Japanese scientists, Kaguya the mouse was named after a mythical princess whose true parentage was unknown – she was found inside a bamboo reed. In contrast, Kaguya the mouse’s heritage could not have been better recorded. She was the first mammal to be born without a father and, what is more, the first animal in history to be born to two mothers.

The team, led by Tomohiro Kono at the Tokyo University of Agriculture, suspected that certain portions of the genome were posing the critical stumbling blocks when it came to imprinting. To circumvent these two problem regions, they realized they could turn to the biology of egg development. Remember that the genes silenced by imprinting are only silenced as the egg grows to maturity. By using DNA from an egg at an early stage of development, the scientists could gain access to these genes before they were locked. Kaguya was