These days, we don’t think twice about the continuity of life on earth, and accept without question that cells only arise from pre-existing cells; this is all so integral to the biologist’s world view that a number of great mysteries hardly ever come to light. Broadly speaking, these underlie the topic of this collection of essays about oogenesis. How does the germ-plasm manage to avoid the body’s mortality?
The details of how eggs comes to be eggs are fascinating and instructive well beyond the relatively narrow field of reproductive biology. Likewise the events just before and after fertilization, when the egg meets the sperm and starts to become a new body.
The follow a kind of chronological or developmental order from questions about sex determination to assisted reproduction. The simple-sounding decision of what sex to become is anything but, and we are reminded that it is quite possible to be a hermaphrodite and survive perfectly successfully. The setting-aside of germ cells from the soma appear early in development as well as the surprisingly complicated decision-making processes that lead to the differentiation of eggs or sperm. Meiosis is a necessary common process for both kinds of gamete, and we have known about meiotic chromosome pairing and homologous recombination. In oocytes, the meiotic divisions often take place shortly before the cell becomes a fully-fledged, fertilizable egg, and is subject to some elaborate controls that are still far from completely understood.
The regulation of the cell cycle during the life of an oocyte is extremely interesting, with multiple arrest points. There is tremendous specificity and variability from organism to organism, bewildering to the unwary. In some species, it is the arrival of the sperm that reinitiates meiosis. In others, hormonal signals prepare the oocyte for fertilization, and elaborate mechanisms exist to ensure that the sperm hits the egg at the right phase of the cell cycle. So clams release into the sea and the arrival of the sperm initiates completion of meiosis; frogs and women lay eggs that are arrested in second meiotic metaphase waiting for the sperm to arrive, but sea urchins complete both meiotic divisions and arrest in a dormant G-zero state to await fertilization. Limpets and starfish eggs like to be fertilized while meiotic divisions are in progress; sometimes one marvels that there are any successful matings at all. Extensive studies have gradually revealed the core signaling components required for oocytes to wait for the sperm, and show how common components can be used and reused in different ways to achieve the same end by a variety of routes.