ova

ova An ovum (plural ova) is a mature egg released at ovulation. In humans only one egg is normally shed, from one of the ovaries, about 14 days after the start of each 28-day menstrual cycle. This contrasts with the massive output of sperm from the testes, which begins at puberty and continues throughout life. Thus females have a different approach to processing germ cells for fertilization — a process known as oogenesis (see figure).

In both male and female fetuses ‘primordial germ cells’ migrate from the yolk sac into the site where the gonads will develop; this process is complete about 30 days after conception. In a female fetus, in the absence of male sex hormones, the ‘indifferent’ gonad develops into an ovary and the primordial germ cells begin to divide by a process of mitosis giving rise to oogonia (compare spermatogonia in the testes). This phase of mitotic proliferation, in which daughter cells have the normal 23 pairs of chromosomes (including the X-sex chromosomes), begins at 25–30 days of fetal life and continues almost up until the time of birth. Once the oogonia have been formed they begin their first meiosis. This type of cell division is also known as reduction division, because it gives rise to two ‘haploid’ daughter cells containing half the normal number of chromosomes — 23 single ones instead of 23 pairs. However, as the oogonia embark on this process, they become surrounded by a layer of ovarian cells, forming ‘primordial follicles’, and the meiosis is arrested: the oogonia do not actually divide, and are now called primary oocytes. At birth each ovary contains about 200 000 primary oocytes and by puberty this number is reduced to about 40 000. Throughout life there is a continual degeneration of ova and their follicles, and during a woman's reproductive years less than 400 mature eggs will ever be released from the ovary at ovulation. Just before ovulation the ovum completes its first meiotic division which may have begun two, three or four decades earlier. This ends with the most extraordinary inequality. Half of the chromosomes but almost all of the cellular substance (cytoplasm) goes to one cell, which becomes the secondary oocyte. The other half of the chromosomes are discarded in a very small bag of cytoplasm and form the polar body. However, unlike the male, where this type of division results in one cell containing an X-sex chromosome and the other the Y male-determining chromosome, both the oocyte and the polar body contain an X-sex chromosome.

The secondary oocyte and the polar body remain surrounded by the zona pellucida and the cumulus oophorus. Almost immediately after this event the secondary oocyte begins its second meiotic division but, just like the first meiotic division in the fetus, this process is arrested. The follicle ruptures and the secondary oocyte, surrounded by cumulus cells, bursts from the follicle. The ovulated egg plus cumulus is picked up from the peritoneal cavity by the nearby finger-like projections which extend from the open end of the Fallopian tubes. It is in the Fallopian tubes that fertilization takes place, and it is not until fertilization has occurred that the ovum completes its second meiotic division with the formation of a second polar body.

Saffron Whitehead

See also menstrual cycle; ovaries; sex hormones.