Oogenesis

In the foetus

Like in spermatogenesis, oogonia regenerate using mitosis – but only in the foetus. Before birth, the oogonia all develop into primary oocytes, and begin meiosis – but they do not get past DNA replication before stopping in meiotic arrest. This continues until puberty, when ovaries begin activity again. The following process happens inside a follicle:

Secondary oocytes

These form by completing the first stage of meiosis from the primary oocytes. One daughter cell receives all the cytoplasm, the other become a small and useless polar body. The good daughter cell begins the second stage of meiosis at ovulation and becomes…

Ovum

Again, the other part of the division has no cytoplasm, and is called a polar body. The other daughter cell is the ovum. To compare with the process with sperm, in men, unlimited spermatogonia produce primary spermatocytes, which undergo meiosis to form 4 spermatozoa. In females, a limited number of primary oocytes undergo meiosis to form just 1 ovum.

Follicles

The last stage of that oogenesis happens inside a follicle. Its pretty complex, but lets just deal with the fact there are five main things in it:

• one ovum – obviously.
• granulosa cells – sensitive to FSH, and produces oestrogen and inhibin.
• theca cells – sensitive to LH, which makes them proliferate, and make androgens which the granulosas use to make oestrogen.
• zona pellucida – kind of shell round the ovum.
• antrum – fluid filled space.

At the beginning of the menstrual cycle, 10-25 follicles start to develop, but generally only one matures, due to falling FSH levels. Don’t worry why it falls. The dominant one, as it is called, survives because its granulosa cells become more sensitive to FSH due to having more receptors, and they become sensitive to LH too. Read more in the menstrual cycle section.