Oogenesis

In humans mature eggs are generated through the process of oogenesis.

Oogenesis commences during embryonic development in the forming female ovaries. Here diploid precursor cells individually termed oogonium enter mitosis, and divide to form primary oocytes. Each of which is surrounded by supporting cells.

Such oocytes begin the first stage of meiosis. Meiosis I, which proceeds until maternal and paternal chromosomes exchange information. At this point meiosis halts due to support cell derived inhibitory factors.

The process only restarts during puberty. Here menstrual cycle signals cause a primary oocyte to enlarge and reinter Meiosis I, producing two haploid cells of unequal sizes. The larger cytoplasm rich cell is termed a secondary oocyte, while the smaller first polar body eventually dissolves.

The secondary oocyte briefly enters the second phase of meiosis, meiosis II. But this process again stops. Ovulation then releases this cell and those surrounding it from the ovary. And the complex migrates into a fallopian tube where fertilization occurs if sperm are present.

Meiosis resumes post fertilization and ends with a secondary oocyte dividing forming an undersized second polar body which similarly dissipates, and a larger ovum. The latter is the haploid mature egg in which distinct paternal and maternal nuclei are transiently visible.

Thus oogenesis only yields a single mature egg cell for every primary oocyte that successfully completes the process.