Oogenesis

Oogenesis involves the formation of egg cells, or female gametes, in the ovaries.

In fetal development, diploid oogonia in the ovaries divides mitotically to become primary oocytes.

These primary oocytes start meiosis I but are arrested in late prophase I until puberty.

Each primary oocyte is enveloped by a layer of pre-granulosa cells, forming a primordial follicle.

By puberty, the ovaries have a fixed number of about 400,000 primordial follicles, each containing a primary oocyte.

Since menarche, follicle-stimulating hormone prompts a few follicles with primary oocytes to restart meiosis I each month.

Only the primary oocyte in the dominant follicle finishes meiosis I, resulting in two cells of unequal size: a secondary oocyte and a polar body.

The secondary oocyte begins meiosis II, pausing at metaphase II, and is released from the ovary following a luteinizing hormone surge.

If fertilized in the fallopian tube, it completes meiosis II, producing a haploid ovum and a polar body. If unfertilized, it degenerates without finishing meiosis II.