oogenesis
Summary
Oogenesis is the formation of female gametes by meiosis in the ovary. Primary oocytes arrest in prophase I, and the ovulated secondary oocyte arrests in metaphase II. Meiosis II is completed only if a sperm penetrates it.
Detail
Meaning: Oogenesis is the development of ova from oogonia in the ovary. Oogonia divide by mitosis before birth, then become primary oocytes (2n) that begin meiosis I and arrest in prophase I until puberty. Each cycle, FSH and the LH surge push one follicle's oocyte to finish meiosis I, producing a large secondary oocyte (n) and a small first polar body. The secondary oocyte starts meiosis II, arrests in metaphase II, and is ovulated. Meiosis II is completed only if a sperm penetrates it, giving the ovum and a second polar body. Unequal cytoplasmic division gives the one functional gamete most of the nutrients; polar bodies degrade.
On the MCAT: Expect questions on the two arrest points, ploidy at each stage, and comparison with spermatogenesis (one gamete per meiosis versus four; oocytes formed before birth versus sperm made continuously after puberty). Passages may link the long prophase I arrest to more nondisjunction with maternal age.
Common confusion: The ovulated cell is a secondary oocyte, not a mature ovum; meiosis II finishes after sperm entry.
Sources
- Campbell Biology
- Biology (Raven, Johnson, Mason)
- Human Physiology: An Integrated Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.