Overview
This lecture explains the process of egg (oocyte) development in the female reproductive cycle, focusing on oogenesis, meiotic arrest, ovulation, and fertilization.
Egg Formation and Oogenesis
- Oogenesis is the creation of eggs (oocytes) in the ovaries during fetal development.
- All eggs are produced before birth; a baby girl has her full lifetime supply of eggs at birth.
- Germ cells called oogonia undergo mitosis, producing 2-4 million cells by the 7th month of fetal life.
- Oogonia then develop into primary oocytes, each with two sets of chromosomes (2n).
- Primary oocytes begin meiosis 1 but stop partway; this arrest is called "meiotic arrest."
- At birth, all primary oocytes are in meiotic arrest.
The Reproductive Cycle and Ovulation
- At puberty, the reproductive cycle starts, and primary oocytes resume development monthly.
- Each month, typically one primary oocyte completes meiosis 1, becoming a secondary oocyte and a smaller polar body.
- The secondary oocyte retains most of the cytoplasm; the polar body degenerates.
- Ovulation is the release of the mature secondary oocyte from the ovary into the uterine tube.
Fertilization and Completion of Meiosis
- Meiosis 2 is completed only if a sperm fertilizes the secondary oocyte.
- Upon fertilization, the secondary oocyte divides, forming another polar body and an ovum.
- The egg is termed an ovum only after fertilization; before that, it's a secondary oocyte.
- The nuclei of sperm and ovum fuse, forming a zygote.
- If fertilization does not occur, the secondary oocyte is expelled during menstruation.
Hormonal Functions of Ovaries
- Ovaries secrete female sex hormones: estrogen, progesterone, and inhibin.
Key Terms & Definitions
- Oogenesis — The process of egg formation in the ovaries.
- Oocyte — An immature egg cell.
- Oogonia — Precursor germ cells that become oocytes.
- Primary oocyte — An oocyte arrested in meiosis 1.
- Meiotic arrest — Pause in the completion of meiosis in primary oocytes.
- Secondary oocyte — The cell formed after meiosis 1, ready for ovulation.
- Polar body — Small cell formed during meiosis that usually degenerates.
- Ovum — The fertilized egg cell.
- Zygote — The cell formed when sperm and egg nuclei fuse.
Action Items / Next Steps
- Review and memorize the stages of oogenesis and the roles of primary and secondary oocytes.
- Study the hormonal functions of the ovaries (estrogen, progesterone, inhibin).