When a girl is born, she typically has between 1 and 2 million eggs in her ovaries (primary oocytes.) She will begin to lose these eggs early in life leaving her only 300,000 to 500,000 by the age of puberty.
As the eggs are formed, thin layers of cells (granulose cells) grow around them. The granulose cells and the oocyte are contained within a follicle. The follicle supports the egg it encloses for 50 years or more, providing it with nourishment but preventing it from maturing. The great majority of follicles and eggs never grow to maturity and ovulation. Follicles begin to develop, but if conditions for continued growth are not optimal, the eggs lose nourishment and die. The follicle cells then are absorbed back into the ovary (atresia.) Atresia continues throughout a woman’s life until all of the follicles are gone and she enters menopause.
During the hours leading up to ovulation when the mature egg is released from the dominant follicle, some important processes begin:
- The production of the LH hormone (lutenizing hormone rises in the blood).
- A few hours before ovulation, the primary oocyte undergoes a cell division process (meiosis) which results in an egg that contains 23 chromosomes
- The follicle cells surrounding the oocyte secrete mucous, forming a circular barrier (the cumulus) around the oocyte. Beneath the cumulus is a glassy looking membrane the zona pellucida. Sperm must be able to go through the cumulus and the zona pellucida of the egg in order to fertilize it.
- The egg is released and is picked up by the fimbriated ends of the fallopian tube.
- Fertilization usually occurs in the fallopian tube 12-24 hours after ovulation.