Oogenesis and the Ovarian Cycle

Basic Facts Females are born with all of the eggs that they will be capable of releasing during their lives The ability to release eggs begins at the onset of puberty It will end during menopause which occurs somewhere in the 50’s for most women

Oogenesis Uses meiosis to produce the primary oocyte Oogonia (stem cells of females) produce the daughter cells called primary oocytes during development Primary oocytes push their way into the ovary where they are surrounded by a single layer of cells These cells become the primary follicles

Cont. By the time a female is born the oogonia no longer exist Around 2 million primary oocytes are left for release FSH is responsible for the monthly maturing and releasing of a number of primary oocytes And this release is the ovarian cycle

The Ovarian Cycle Each month a small number of oocytes are activated to grow larger and get ready for release Each follicle accumulates fluid in a central chamber called the antrum The primary oocyte will undergo replication so that it can begin meiosis The first round of this cell division produces 2 cells that are different in size The larger cell is the secondary oocyte The tiny sister cell is the polar body

Stages cont. At this point the mature follicle (vesicular follicle) will stick out from the surface of the ovary Process takes around 14 days and ovulation will occur at this point due to an increase in LH The follicle is surrounded by a capsule called the corona radiata Any left over mature follicles that are not released will deteriorate and be removed via the immune system In response to LH the follicle changes as well and becomes the corpus luteum

Cont. If it is fertilized the secondary oocyte will undergo a second meiotic division and will produce another polar body and the ovum The ovum is what will combine with the sperm to produce the fertilized egg If the secondary oocyte is not fertilized it will deteriorate as well as the polar bodies and be removed as part of the menstrual cycle.