|MadSci Network: Cell Biology|
Cat: Thank you for your question. The development of the female reproductive system is very complicated, but also very interesting. This process begins very early in the development of the embryo. Specialized cells called Primordial Germ Cells (PGC)have been growing and developing in the tissue of an area called the Yolk Sac. About one month (4-6 weeks) after conception, these cells start moving into the embyro and take up residence in an area of the embryonic body wall. Once here, they make close associations with cells already there and form "primitive sex cords." The primitive sex cords grow and develop and become the "genital ridges". Each embryo will have genital ridges on both sides of the body. In female embryos (those without a Y-chromosome), the PGC's associated with the sex cords undergo mitosis forming ~200,000 oogonia (stem cells which will give rise to adult oocytes) by 8 weeks of development. By the 5th month of development a peek of 7,000,000 oogonia have formed. At this point, mitosis stops. Some oogonia will degenerate while others begin the process of meiosis. Meiosis is the process, by which the number of chromosomes is reduced from 23 pairs (diploid)to 23 individual (haploid) chromosomes, thus preparing the oocyte for eventual fertilization and return to the adult condition of diploidy. Once the oogonia begin meiosis they are called Primary Oocytes. Primary oocytes will proceed only part way into the process of meiosis. The development is stopped or arrested in the first phase of meiosis called "diplonema of Prophase I". There are approximately 2,000,000 Primary Oocytes arrested in prophase I at birth. As the girl grows and matures, most of these Primary Oocytes will degenerate, a process called atresia. Leaving about 400,000 to 500,000 Primary Oocytes in the ovary at puberty (the onset of sexual maturity). Puberty is when the menstrual cycle begins. Each month several primary oocytes will be provoked to start developing again and meiosis resumes. Usually only 1 of the group of oocytes which begins developing each month progresses to the point of maturation and is ovulated on day 14 of the menstrual cycle. (The average ovulation rate for women is 1.2 ovulations/month, meaning that most months only one cell is ovulated, but some months more than one may be released). I have included URLs for web sites which you may find helpful. http://www.student.loretto.org/anatomyphys/reproductive% 20system.htm http://cna.uc.edu/embryology/ http://zygote.swarthmore.ed u/chap20.html Thanks for you question. David Mallory Associate Professor of Biological Sciences Marshall University Huntington, WV USA 25755-2410
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