|Oogenesis: the production of female gametes, the ova|
Point to the paragraphs below and the described element will be
|1. Early in embryogenesis, primordial germ cells migrate from the yolk sac endoderm to the genital ridge (developing ovary) where they take up residence and are called oogonia.||3. When oogonia begin the first
meiotic division, they are called primary oocytes.
4. Primary oocytes are arrested in prophase of Meiosis I until the female reaches sexual maturity. They grow in size during this arrested phase, but do not divide. A human female is born with about 2 million primary oocytes in her ovaries, but by the time of puberty only about 400,000 are left due to atresia (degeneration).
When the female reaches
sexual maturity and under the influence of follicle stimulating hormone (FSH), a small
number of primary oocytes are stimulated to continue through Meiosis I.
process the number of chromosomes is reduced from the diploid number (2N) to the haploid
10. If fertilization and pregnancy do not occur, a new cycle will begin in which FSH from the pituitary gland will stimulate a few more primary oocytes to continue through Meiosis I. 11. The process is the same as previously described and a secondary oocyte is formed.
12. However, some of the time a sperm will penetrate the zona pellucida and the secondary oocyte is stimulated to continue through Meiosis II, forming a second polar body and a mature ovum (1N). Again, the polar body contains half of the chromosome material, but little cytoplasm, and it eventually degenerates.
|14. Fertilization is complete when the two pronuclei fuse and restore the diploid chromosome number.||15. If fertilization is completed, the zygote undergoes several mitotic changes to become an embryo; otherwise it degenerates.|
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