Histology by Charlotte L. Ownby, PhD. Links to beginning of Histology web site.

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Oogenesis: the production of female gametes, the ova   

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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.

2. These diploid oogonia undergo several mitotic divisions prior to or shortly after parturition, thus providing the developing ovary with a large supply of future ova (eggs). 

 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).
5. 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.

6. During this process the number of chromosomes is reduced from the diploid number (2N) to the haploid number (1N). 
7. This division is uneven in that although the chromosomes are divided equally, most of the cytoplasm stays with the oocyte.  The smaller polar body contains half the chromosomes but only a small amount of cytoplasm and will eventually degenerate.

8.After a primary oocyte completes the first meiotic division, it is called a secondary oocyte (1N).  In most species Meiosis I is completed just before ovulation (release of the ovum from the ovary).  However, in horses and dogs Meiosis I is completed after ovulation.  

9. If a secondary oocyte is not penetrated by a sperm, it will degenerate. 

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.

13. After a sperm enters the cytoplasm of the ovum, two pronuclei form, containing genetic material from the ovum or the sperm. 

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.

Quick Checks:

1.  Oogenesis is the process by which the following occurs:
Female gametes leave the surface of the ovary
Female gametes develop from small cells with a diploid number (2N) of chromosomes to larger cells with a haploid number (1N) of chromosomes.
Female gametes grow in size and number, but remain diploid (2N).

2.  A female animal is born with all of the female gametes it will ever have – some 2 million ova.  This large number of ova in the ovary is due to which process:
Meiosis
Mitosis

3.  In most female animals, including the human, developing ova are arrested in this phase of cell division until puberty.  Then under the influence of this hormone, a small number complete the cell division and then develop into mature ova ready for ovulation.
Prophase of Mitosis; FSH
Prophase of Meiosis I; FSH

4.  In most species fertilization occurs prior to the completion of:
Mitosis
Meiosis I
Meiosis II

         

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