MadSci Network: Cell Biology

Re: When a woman is born how many cells are capable of becoming ova?

Date: Mon Nov 3 13:09:56 2003
Posted By: David Mallory, Faculty, Biological Sciences, Marshall University
Area of science: Cell Biology
ID: 1067164613.Cb

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.
20system.htm http://zygote.swarthmore.ed

Thanks for you question.

David Mallory
Associate Professor of Biological Sciences
Marshall University
Huntington, WV USA  25755-2410

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