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Sunday, January 06, 2008

Polar bodies are formed during Oogenesis

010. Polar bodies are formed during :

1. Spermatogenesis.

2. Organogenesis.

3. Oogenesis.

4. Morphogenesis.


3. Oogenesis.


Gray 38th Edition Page 123





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The poles of the spindle in oöcytes lack centrioles and are composed of pericentriolar material. As anaphase approaches, the homologous pairs of chromosomes move towards the poles of the spindle. A bulge forms at the site of the spindle which is destined to become the first polar body. The midbody forms around the spindle and initiates cleavage of the first polar body and final separation of the homologous chromosomes occurs. Unlike the equal division of the nucleus, the division of the cytoplasm is highly unequal, the polar body carrying with it its numerically equal chromosomal complement and an exiguous share of the cytoplasm. (The first polar body is occasionally seen to cleave into two equal fragments before degenerating sometime after ovulation.)


The oöcyte resulting from this reduction division is known as the secondary oöcyte; it contains 23 double-stranded chromosomes. In the absence of an interphase the chromosomes are rearranged around the equator of a second spindle. At metaphase of the second meiotic division the secondary oöcyte arrests until fertilization or parthenogenetic activation stimulates the completion of meiosis which is marked by extrusion of the second polar body.


Accumulation of lecithin like substance the deutoplasm within the cytoplasm of oocytes makes the nucleus eccentric in position. This explains why in female, the primary oocyte gives rise to a single mature ovum with the extrusion of first and second inactive polar bodies in the process of maturation of female gametes. But in similar process of maturation of male gametes, one primary spermatocyte gives rise to four active spermatids because nucleus of the primitive male sex cells is central in position.

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