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Gametogenesis:
  • sperms and oocyte,the male and female gametes,are highly specialized sex cells.
  • the formation of male gametes is called spermatogenesis and formation of female gametes is called oogenesis.
  • gametes are formed during meiosis in which number of chromosomes are reduced by half.
  • primordial germ cells originate in the wall of the yolk sac of the embryo and migrate into the gonad region.
Meiosis:
  • it is a special type of cell division that involve two meiotic cell divisions(first meiotic division and second meiotic division),which occurs only during the production of gametes.
  • it result in the formation of four gametes containing 23 chromosomes and 1N amount of DNA.
  • it provides constancy of the chromosome number from generation to generation by reducing the chromosome number from diploid to haploid,thereby producing haploid gametes.
  • Allows random assortment of maternal and paternal chromosomes between the gamtes.
  • In meiosis crossing over of chromosome segments shuffles the genes and produces a recombination of   genetic material.
Disturbances of meiosis during gametogenesis.e.g nondisjunction,result in the formation of abnormal gametes.These gamtes can cause abnormal development such as occur in infants with Down syndrome.

Oogenesis:
  • Primordial germ cells(46,2N) arrive in the ovary at week four of embryonic development and differentiate into oogonia(46,2N).
  • Oogonia enter first meiotic division and undergo DNA replication to form primary oocytes(46,4N).
  • All primary oocytes are formed by the fifth month of fetal life and  remain dormant in prophase (diplotene) of meiosis 1 until puberty.
  • during women ovarian cycle.a primary oocyte complete the meiosis1 to form the secondary oocyte(23,2N) and first polar body which will degenerate.
  • secondary oocyte enters second meiotic division and ovulation occur when chromosomes align at metaphase.The secondary oocyte remains arrested in metaphase of second meiotic division until fertilization occur.
  • At fertilization,secondary oocyte completes second meiotic division to form a muture oocyte(23,1N) and second polar body.
Spermatogenesis:
  • Primordial germ cells(46,2N) arrive in the indifferent gonad at week four of embryonic development and remain dormant until puberty.
  • At puberty, Primordial germ cells differentiate into type A spermatogonia.which serve as stem cell throughout adult life.
  • some type A spermatogonia differentiate into type B spermatogonia.
  • Type B spermatogonia enter first meiotic division to form primary spermatocytes.
  • Primary spermatocytes form two secondary spermatocytes.
  • Secondary spermatocytes form two spermatids.
  • Spermatids undergo spermatogenesis,which result in mature sperms.

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