- 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.
- 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.
- 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.
- 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.