端木·宇 2008-6-19 22:14
Spermatogenesis and Oogenesis
Meiosis, the process by which gametes are formed, can also be called [b]gametogenesis[/b],literally “creation of gametes.” The specific type of meiosis thatforms sperm is called spermatogenesis, while the formation of eggcells, or ova, is called oogenesis. The most important thing you needto remember about both processes is that they occur through meiosis,but there are a few specific distinctions between them.
[b] Spermatogenesis[/b]
The male testes have tiny tubulescontaining diploid cells called spermatogonium that mature to becomesperm. The basic function of spermatogenesis is to turn each one of thediploid spermatogonium into four haploid sperm cells. This quadruplingis accomplished through the meiotic cell division detailed in the lastsection. During interphase before meiosis I, the spermatogonium’s 46single chromosomes are replicated to form 46 pairs of sisterchromatids, which then exchange genetic material through synapsisbefore the first meiotic division. In meiosis II, the two daughtercells go through a second division to yield four cells containing aunique set of 23 single chromosomes that ultimately mature into foursperm cells. Starting at puberty, a male will produce literallymillions of sperm every single day for the rest of his life.
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[b] Oogenesis[/b]
Just like spermatogenesis, oogenesis involvesthe formation of haploid cells from an original diploid cell, called aprimary oocyte, through meiosis. The female ovaries contain the primaryoocytes. There are two major differences between the male and femaleproduction of gametes. First of all, oogenesis only leads to theproduction of one final ovum, or egg cell, from each primary oocyte (incontrast to the four sperm that are generated from everyspermatogonium). Of the four daughter cells that are produced when theprimary oocyte divides meiotically, three come out much smaller thanthe fourth. These smaller cells, called polar bodies, eventuallydisintegrate, leaving only the larger ovum as the final product ofoogenesis. The production of one egg cell via oogenesis normally occursonly once a month, from puberty to menopause.
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