Spermatogenesis Oogenesis

Spermatogenesis and Oogenesis

Meiosis, the process by which gametes are formed, can also be called gametogenesis, literally “creation of gametes.” The specific type of meiosis that forms sperm is called spermatogenesis, while the formation of egg cells, or ova, is called oogenesis. The most important thing you need to remember about both processes is that they occur through meiosis, but there are a few specific distinctions between them.

Spermatogenesis

The male testes have tiny tubules containing diploid cells called spermatogonium that mature to become sperm. The basic function of spermatogenesis is to turn each one of the diploid spermatogonium into four haploid sperm cells. This quadrupling is accomplished through the meiotic cell division detailed in the last section. During interphase before meiosis I, the spermatogonium’s 46 single chromosomes are replicated to form 46 pairs of sister chromatids, which then exchange genetic material through synapsis before the first meiotic division. In meiosis II, the two daughter cells go through a second division to yield four cells containing a unique set of 23 single chromosomes that ultimately mature into four sperm cells. Starting at puberty, a male will produce literally millions of sperm every single day for the rest of his life.

Oogenesis

Just like spermatogenesis, oogenesis involves the formation of haploid cells from an original diploid cell, called a primary oocyte, through meiosis. The female ovaries contain the primary oocytes. There are two major differences between the male and female production of gametes. First of all, oogenesis only leads to the production of one final ovum, or egg cell, from each primary oocyte (in contrast to the four sperm that are generated from every spermatogonium). Of the four daughter cells that are produced when the primary oocyte divides meiotically, three come out much smaller than the fourth. These smaller cells, called polar bodies, eventually disintegrate, leaving only the larger ovum as the final product of oogenesis. The production of one egg cell via oogenesis normally occurs only once a month, from puberty to menopause.

As we all know spermatogenesis and oogenesis are two processes that are initiated in the human gonads to produce gametes.

Let us highlight some facts about these two series of events: facts about the number of cells produced per each germ stem cell, the number of chromosomes in each cell and the number of chromatids per each chromosome.

- Each human Spermatogonium ( germ stem cell) gives rise to manySpermatogonia by mitosis. So the number of chromosomes in eachSpermatogonium cell is conserved at 46 chromosomes.

- Each Spermatogonium will then grow and develop into a Spermatocyte Iwhich will duplicate its DNA so that each of its 46 chromosomes will have two chromatids per each chromosome at the end of the growth stage.

- Each Spermatocyte I will divide by Meiosis I ( reductional meiosis) to give rise to two haploid Spermatocyte II each having 23 chromosomes and each chromosome is made up of two chromatids.

- Each Spermatocyte II will then divide by Meiosis II ( equational meiosis) to give rise to two Spermatids. Each spermatid will have 23 chromosomes but the number of chromatids per chromosome is ONE.

- Since the Spermatid only changes in shape and does not divide to produce aSperm, the number of chromosomes and chromatids will stay the same as in the spermatid. The Sperm will have 23 chromosomes ( one chromatid per each chromosome).

What about the cells of the oogenesis process?

Number of Chromosomes Number of

Chromatids Per each Chromosome

Oogonium 46 1 before interphase

2 after interphase

Oocyte I 46 2 (at the end of the growth phase)

...