Spermatogenesis and Oogenesis Made Simple!

gametogenesis

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 many Spermatogonia by mitosis. So the number of chromosomes in each Spermatogonium cell is conserved at 46 chromosomes.

– Each Spermatogonium will then grow and develop into a Spermatocyte I which 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 a Sperm, 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)
Oocyte II /
First Polar body
23 2
Ootid /
Second Polar body
23 1
Ovum 23 1
– Each Oogonium will give rise to many oogonia.
– One Oogonium will grow into One Oocyte I.
– One Oocyte I will divide into One Oocyte II and One Polar body ( 1st polar body).
– One Oocyte II will divide into One Ootid ( quickly changes into Ovum) and one polar body (the second polar body).The first polar body will also divide into 2 polar bodies that will stay near the Ootid that will later change into the ovum.
– At last, each Oocyte I will have given rise to only One functional Ovum and 3 polar bodies. Whereas, each spermatocyte I will give rise to four functional Spermatozoa!
Note that after fertilization, the female pronucleus ( that of the ovum)  and the male pronucleus ( coming from the sperm) will fuse and mix their chromosomes.
During this process, DNA replication takes place so that the Zygote cell will have – after the fusion – 46 chromosomes , each chromosome made up of two chromatids and this way it can enter mitosis quickly and efficiently!
Good Luck in your Exam!
Yusser CHIDIAC 🙂
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9 responses to “Spermatogenesis and Oogenesis Made Simple!

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  3. Thanks! Very helpful! !

  4. What is the change that happened to the chromosomes in the spermatids ?

  5. What was the shape of chromosomes in the secondary spermatocyte and the spermatids ?

    • The number of chromosomes in the secondary spermatocyte are 23 but each chromosome having two chromatids, while the chromosomes in the spermatid are also 23 but each chromosome is made up of one chromatid.

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