Introduction to Human General Embryology Developmental Genetics

The phenomenon of human development begins with the fusion of the males and female germ cells (the gametes). Prior to that however, the gametes must develop and the series of events associated with Gamete development is termed Gametogenesis and occurs in the Gonads.

In males, gamete development occurs in the Testis and it is called Spermatogenesis, while in the females, it occurs in the ovary and called oogenesis.

In both cases, a pluripotent cell called the Primordial germ Cell (PGC) is involved. This cell develops early by the 3^rd to 4^th week of intrauterine life from the wall of the Yolk Sac and migrates though the embryonic dorsal mesentery by amoeboid movement, to the genital ridge from which the indifferent gonad develops. The cell is easily recognized because it contains lots of alkaline phosphatase. It is also considered that the determination of the PGC’s is one of the earliest events of embryogenesis.

Under the influence of Testis Determining Factor (TDF), the PGC’s in males transforms into spermatogonia while in the absence of TDF as it is the case for females, PGC’s transforms into Oogonia. Both spermatogonia and Oogonia undergo multiplicative cell division (mitosis) thereby ensuring a reservoir of cells for gamete development.

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  Differentiation of primordial germ cells in the male begins at puberty.
  

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  In the female, it begins in utero during the 3rd month of development.
  

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  At puberty as the testes descend into scrotum, the sex chords develop a central lumen and differentiate into the seminiferous tubules.
  

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  Germ cells in the male at birth in the sex cords of the testis are large, pale cells surrounded by supporting cells.
  

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  The supporting cells become sustentacular cells or sertoli cells.