Origin of the germ cells

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(Bài của Dương Minh Hoàng, yk34) The gonadal ridge

For both sexes the gonads arise in the gonadal ridges (inset in figure 5). These arebilateral, ridge-like protrusions that appear ventromedially to the nephrogenic cord. They are generated in the 5th week through the proliferation of the coelomic epithelium and the thickening of the underlying mesenchyma. At this point, the gonadal ridge represents the primitive gonadal primordium. In order for this to develop into the definitive and gender-specific gonads, the immigration of the primordial germ cells is necessary.
Indifferent gonadal primordium
In the 6th week, the primordial germ cells infiltrate into both gonadal ridges. The primordial germ cells become surrounded by the coelomic epithelial cells that have proliferated and advanced into the depths of the mesenchyma. These germinal cordsare still connected with the surface of the coelomic epithelium. At this point, the male and female gonadal primordia cannot be distinguished and, for this reason, this condition is referred to as the indifferent gonadal primordium.

Commentary
The gonads (= gonadal ridge + primordial germ cells) are extended, arched bodies in the somatic cavity.
Their cranial and caudal parts later form themselves back into gonadal ligaments, while the remainder proliferates further to testis and ovary, which soon exceeds the mesonephros in size.




Determining the gender

The key to sexual differentiation lies on the Y chromosome in the SRY (sex determining region of the Y chromosome). There the testis-determining factor (TDF) is found that induces male development. Among other substances, testosterone is formed beginning with the 7th week.
If no Y chromosome - and thus no SRY - is present, a feminine phenotype is engendered.

Summary

The gender of an embryo is determined at the moment of fertilization and depends on whether the spermatozoon carries an X or a Y chromosome. In XX embryos the germinal cords do not grow as far as the medulla and the cortical cords envelop the oogonia. In XY embryos, on the other hand, the medullary cords become the testicular cords that also grow into the depths and establish contact with the mesonephros. .