3.1 The germline - origin of the germ cells



The gonadal ridge

Quiz

Quiz 05



For both sexes the gonads arise in the gonadal ridges (inset in figure 5). These are bilateral, 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.



Fig. 5 - Immigration of the primordial germ cells into the gonadal ridge
in the 5-6th week
 Legend

1
2
3
4
5
6
7
Proliferating coelomic epithelium
Thickening of the mesenchyma
Germinal cords
Primordial germ cells (red dots)
Mesenchyma
Allantois
Vitelline
8
9
10
11
12
13
14
Intestinal tube
Dorsal mesentery
Gonadal ridge
Nephrogenic cord
Mesonephric (Wolffian) duct
Mesonephric tubule
Aorta

Fig. 5
Inset:

primordium of the gonadal ridge at around the 5th week.

Large picture: formation of the undifferentiated gonadal primordium in the 6th week with the immigration of the primordial germ cells (red dots)



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 cords are 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.



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