gonadal ridge

In embryology, the gonadal ridge (or genital ridge) is the precursor to the gonads and appears at approximately five weeks. The gonadal ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin.

Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells (pre-granulosa cells). It gives rise to the sex cords.

Formation : The urogenital folds (or urogenital ridges, or urethral folds) are an embryological structure which give rise to a portion of the external genitalia. The urogenital ridge, from which the urogenital system will derive, arises at approximately the 4th week of gestation in the intermediate mesoderm. It differentiates into two structures:

 the gonadal ridge is medial and gives rise to primarily reproductive system
 the nephrogenic cord is lateral and gives rise to primarily urinary system.

The indifferent gonad (gonadal ridge), identical in females and males, emerges on the ventromedial surface of the mesonephros as a derivative of the intermediate mesoderm.

The indifferent or bipotent gonad is formed by proliferation of the coelomic epithelium and a condensation of mesenchymal cells of mesonephric origin.

Primordial germ cells (PGCs) derive from the epiblast, the outer ectodermal layer of the embryo; they subsequently move to the yolk sac wall and then migrate along the dorsal mesentery of the hind gut to the gonadal ridge.

During migration, PGCs undergo cell division and, once in the genital ridge (by the end of the 5th week), lose their motility, begin to aggregate and continue to proliferate by mitosis.

See also

 urogenital development
 gonadal development
 urogenital folds
 sex cords