Development of the Reproductive Organs
The Gonads
Indifferent Stage
In the first stage of gonadal development, it is impossible to distinguish between the male and female gonad. Thus, it is known as the indifferent stage.
The gonads begin as genital ridges – a pair of longitudinal ridges derived from intermediate mesoderm and overlying epithelium. They initially do not contain any germ cells.
In the fourth week, germ cells begin to migrate from the endoderm lining of the yolk sac to the genital ridges, via the dorsal mesentery of the hindgut. They reach the genital ridges in the sixth week.
Simultaneously, the epithelium of the genital ridges proliferates and penetrates the intermediate mesoderm to form the primitive sex cords. The combination of germ cells and primitive sex cords forms the indifferent gonad – from which development into the testes or ovaries can occur.
Embryonic development of the indifferent gonads to testes is dependent on the SRY gene (sex-determining region on the Y chromosome). If the SRY gene is present on the Y chromosome, SRY protein, testis-determining factor (TDF) is expressed, resulting in the formation of testes. In the absence of the SRY gene, gonads will differentiate into ovaries