Embryology

• The reproductive system is a collection of internal and external sex organs which work together for the purpose of sexual reproduction.

  The development of these reproductive organs begins at an early stage in the embryo. There is a close link throughout with the development of the urinary system.
• The Gonads: 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.
• The Gonads: In the fourth week, germ cells begin to migrate from the endoderm lining of the yolk sac to the genital ridges, via the dorsal mesentary of the hindgut. They reach the genital ridges in the sixth week.
• The Gonads: 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.
• The Gonads: In a male embryo, the XY sex chromosomes are present. The Y chromosome contains the SRY gene, which stimulates the development of the primitive sex cords to form testis (medullary) cords. The tunica albuginea, a fibrous connective tissue layer, forms around the cords.
• The Gonads: A portion of the testis cords breaks off to form the future rete testis. The remaining cords contain two types of cells:

  • Germ cells
  • Sertoli cells (derived from the surface epithelium of the gland).
• The Gonads: In puberty, these cords acquire a lumen and become the seminiferous tubules – the site within which sperm will be formed.
• The Gonads: Located between the testis cords are the Leydig cells (derived from the intermediate mesoderm). In the eighth week, they begin production of testosterone – which drives differentiation of the internal and external genitalia.
• The Gonads: In a female embryo, the XX sex chromosomes are present. As there is no Y chromosome, there is no SRY gene to influence development. Without it, the primitive sex cords degenerate and do not form the testis cords.