Chapter 98. The Genital or Reproductive System: The Primitive Genital System - Review of Medical Embryology Book

Introduction: the genetic sex of an embryo is determined at fertilization by the sperm that fertilizes the oocyte, but the gonads do not acquire male or female morphologic characteristics until week 7 of development. The early genital system is similar in both sexes, and in the beginning all human embryos are potentially bisexual. The period of early genital development is called the indifferent or primitive stage of the reproductive organs
Primitive genital system

PRIMORDIAL GERM CELLS: the genital glands or gonads, the testes and ovaries, are formed from 2 types of cells, the reproductive germinal cells or primordial germ cells and the nutrient supporting cells (follicular cells of the ovary; the Sertoli cells of the testis)

The primordial germ cells are large, spherical primitive sex cells of about 25 to 30 mm, with a granular cytoplasm, rich in lipids, and containing a large attraction sphere or idiozome consisting of 2 centrioles surrounded by Golgi apparatus

The human primordial germ cells are discernible at about day 21 of embryonic life and are seen among the entodermal cells in the wall of the yolk sac near the origin of the allantois. Thus, they are at first at some distance from their eventual definitive location in the genital or gonadal ridge

The primordial germ cells migrate, by ameboid movement, along the dorsal mesentery of the hindgut during week 5 and reach the lumbar region of the developing embryo, the future gonadal ridge

The coelomic epithelium which lines the anterior internal side of the mesonephric (wolffian) body, thickens to form the genital or gonadal ridge and provides the nutrient supporting cells of the gona
If the cells fail to reach the ridges, the gonads do not develop and gonadal dysgenesis occurs, a well-known syndrome in the female

In week 6, the primordial germ cells invade the genital ridges and are incorporated into the primary sex cords, which proliferate and grow from the coelomic epithelium into the underlying mesenchyme to form the primary sex cords

The gonad is called "indifferent" or undifferentiated at this stage because it has the same morphologic appearance in both male and female
The indifferent gonad now consists of an outer cortex and an inner medulla

In embryos with an XX sex chromosome complex, the cortex forms an ovary, and the medulla regresses; in one with an XY chromosome complex, the medulla differentiates into a testis, and the cortex regresses

The sex cords eventually become the seminiferous tubules in the male and the medullary cords in the female

The primary sex cords continue to proliferate actively, anastomose deep in the mesenchyme, and produce a complex network called the rete, which is seen as a bulge under the coelomic epithelium on the anterointernal side of the mesonephric (wolffian) body.

UROGENITAL CONNECTIONS: the rete anastomoses with the adjacent part of the mesonephric proximal convoluted tubules establishing the initial or first urogenital connections

Toward the end of month 2, the mesonephric (wolffian) body begins to regress, the, glomeruli disappear, and only the mesonephric tubules remain linked with the genital gland