The testes develop in close association with the urinary system retroperitoneally on the posterior wall of the abdominal cavity. Testes (like ovaries) are derived from three sources:  Intermediate mesoderm that forms the urogenital ridges on the posterior abdominal wall, giving rise to Leydig cells (interstitial cells) and myoid cells (peritubular contractile cells) that lines the urogenital ridges and gives rise to Sertoli cells  Primordial germ cells that migrate from the yolk sac into developing gonads, where they divide and differentiate into spermatogonia
Migration of the primordial germ cells into the genital ridges induces mesodermal cells of the urogenital ridges and cells of the coelomic mesothelium to proliferate and form the primary sex cords. At this stage, these cords are comprised of primordial germ cells, pre-Sertoli cells, and a surrounding layer of myoid cells. Later, primary sex cords duct of epididymis degenerating paramesonephric duct
FIGURE 22.2 • Schematic diagram of the stages of testicular development. a. This diagram shows the 5-week embryo in the stage of indifferent gonads. The gonadal ridges visible on the posterior abdominal wall are being infiltrated by primordial germ cells (green) that migrate from the yolk sac. Most of the developing gonad is formed by mesenchyme derived from the coelomic epithelium. The primordial germ cells become incorporated in the primary sex cords. b. At a later stage, under hormonal influence of testis-determining factor (TDF), the developing gonad initiates production of testosterone. This is followed by differentiation of the primary sex cords into seminiferous cords. At the same time, the developing gonad produces Mlerian-inhibiting factor (MIF), which causes regression of the paramesonephric duct and those structures derived from it. Note that the mesonephric tubules come in close contact with the developing rete testis. c. Final stages of testicular development. The tunica albuginea surrounding the testis contributes to development of the testicular septa. The rete testis connects with the seminiferous cords and with the excurrent duct system that develops from the mesonephric duct and tubules.
differentiate into the seminiferous cords, which give rise to the seminiferous tubules, straight tubules, and rete testis (Fig. 22.2).
In the first stage of development, the testes develop on the posterior abdominal wall from indifferent primordia of urogenital ridges that are identical in both sexes. During this indifferent stage an embryo has the potential to develop into either a male or female. However, expression of the SRY gene—exclusively in the pre-Sertoli cells—orchestrates male development of the embryo. Early in male development, mesenchyme separating the seminiferous cords gives rise to Leydig (interstitial) cells that produce testosterone to stimulate development of the indifferent primordium into a testis. Testosterone is also responsible for the growth and differentiation of the mesonephric (Wolffian) ducts that develop into the male genital excurrent ducts. Also in this early stage, the Sertoli (sustentacular) cells that develop within the seminiferous cords produce another important hormonal substance, called Mlerian-inhibiting factor (MIF). MIF’s molecular structure is similar to that of transforming growth factor(TGF-). It is a large glycoprotein that inhibits cell division of the paramesonephric (Mlerian) ducts, which in turn inhibits development of  female reproductive organs (Fig. 22.3).