Embryology of the Male Reproductive System in Animals

Embryology of the male reproductive system refers to the study of how the male reproductive organs develop during the embryonic and fetal stages of life.

The urogenital system is formed mainly from mesodermal tissue that in early embryonic life forms the nephric and genital regions.

In the cranial portion of the nephric region segmental tubules initially form the pronephros each with a pronephric duct that runs to the primitive cloaca.

Later other segmental tubules form caudal to the pronephros and unite with the pronephric ducts to form another temporary excretory organ, the mesonephros and mesonephric ducts or Wollffian body and ducts and the pronephros degenerates.

Later a third and more permant excretory organ forms more caudally from an out growth of the mesonephric duct to become the metanephros or true kidney with its ureter and bladder.

The mesonephros then degenerates but its duct system is utilized in the male to transport spermatozoa from the testes to the pelvic urethra.

Remnants of the paramesonephric or mesonephric ducts may persist in the adult male as the appendix testis and the uterus masculinus.

In the fetus most of the urine from the fetal kidneys passes into the bladder and through the urachus into the allantoic cavity.

The testes form initially as undifferentiated gonads late in the embryonic period in the genital region or gonadal or genital ridge between the dorsal mesentery and the mesonephros.

Although the sex of the embryo is determined at the time of fertilization, sexual differentiation doesn’t occur until early fetal period after the primordial germ cells have migrated to the gonadal ridge from the wall of the yolk sac in the region of the hind gut and structural changes occur in the gonad.

Testis cords, rete testis cords and the tunica albugeniea form early in the fetal period. Primary medullary cords are characteristic of the early male gonad.

These testis cords of primitive germ cells and supportive sertoli cells remain solid until just before puberty when the seminiferous tubules are formed and join the rete testis by means of tubuli recti.

The rete testis in turn enters the efferent tubules, epididymis and ductus deferens that are formed from the mesonephric tubules and duct.

Certain tubules of the mesonephros that do not form the testicular duct system may remain as small remnants called the ductuli aberrants and appendix testis near the head and the paradidymis near the body or tail of the epididymis.

During the middle trimester of gestation the interstitial or Leydig cells are numerous especially in the equine fetus.

In the very early fetal period the external genitalia in the cloacae region are also in the indifferent stage characterized by a genital tubercle which elongates to form the phallus or penis with two genital folds that form the urethra and the two genital swellings that form the scrotum.

Development of the external genitalia in males is hormonally controlled by androgens presumably from the fetal gonad.

Undifferentiated Genital Structures in the Embryo and their Adult Male Counterparts