Origin and Migration of Germ Cells
Primordial germ cells, known as gonocytes, appear during early embryonic development at the zygote cleavage stage.
During gastrulation, they reside in the epiblast, then migrate to the hypoblast, and finally settle in the wall of the yolk sac. From there, gonocytes enter the embryonic bloodstream and are carried to the mesonephros (primitive kidneys).
On the medial side of the mesonephros, the coelomic epithelium thickens to form the genital ridges. This is where the migrated gonocytes settle, preparing for further differentiation.
Formation of Testicular Primordia
Epithelial cells of the genital ridges begin to grow inward into the tissue of the mesonephros, forming epithelial cords called sex cords. Gonocytes penetrate inside these growing structures.
Under the influence of surrounding somatic cells, sex determination occurs: gonocytes begin to develop along the male pathway and are termed prespermatogonia. The sex cords gradually transform into seminiferous tubules. At this stage, prespermatogonia enter a prolonged resting phase that continues until puberty.
Histogenesis: Tissue Elements
The various structures of the testis have unique embryonic origins:
- Gonocytes (from the yolk sac) give rise to the spermatogenic epithelium (from prespermatogonia to spermatogonia).
- Coelomic epithelium gives rise to supporting Sertoli cells (sustentocytes). It also contributes to the walls of the convoluted seminiferous tubules and the rete testis.
- Mesenchyme serves as the source of interstitial tissue, including endocrine Leydig cells.
- Mesonephric (Wolffian) elements: mesonephric tubules transform into the efferent ductules of the testis, while the mesonephric duct becomes the basis for the epididymis and ductus deferens.
Embryogenesis of the Sperm Ducts
The duct system forms from multiple embryonic sources.
Straight tubules and the rete testis form from the sex cords. In these segments, gonocytes disappear, and the wall consists exclusively of epithelial cells. Until ages 6–8, the straight and convoluted tubules and the rete testis lack an internal lumen, appearing as solid cellular cords.
Mesonephric tubules transform into efferent ductules, and the paired mesonephric duct gives rise to the epididymis and ductus deferens.
Postembryonic Development
Postnatal testicular development proceeds through two main stages:
- Prepubertal period (ages 7–8). A lumen forms within the seminiferous tubules. Prespermatogonia actively divide, forming isolated spermatogonia. These cells temporarily enter a resting phase and lack cytoplasmic bridges.
- Pubertal period (ages 10–15). Rapid growth occurs: the tubules lengthen and become convoluted. Leydig cells activate and begin synthesizing testosterone. Isolated spermatogonia exit the resting state, initiating full spermatogenesis, during which spermatocytes, spermatids, and ultimately spermatozoa appear.