Origin and migration of germ cells
Primordial germ cells, known as gonocytes, appear as early as the early stages of embryonic development, during zygote cleavage.
During gastrulation, they are located in the epiblast, then move to the hypoblast, and finally localize in the wall of the yolk sac. From there, gonocytes penetrate the embryo's bloodstream and are carried by blood flow to the mesonephros.
On the medial side of the mesonephros, the coelomic epithelium thickens to form the genital ridges. It is here that the migrated gonocytes settle, preparing for further differentiation.
Formation of testicular primordia
Epithelial cells of the genital ridges begin to grow deep into the tissue of the mesonephros. As a result, epithelial cords—sex cords—are formed. 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 called prespermatogonia. The sex cords themselves gradually transform into seminiferous tubules. At this stage, prespermatogonia enter a prolonged resting phase that lasts until puberty.
Histogenesis: tissue elements
The various structures of the testis have unique embryonic sources:
- Gonocytes (from the yolk sac) become the precursors of the spermatogenic epithelium (from prespermatogonia to spermatogonia).
- Coelomic epithelium gives rise to supporting Sertoli cells (sustentocytes). It also participates in the formation of the walls of the convoluted seminiferous tubules and the rete testis.
- Mesenchyme serves as the source of interstitial tissue, including endocrine Leydig cells.
- Elements of the mesonephros: urinary tubules transform into the efferent ductules of the testis, while the mesonephric (Wolffian) duct becomes the basis for the epididymis and ductus deferens.
Embryogenesis of the spermatic ducts
The duct system is formed from different primordia.
Straight tubules and the rete testis (rete testis) form from the sex cords. In these segments, gonocytes disappear, and the wall consists exclusively of epithelial cells. Until 6–8 years of age, the straight tubules, convoluted tubules, and rete testis lack an internal lumen, appearing as dense cords of cells.
The tubules of the mesonephros transform into efferent ductules, and the paired mesonephric duct gives rise to the epididymis and ductus deferens.
Postembryonic development
Postnatal testicular development goes through two main stages:
- Prepubertal period (7–8 years). A lumen forms within the seminiferous tubules. Prespermatogonia actively divide, forming isolated spermatogonia. These cells temporarily enter a resting phase and are not connected by cytoplasmic bridges.
- Pubertal period (10–15 years). 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.
Briefly: answers to topical questions
Which hormone do Sertoli cells synthesize during embryogenesis to cause the regression of the Müllerian ducts?
During embryogenesis, Sertoli cells synthesize anti-Müllerian hormone (Anti-Mullerian Hormone).
- Anti-Müllerian hormone (AMH) ensures the suppression and regression of the Müllerian ducts during the morphogenesis of internal organs.
This hormone is produced starting from the 7th week of intrauterine development and continues to be secreted until the pubertal period. The genetic control of its synthesis is carried out by the AMH gene, which is localized on the short arm of chromosome 19.
From which three main primordia is the gonad formed during the indifferent stage of development?
During the indifferent stage of development, the gonad primordium is formed with the participation of somatic structures and primordial germ cells.
- Mesonephros (mesonephros) acts as the foundation of the developing gonad;
- Coelomic epithelium thickens on the surface of the mesonephros, resulting in the formation of the genital ridges;
- Gonocytes infiltrate the formed genital ridges, interacting with the cells of the coelomic epithelium, which in turn grow into the tissue of the mesonephros and form epithelial sex cords.
Class and exam questions
What do Leydig cells form from?
Leydig cells, or endocrinocytes, develop from the mesenchyme, which is the source of all interstitial testicular tissue.
What happens to the seminiferous tubules before puberty?
Up to the age of 6–8 years, the straight and convoluted seminiferous tubules, as well as the rete testis, lack a lumen and appear as solid cellular cords.
What source gives rise to the ductus deferens?
The ductus deferens, as well as the epididymal duct, are formed from the mesonephric (Wolffian) duct.
Where are gonocytes initially located before migration?
Before beginning their migration into the embryonic bloodstream, gonocytes are localized in the wall of the yolk sac.
What else to study on this topic
- Morphology of primordial germ cells
- Structure and function of Sertoli cells
- Mechanisms of male sex determination
- Stages of spermatogenesis in puberty
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