Testes and Supportive Tract Development

Formation of the Testes

< Week 5

Early in development, primordial germ cells migrate from the epiblast to the extraembryonic yolk sac.

Week 5+

The primordial germ cells migrate from the yolk sac to the posterior body wall, medial to the mesonephros in the intermediate mesoderm.
The somatic cells of the gonadal ridge produce transcription factors that induce cell differentiation.

— Produce Sertoli cells (aka, nurse cells). The Sertoli cells are integral to typical male anatomical development and physiologic functioning. — SRY is the transcription factor gene responsible for differentiation; SRY is found on the Y chromosome of the somatic cells. Thus, the presence of the Y chromosome determines early testes development.

The Sertoi cells "recruit" other cells that transform to become Leydig (aka, interstitial) cells.

— The Leydig cells produce testosterone, which plays a role in male reproductive tract formation during gestation; eventually, these fetal Leydig cells will disappear, and a new adult generation will form during puberty, when full maturation of the reproductive system occurs.

The somatic and primordial germ cells form solid testis cords.

— Primordial germ cells lie at the center of the cords. — Sertoli cells surround them. — The outermost layer of the cord comprises peritbular myoid cells. Later in life, the peritubular myoid cells contract to propel the contents of the cords (for example, during ejaculation). The testes cords remain solid until puberty, when they become canalized to form the seminiferous tubules; likewise, the primordial germ cells are developmentally arrested until onset of meiosis during puberty.

Formation of the male reproductive tract and accessory gland formation

Weeks 8-12

First, we draw the gonads in their original position. Indicate the location of the urogenital sinus; recall it is derived from endoderm. Then, show the location of the gonads (in a separate tutorial, we'll show their descent through the inguinal canal). Show the mesonephric, aka, Wolffian, ducts nearby, and, show that the paramesonephric, aka, Mullerian, ducts emerge lateral to the gonads, cross over the mesonephric ducts, and run caudally towards the urogenital sinuss.

Anti-Mullerian hormone (AMH) causes regression of the paramesonephric ducts.

— AMH is produced by the Sertoli cells of the testis (small, non-functional remnants of the paramesonephric ducts may appear as the appendix testis). — Testosterone produced by the Leydig cells of the testes maintains the mesonephric structures, which are incorporated into the male reproductive tract.

To show incorporation of mesonephric structures: draw a mesonephric duct and its tubules, and, indicate the testis, which comprises the solid testis cords and interstitial cells.

Testosterone and genetic factors induce the following changes:

— The mesonephric tubules give rise to the rete testes and efferent ductules; — The mesonephric duct gives rise to the epididymis, which is a tightly coiled tube, and, the ductus deferens (aka, vas deferens). — After canalization of the testes cords, sperm cells and seminal fluids will pass from the seminiferous tubules, though the rete testis and efferent ductules, into the epididymis and to the ductus deferens, which will ultimately deliver the seminal products to the urethra for ejaculation.

Connection to the urethra and the formation of the male accessory glands

First, we indicate the urinary bladder and urethra; draw the distal end of the ductus deferens, and show that it empties into the urethra via the ejaculatory duct.

The seminal vesicles arise as outgrowths of the ductus deferens near its connection to the urethra.

— Notice that the contents of both the ductus deferens and the seminal vesicles will pass through the ejaculatory duct to reach the urethra.

Prostatic tissue arises from the endoderm of the urethra as a series of outgrowths that eventually form the singular gland.
Segments of the Urethra:

Prostatic, which passes through the prostate and receives its secretions. Membranous, which passes through the pelvic diaphragm. Penile, which passes through the penis.

Summary:

The primordial germ cells give rise to the spermatogonia, the early-stage male gametes. The genital ridge and coelomic epithelium give rise to the Leydig and Sertoi cells. The mesonephros gives rise to the rete testis, ductus deferens, epididymis, and seminal vesicles. The endoderm of the urethra gives rise to the prostate and bulbourethral glands (not shown in our diagram).

Testes and Supportive Tract Development

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