Male Reproductive System
Testicles
Embryology
Development
During the fifth week of development, proliferation of coelomic epithelium and condensation of underlying mesenchyme on the medial aspect of the mesonephric ridge form the genital ridges.
The ‘indifferent gonad’ is formed when coelomic epithelium penetrates the mesenchyme to form primitive sex cords.
Differentiation
The presence of Y-chromosome directs further development of primitive gonad into testis, with the sex cords proliferating further to form testis cords. At puberty, these canalise to produce the seminiferous tubules, and, at the hilum of the gland, the tubules of the rete testis.
The testis cords become separated from the surface epithelium by fibrous connective tissue: tunica albuginea.
Ducts
Under the influence of the developing testis, the mesonephric duct persists and elongates to form the epididymis, vas deferens and ejaculatory duct (an out-pouching forms the seminal vesicle.
The paramesonephric duct degenerates completely except for its cranial end: the appendix testis.
Descent of Testes
In the second month, the testis is attached to the urogenital mesentery, the caudal extent of which is known as the caudal genital ligament. This continues as a mesenchymal band that condenses in the genital swelling and together it is known as the gubernaculum testis.
As a result of rapid body growth, the testis descends to the inguinal region, bringing its blood supply with it.
Around birth, the testis descends through the inguinal ring into the scrotal swelling.
Tunica Vaginalis
A protrusion of peritoneum from the anterior abdominal wall forms the processus vaginalis, which together with the muscular and fascial layers of the body wall, invaginates into the scrotal swelling to form the inguinal canal.
As it descends, the testis is covered by a reflected fold of the processus vaginalis that becomes the visceral layer of the tunica vaginalis.
The remainder of the peritoneal sac forms the parietal layer, with a bare area at the attachment to the urogenital mesentry.
Adult testicles
The adult testis is ovoid in shape measuring 3-5 cm in length, 2-4 cm in width and 3 cm antero-posterior, although this decreases with age. A fibrous capsule known as the tunica albuginea surrounds the testis, from the inner aspect of which arise multiple septa (septula) that converge posteriorly to form the mediastinum testis. These septa divide the testis into a number of cone-shaped lobules.
Each lobule contains 1-3 convoluted tubules (seminiferous tubules), along with their blood supply nerves and lymphatics. (Sertoli cells form one of the layers of cells in the tubules, while Leydig cells are found in the interstitium). At the apex of each lobule, the tubules straighten before entering the mediastinum testis where they form the rete testis.
The normal testis is of a smooth ovoid shape and homogeneous echo texture, similar to that of the thyroid gland. Visualisation of a small amount of fluid around the testes is a normal finding.
The rete testis is located in the mediastinum testis and connects the seminiferous tubules to the efferent ducts that in turn drain out into the epididymis. Microscopically it is composed of three parts:
- Interlobular tubuli recti
- A mediastinal network of channels
- Extra-testicular lacunar spaces
Testicular Appendages
There are four testicular appendages:
- The appendix testis is a remnant of the paramesonephric duct. It is usually located on the superior pole of the testis, in the groove between the head of the epididymis and testis, and is seen sonographically in approximately 80% of testes.
- The appendix epididymis is a remnant of the mesonephric duct, seen in a third of men projecting from the medial aspect of the head of the epididymis, and is sometimes duplicated.
- The paradidymis, also known as the organ of Giraldes (rarely present)
- The inferior and superior vas aberrans of Haller (rarely present)