Corpus: Testicle

The paired testes are the male gonads. They are located in the scrotum with the epididymis attached to them. Sperm and the male sex hormone testosterone are produced in them.

Gonadal development[Bearbeiten]

The development of the gonads begins in both sexes with the proliferation of cells of the coelomic epithelium in the middle section of the gonadal ridge. This process takes place in the embryonic period during the 4th-6th week after fertilisation. As the primordial germ cells migrate into the thickened coelomic epithelium, the so-called gonadal cords develop. They surround the primordial germ cells and together with them form the gonadal anlage. The male and female gonadal anlage cannot be distinguished until the 6th week. The subsequent different development of the gonads in male embryos is controlled by the expression of the SRY gene.

Differentiation of the testes[Bearbeiten]

The testicles begin to differentiate in the course of the 7th week - somewhat earlier than the ovaries. The first developmental step in testicular development is the differentiation of the Sertoli cells, which probably form from the pluripotent cells of the zoelomic epithelium under the influence of SRY. Together with immigrated cells from the mesonephric duct (Wolff duct), they organise themselves into the testicular cords (chordae testiculares) and lose their connection to the zoelomic epithelium in the process.

At the same time, the Sertoli cells form cell connections with each other and grow around the primordial germ cells. The migrated cells from the mesonephric duct, on the other hand, later differentiate into peritubular myoblasts. Further proliferation of the Sertoli cells causes the testicular cords to elongate in depth. They differentiate further into the seminiferous tubules contorti and the seminiferous tubules recti and form the rete testis with the deepest parts that no longer surround the primordial germ cells.

Development of the testicular connective tissue[Bearbeiten]

From around the 8th week of embryonic development, the mesenchyme between the testicular cords thickens and forms connective tissue septa that divide the testicles into individual lobules. This process also takes place at the same time between the testicular cords and the zoelomic epithelium, forming the later tunica albuginea. Some mesenchymal cells between the testicular cords differentiate into Leydig cells (intermediate cells). Their exact origin is still the subject of speculation. They produce testosterone.

Morphology[Bearbeiten]

The ovoid testis is approx. 4-5 cm long, 3 cm wide, 3 cm thick, weighs approx. 15-20 g and has a volume of approx. 20-25 ml. It is located in the scrotum and is attached to the spermatic cord (funiculus spermaticus). The left testicle is usually slightly larger than the right and lies deeper in the scrotum.

The cranial end of the testis, which points towards the head of the epididymis, is known as the extremitas capitata (head end), the caudal end as the extremitas caudata (tail end). A distinction is also made between the margo epididymalis pointing towards the epididymis and the margo liber pointing in the opposite direction, the so-called free testicular margin. Between these two edges are the "facies medialis" pointing towards the centre of the body and the "facies lateralis" pointing laterally.

Testicular sheaths[Bearbeiten]

On the outside, the testicle is surrounded by a serous skin consisting of two sheets, the tunica vaginalis testis. It is to be understood as a peritoneal duplicate that passes through the inguinal canal into the scrotum during descent of the testicles (descensus), where it forms an abdominal exclave. The testicular sheaths can be assigned to the corresponding layers of the abdominal wall:

• The abdominal skin with its subcutaneous fatty tissue continues as scrotal skin (cutis scroti) with underlying tunica dartos
• The superficial body fascia and the fascia of the external abdominal oblique muscle continue as the external spermatic fascia, which lies on the outside of the cremaster muscle.
• The obliquus internus abdominis muscle and the transversus abdominis muscle continue as the cremaster muscle with its muscle fascia, the cremaster fascia.
• The transversalis fascia continues as the internal spermatic fascia, which lies against the cremaster muscle on the inside.

The peritoneum continues in the same way. The visceral leaf covers the testicle as the epiorchium, while the parietal leaf lines the inside of the scrotum as the periorchium. The space between the two leaves (vaginal cavity) is filled with a thin film of fluid and allows the testicle to move. The fold between the two leaves is known as the mesorchium and fixes the testicle to the scrotum.

Vascular supply[Bearbeiten]

The testicle receives arterial blood from the testicular artery, a branch of the abdominal aorta. The testicular artery runs caudally along the edge of the epididymis to the extremitas caudata, the tail end of the testicle. There it pierces the tunica albuginea and runs along the free edge of the testicle within the testicular capsule back to the head end of the testicle.

In the tunica albuginea it branches off into numerous tortuous branches, which run via the testicular septa into the interior of the testicle to the mediastinum testis and from there back to the seminiferous tubules. They form a widely branched capillary network in the testicular parenchyma.

The venous blood of the testicle collects on both sides in a venous plexus, the pampiniform plexus. This first surrounds the testicular artery like a tendril and then drains into the testicular vein dextra or sinistra. On the right, the testicular vein flows directly into the inferior vena cava at an acute angle, whereas on the left it flows into the renal sinus vein at a right angle.

Innervation[Bearbeiten]

The vegetative innervation of the testis is via the testicular plexus (from the renal plexus) and the deferential plexus, which reach the testis via the testicular artery or the vas deferens. Branches of the genitofemoral nerve are responsible for the sensory supply of the testicular sheaths. The predominant sympathetic fibres regulate testicular blood flow and innervate the smooth muscle cells of the tunica albuginea.

The testicle is enveloped by a tight organ capsule made of connective tissue, which is known as the tunica albuginea. From here, connective tissue septa radiate into the interior of the testis and divide the testis into approx. 300 lobules (lobuli testis), each of which has 1-4 seminiferous tubules (tubuli seminiferi).

The seminiferous tubules open into the rete testis. In the connective tissue between the tubules are the Leydig cells, which produce the androgen testosterone after stimulation by the hormone LH. The epithelium of the seminiferous tubules is formed by the Sertoli cells, which have a supporting and nourishing function for the germ cells (spermatogonia) located between them, from which the spermatocytes develop. The blood-testis barrier runs between the spermatogonia and spermatocytes within the germinal epithelium.

A disorder of testicular descent (maldescensus testis) leads to a positional anomaly (testicular dystopia), such as an abdominal testicle. It can occur unilaterally or bilaterally. Possible consequences are a disruption of spermiogenesis and malignant degeneration.

Testicular cancer is a rare malignant tumour that mainly affects young men between the ages of 20 and 40. A testicular tumour usually develops slowly and without symptoms and is therefore often detected late. The main symptom is painless, usually unilateral swelling of the testicles. In the differential diagnosis, orchitis, epididymitis and hydrocele must be differentiated.

Other diseases that affect the testicles include

• testicular torsion
• benign testicular tumours
• acute scrotum
• testicular dystopia
• maldescensus testis
• testicular rupture

In addition to palpation, the most frequently used testicular examination method is testicular sonography.

• Podcast image source: ©Akhilesh Sharma / Unsplash
• Image source for flexicon quiz: created with ChatGPT