The testes are the paired gonads of the male. The have both 'exocrine' and endocrine functions - the production of gametes (spermatozoa) as well as the male sex hormone testosterone.
Each testis lies on one side of the scrotum, separated by the potential space formed by the tunica vaginalis. The testis is is suspended within the scrotum by the spermatic cord. The testis has three outer layers which surround the functional component:
- The tunica vaginalis is the outer layer, and is continuous with the visceral layer of peritoneum through the processus vaginalis of the spermatic cord. It is reflected onto the inner layer of the scrotum as the parietal layer of the tunica vaganlis. The tunica vaginalis covers most of the testis except posteriorly.
- The tunica albuginea is a thick fibrous coat that covers most of the testis, although is deficient posteriorly where vessels and ducts intervene. The posterior part is thickened and known as the mediastinum testis. Numerous septa arise from the mediastinum testis and pass through the substance of the testis. The testicular vessels run within these septa.
- The tunica vasculosa is formed by vessels and loose connective tissue, and covers the internal surface of the tunica albuginea and the mediastinum testis. It provides the nutrient supply to the testicular lobules.
Each testis is about 5 cm from superior to inferior, 2.5 cm wide and 3 cm from anterior to posterior. The epididymis lies posterior and lateral, interrupting the layers that coat the testis. The spermatic cord is also posterior but lies medially. The lobules of the testis perform both functions of the testis and lie between the septa of the mediastinum testis.
The tunica vaginalis consists of an simple squamous epithelium.
The tunica albuginea is a fibrous layer consisting of collagen fibres and supporting fibroblasts.
The tunica vasculosa contains vessels and loose connective tissue fibres.
The septa of the mediastinum testis divide the testis into about 200 - 300 lobules. Each lobule contains a small number of seminiferous tubules, which are coiled ducts about 80 cm long. The epithelium of the ducts contains developing spermatocytes and supporting Sertoli cells. Between the seminiferous tubules is loose connective tissue that also contains Leydig cells that produce androgens.
The seminiferous tubules becomes less convoluted as they approach the mediastinum, coalescing into a number of anastamosing ductules (the rete testis). From the rete testis, efferent ductules pass to the epididymis.
Sertoli cells are tall, columnar cells that lie between the spermatocytes. They extend from the basal lamina to the lumen of the seminiferous epithelium, supporting the developing gametes as well as supplying supportive hormones. Sertoli cells have cytoplasmic processes linking nearby cells together. This forms a 'blood-testis barrier' above the layer of spermatogonia, preventing antigen detection of the developing spermatocytes.
The remaining epithelium is stratified and contains the developing gametes.
- Spermatogonia are the stem cells for the spermatozoa. They lie along the basal lamina and several types exist.
- Spermatocytes are cells undergoing meiosis. Primary spermatocytes have yet to undergo meiosis I whereas secondary spermatocytes have completed this first stage. Primary spermatocytes are more numerous as meiosis I is very slow (several weeks) relative to meiosis II (several hours)
- Spermatids are formed by the completion of meiosis II in secondary spermatocytes. They mature into spermatozoa while in contact with Sertoli cells
- Spermatozoa (singular: spermatozoon) are released by the Sertoli cell once they have completed maturation.
Tubuli recti and Rete Testis
At the proximal end of each seminiferous tubule the epithelium becomes low columnar and spermatogenesis is not seen. The straighter tubules are known as tubuli recti. When the rete testis is encountered, the epithelium becomes flattened. Efferent tubules to the epididymis are recognisable by the appearance of a ciliated columnar epithelium.
Myoid cells in the connective tissue between the seminiferous tubules have contractile properties but their function is not well understood. Leydig cells produce androgens under the stimulation of leutenising hormone (LH).
During foetal development the testis produces testosterone and anti-Müllerian hormone. These hormones lead to the development of the male phenotype, and are produced by a specialised population of foetal Leydig cells which degenerate after birth.
During childhood the testes grow slowly until spermatogenesis begins with the onset of puberty. A new population of Leydig cells is created at this time.
With advancing age, the seminiferous tubules begin to involute and there is a loss of Sertoil cells, Leydig cells and spermatogonia. Increasing numbers of abnormal spermatogenesis is reported.
The epididymis lies posterior and laterally; the spermatic cord is superior, posterior and medial. The testis is in close contact with the tunica vaginalis of the scrotum. It is separated from the contralateral testis by the median raphe of the scrotum.
Appearance on Imaging
The testis has two known functions:
- Spermatogenesis for the purposes of reproduction
- Hormonal production (testosterone) which has androgenic effects.
Each testicle receives arterial supply from the testicular artery. These paired vessels arise from the aorta just below the renal arteries. The pass inferiorly and laterally beneath the peritoneum on the surface of the psoas muscle. They enter the spermatic cord and descend to the testis through the the inguinal canal. Upon reaching the testis, each artery divides into smaller branches which pierce the tunica albuginea to supply the tunica vasculosa and the testicular lobules.
Some blood may be sourced from the cremasteric branch of the inferior epigastric artery as well as the artery to vas deferens.
Blood from the tunica vasculosa emerges as a number of small veins which combine with those draining the epididymis, forming the pampiniform plexus. This plexus typically drains into a number of testicular veins which accompany the testicular artery along the spermatic cord, gradually reducing in number to one or two larger vessels. In the abdomen, the veins lateral to the testicular artery and beneath the peritoneum. The right vein empties into the inferior vena cava just below the renal veins; the left vein normally ends in the left renal vein.
Lymph collects in a superficial plexus between tunica vaginalis and tunica albuginea as well as a deep plexus within the tunica vasculosa. Several lymphatic vessels drain these plexuses and ascend with the testicular vessels through the spermatic cord and into the abdomen; lymphatics terminate in para-aortic lymph nodes (either lateral or anterior) at the level of the renal vessels.
The testes receive autonomic supply derived from the the renal and aortic autonomic plexuses. Sympathetic nerves typically arrive from the thoracic ganglions at T11 and T12.
Potential Routes of Malignant Spread
Malignancies of the testis may invade through the various covering layers; an important boundary is the tunica albuginea which is a barrier to invasion due to its thickness. Tumours may also invade through the rete testis into the epididymis or spermatic cord. More advanced malignancies can invade through tunica vaginalis to involve the scrotum.
Lymphatic spread of malignancy typically occurs to para-aortic lymph nodes in the abdomen. The most commonly affected levels are from L1 to L3, although nodes from T12 to L4 may be seen.
If surgery has disrupted the lymphatic drainage of the testis then lymph may pass instead to subcutaneous lymphatics of the scrotum. These vessels drain to the medial proximal superficial inguinal nodes of the thigh or to external iliac nodes. This has implications for future radiotherapy.
This mode of spread is usually seen in non-seminomatous germ cell tumours but may occur occasionally in seminoma. The most commonly affected sites are the lung, liver, bone or brain.