Anatomy of the Testis

MARK SIGMAN

An understanding of normal testicular anatomy is important for all urologists involved in male reproductive surgery. Proper knowledge of normal allows recognition of those instances of abnormal anatomy. Inadvertent injury to testicular arteries may result in testicular atrophy and potentially infertility and hypogonadism. Many of the testicular structures are extremely small, which has led to the adoption of magnification provided by an operating microscope by male reproductive surgeons. There is often a close coloration between physical exam findings and findings on surgical exploration. Small soft testes usually indicated impaired or absent spermatogenesis. Small firm testes are often associated with sclerosis and scarring of the testicular parenchyma, which may make testicular microdissection more difficult. Therefore, the surgeon should be familiar not just with normal anatomy but the particular physical exam findings on the patient to be operated on which will help preparation for the actual surgical procedure.

GROSS ANATOMY

Testicular Position

The testes are ovoid organs that lie within the scrotum with the long access of the oval, usually, but not always, in a vertical direction. The epididymis is most commonly aligned in a vertical posterior direction attached to the posterior surface of the testes. Normal testicular size is commonly reported to be 15 to 25 mL in volume (1) when measured with bead-like orchidometers; however, the recorded size depends on the measuring device used and often larger sizes obtained with devices such as punched out elliptical rings with which normal adult sizes commonly exceed 30 mL (2). The right testis is often slightly larger than the left. The testis and epididymis reside within the parietal layer of the tunica vaginalis (Fig. 56.1). Often attached to the caput epididymis is the appendix epididymis, whereas just in front of this is another embryologic remnant, the appendix testis. Either of these structures may torse resulting in pain. The capsule of the testis is actually composed of three layers. The most superficial layer is the visceral layer of the tunica vaginalis followed by the thickest component, the tunica albuginea, which consists of bundles of collagen fibers. From a surgical point of view, these two layers act as one, and surgical incisions into the capsule are closed in one layer. Underneath the tunica albuginea is the tunica vasculosa, which is a thin delicate layer that contains a plexus of small blood vessels. These vessels are easily injured during incisions into the testicular capsule. Delicate septations arise from the tunica albuginea separating groups of seminiferous tubules into lobules. The tunica albuginea is thickest along the posterior portion of the testes where it forms the mediastinum testis which extends into the posterior portion of the testis. Within the mediastinum is the rete testis that connects the seminiferous tubules to the efferent ductules (Fig. 56.2).

BLOOD SUPPLY

The testis has three arterial supplies. Two-thirds of testicular blood supply derives from the testicular artery with the remaining one-third divided between the deferential and cremasteric arteries (3). The testicular artery, also referred to as the
internal spermatic artery (Fig. 56.3), arises directly from the aorta at level L3 below the renal arteries. It crosses anterior to the ureter and lies on the anterior surface of the psoas major muscle. The right testicular artery also crosses anteriorly to the vena cava; however, it is posterior to the bowel including the duodenum, root of the mesentery, terminal ileum, as well as the arteries associated with those portions of the bowel (right colic artery and the ileocolic artery). The left testicular artery is similarly posterior to the distal portion to the descending colon as well as the left colic artery and the inferior mesenteric vein. Both the right and left testicular arteries begin as single arterial structures. Once they enter the spermatic cord, the arteries may branch and thus one to three or more testicular arteries may be found within the spermatic cord (4,5). The numbers of testicular arteries and veins within the spermatic cord increase as the cord travels from the pelvis through the internal ring toward the scrotum. Although there is considerable variation in the pelvis, there is usually one artery and two veins. The mean number of vessels at the level of the inguinal canal is 2 arteries and 8 veins, whereas at the level of a subinguinal incision it is 3 arteries and 11 veins. A single testicular artery is present in 69% of cords within the inguinal canal, whereas this is the case only 25% of the time in the subinguinal region (6). In addition, the arteries in the subinguinal region are much more likely to be surrounded by a complex of veins than in the inguinal region. Thus, varicocele repair in the subinguinal region is technically more demanding than if performed in the inguinal canal (7).

FIGURE 56.1 Anatomic layers of the scrotal wall and testicular capsule.

As with the arterial blood supply within the spermatic cord, there is considerable variation in the course of the arteries within the testis. The arteries enter the testis beneath the epididymis, usually in the corpus region. These capsular arteries travel in the tunica vasculosa in an inferior direction toward the lower pole, sending out other capsular branches interiorly. The arteries also then ascend from the lower pole superiorly along the anterior surface of the tunica vasculosa. Branches extend from the anterior surface over the testicular parenchyma (Fig. 56.4). Centripetal arteries descend within the septa between lobules. Before reaching the rete testis, they leave the septa as recurrent arteries, enter the lobule and travel vertically back towards the tunica—the opposite direction of
the centripetal arteries. Segmental arterioles branch off the recurrent arteries in the interstitium which branch further as as interstitial arterioles. These arterioles form branching capillaries among Leydig cell clusters. Some of these leave the Leydig cells as inter-Leydig cell capillaries and penetrate into the lamina propria of the seminiferous tubules, and after some distance, these intramural capillaries leave the seminiferous tubule wall and are again surrounded by Leydig cells. They then continue as postcapillary interstitial venules which enter intralobular small veins (Fig. 56.5) (8).

FIGURE 56.2 Overview of testicular anatomy.

Often, but not always, the least vascular regions of the testis are the medial and lateral surfaces of the superior pole (6). However, incisions in the upper pole, if under the caput epididymis, may injure the rete testis and interfere with the entry of sperm into the efferent ductules and therefore should be avoided. Often, the midsection of the testis has fewer vessels, and a horizontal incision in this region is commonly used for testicular sperm retrieval. However, because of the variability of arterial blood flow, optical magnification should be used and allows for the identification of less vascular regions of the anterior surface of each individual testis. The deferential artery travels with the vas deferens after branching from the superior or inferior vesicular artery. It travels inferiorly toward the cauda of the epididymis. In general, this remains a single artery throughout the course of the vas deferens. The cremasteric artery branches off from the inferior epigastric artery. It travels on the cremasteric fascia to the surface of the tunica vaginalis.

There are extensive communicating channels between the various arterial supplies to the testis. The communicating channels between deferential and testicular arteries have been reported in 87% of humans, whereas communications between the cremasteric artery and the testicular artery have been found in 50% of specimens (9). The testicular artery anastomoses may occur at any point from the internal inguinal ring to the tunica albuginea. In contrast, the deferential artery tends not to demonstrate anastomoses until the level of cauda epididymis. In general, the testicular arteries tend to be larger than either the cremasteric or deferential arteries. Considerable variation in size may exist between branches of the testicular artery, with some branches being quite small and therefore easily injured during surgery. Optical magnification is very helpful in identifying these small branches.

There are four potential venous outflow systems from the tests: testicular (spermatic), vasal (deferential), external cremasteric, and gubernacular veins. Intratesticular veins travel between the lobules of the seminiferous tubules interconnecting in the tunica vasculosa, but they do not follow the course of the intratesticular arteries. They exit posteriorly either near the mediastinum following the course of the rete testis or exit through the tunica to superficial veins on the testicular capsule (10). These veins merge with deferential veins forming the pampiniform plexus in the spermatic cord. Within the spermatic cord, there are more testicular veins than arteries. The testicular arteries are typically closely adherent to veins that have multiple interconnecting anastomoses (11). This is thought to be important for countercurrent heat exchange. As a result, intratesticular temperatures are 3° to 4° lower than rectal temperature (12). The deferential veins follow the vas deferens proximally. External cremasteric veins lie within the cremasteric muscle fibers and may exit the cord through the floor of the inguinal canal or in the subinguinal region, draining into the inferior epigastric vein (13). Finally, gubernacular veins exiting the tunica vaginalis through the gubernaculum have been identified in 79% of varicocelectomy patients (4).
The significance of the gubernacular veins is unclear—some have suggested they are a potential source of varicocele recurrence if they are not ligated (4); however, others have demonstrated no benefit in clinical outcomes to ligation of these veins (14).

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