The testis, or male gonad (), is oval in shape and located within the scrotum. The length of the testis in a 25-pound dog averages 3 cm and the width 2 cm. The fresh organ weighs approximately 8 g. In normal position, the testis of the dog is situated obliquely, with the long axis running dorsocaudally. The epididymis is adherent to the dorsolateral surface of the organ, with its head located at the cranial end and its tail at the caudal extremity of the testis.
The surface of the testis is invested by the tunica albuginea, a dense, white fibrous capsule. Covering the testis most immediately is the visceral vaginal tunic, a serous membrane continuous with the peritoneum of the spermatic cord and the abdominal cavity. The tunica albuginea joins the centrally located mediastinum testis by means of interlobular connective tissue lamellae (septula testis), which converge centrally. The mediastinum testis is a cord of connective tissue running lengthwise through the middle of the testis. The lobuli testes (wedge-shaped portions of testicular parenchyma) are bounded by the septula. The lobuli contain the convoluted seminiferous tubules (tubuli seminiferi contorti), a large collection of twisted canals. Spermatozoa are formed within the epithelial lining of the tubules, which contains spermatogenic cells and susten-tacular (Sertoli) cells. The organization, motility and structure of sperm cells have been reviewed by Andre (1982). The longevity of spermatozoa in the reproductive tract of the bitch can be several days (), and at least 6 days as shown by Concannon, Whaley, and Lein (1983).
Straight seminiferous tubules (tubuli seminiferi recti) are formed by the union of the convoluted seminiferous tubules of a lobule. The mediastinum testis contains a network of confluent spaces and ducts called the rete testis. These connect the straight tubules with the efferent ductules (ductuli ejferentes testis). Testicular blood vessels and lymphatics enter and leave through the mediastinum. The lobuli testis also contains interstitial cells (of Leydig) between tubular elements. Johnson et al. (1970) and Setchell (1978) consider the anatomy, physiology, biochemistry, and other parameters of the testis.
The short proper ligament of the testis (lig. testis proprium) attaches the testis to the tail of the epididymis. This is continued by the short ligament of the tail of the epididymis (lig. caudae epididymidis) that attaches the tail of the epididymis to the reflections of the layers of the vaginal tunic at its most distal extent as well as to the spermatic fascia. These two ligaments are remnants of the two divisions of the embryonic gubernaculums testis. The scrotal ligament (lig. scrott) is a thin layer of connective tissue between the tunica dartos and the tail of the epididymis. These latter two ligaments are difficult to distinguish from each other. These ligaments as well as the spermatic cord provide stability to the testis.
Testes: Vessels and Nerves
The testicular artery and the artery of the ductus deferens supply the testis and epididymis. The testicular artery (homo-logue of the ovarian artery of the female) arises from the ventral surface of the aorta at the level of a transverse plane through the fourth lumbar vertebra. The right artery originates cranial to the left, corresponding to the embryonic positions of the testes. The artery of the ductus deferens, a branch of the prostatic artery from the internal pudendal, follows the ductus deferens into the spermatic cord to the level of the epididymis. It sends branches to the epididymis and anastomoses with the testicular artery. The testicular vein follows the arterial pattern but forms an extensive pampiniform plexus (plexus pampiniformis) in the spermatic cord, surrounding the testicular artery lymphatics, and nerves. The right testicular vein empties into the caudal vena cava at the level of the origin of its arterial counterpart. The left drains into the left renal vein. Harrison (1949) made a detailed comparative study of the vascularization of the mammalian testis.
The testicular and epididymal lymphatics anastomose into a variable number of trunks that drain into the lumbar lymph nodes ().
The nerve supply to the testis is derived from the sympathetic division of the general visceral efferent component of the autonomic nervous system. The nerves of the testicular plexus accompany the testicular arteries distally and enter the testis with either the blood vessels or the efferent ducts. Indirectly, they are derived from the fourth, fifth, and sixth lumbar sympathetic trunk ganglia. The testicular plexus is derived from the abdominal aortic plexus at the level of the origin of the testicular arteries. These testicular vessels, lymphatics and nerves are suspended in the spermatic cord by the mesorchium, which is a fold of the visceral layer of the vaginal tunic. This is continued in the abdomen as a fold of the parietal layer of the peritoneum. The blood vessels and smooth muscle fibers in the testis receive a sympathetic nerve supply, but the seminal epithelium and the interstitial secretory tissue do not. Elimination of the sympathetic nerve supply to the testis is followed by degeneration of the seminal epithelium and hypertrophy of the interstitial secretory tissue (). The degenerative changes are considered to be the result of paralysis of the blood vessels in the spermatic cord and testis.
Cryptorchidism, or failure of the testis to descend, is the most important congenital anomaly of the testis. This condition is comparatively frequent and is believed to be hereditary in some instances. Cox et al. (1978) investigated 12 cases of cryptorchidism in Miniature Schnauzers. Five were unilateral and seven were bilateral. All of the unilateral cases had retained testes on the right side. When retained testes were bilateral the right testis was always smaller. Their observations suggested a multigene defect. In a cryptorchid animal one or both testes are retained either in the abdominal cavity (in the region of the inguinal canal) or between the superficial inguinal ring and the scrotum. Sterile, cryptorchid dogs usually possess normal sexual desire. For a discussion of cryptorchidism see Wensing and van Straten (1980). Hayes et al. (1985) studied 1.8 million documented medical records and identified 2912 dogs (in 104 different breeds) that had cryptorchid testes. There were 14 breeds with significantly high risk.
According to Runnells (1954) testicular tumors of dogs have been reported to cause anatomic alterations, such as atrophy of the opposite testis and enlargement of the prepuce and prostate gland. Hayes et al. (1985) reported that testicular tumors were found in 5.7% of the 2912 cryptorchid dogs whose records they reviewed. Half had Sertoli cell tumors, and one-third had seminomas.
Male pseudohermaphroditism and true hermaphroditism have been reported in the dog by Lee and Allam (1952), Brodey et al. (1954), and Bodner (1987). Female pseudohermaphroditism, considered rare (), was reported by Olson et al. (1989) in three sibling Greyhounds.