Functional Anatomy of the Normal Male Dog
The male reproductive system in the dog has developed to meet the needs of canids to transfer spermatozoa to the female and the size and structure matches this.
The scrotum of the dog is located between the back legs, and is mostly lacking in hair. The dog is capable of licking its scrotum, so self inflicted trauma is possible. The testes and epididymides are arranged with one slightly forward of the other.
The skin is mostly hairless and there is no real subcutaneous region - the dermal collagen finishes at the tunica vaginalis (parietal part). There are smooth muscle bundles dispersed in the dermal collagen.
The cavity of the tunica vaginalis is in direct communication with the peritoneal cavity. The lining of this cavity is a reflection of the peritoneum, and has the visceral and parietal surfaces. The tunica vaginalis is fused with the covering of the testis, the tunica albuginea. It is a serous surface lined by mesothelium.
The prostate of the dog is the only accessory genital gland. Early reports indicate 2 components - the bulbous part, and a disseminate part composed of prostatic lobules in the wall of the urethra. Macroscopically, it is a bilobed structure, and it increases in size with age. This hyperplasia is discussed under 'Diseases of the Prostate'. The prostate of the Scottish Terrier is larger than one would expect of a dog of that size.
The testis descends into the scrotal sac by 10 days and certainly by 6 to 8 weeks. The initial phase of descent is the transabdominal phase and is probably influenced by Mullerian inhibitory substance. The second phase is the inguinoscrotal phase. During this phase, the inguinal canal expands and the testis and epididymis move distally. The testes go through the inguinal canal three to four (3-4) days after birth. Next the gubernaculums regresses which draws the testicle into the scrotum. The testes are typically in its final scrotal position in the scrotum by day 35. Because the inguinal canal does not close until 6 months, it can potentially move back and forth.
Each testis and epididymis is arranged in a horizontal plane, with the head slightly higher than the tail. The head is cranial and the tail is caudal. The body of the epididymis is dorsal. The deferent duct is medial to the body, and forms part of the spermatic cord (deferent duct, testicular artery, vein and nerves). The spermatic cord is always dorsocranial.
The testis is similar to other that of other species. The seminiferous tubules are coiled, but basically run from the rete testis in the mediastinum to the tunica albuginea and back, this in a vertical axis. A histological section taken from the tunic to the mediastinum will often capture several cross sections of the same tubule.
The completely normal testis should have normal spermatogenesis in each seminiferous tubule, however there are reports of otherwise normal dogs with some degree of spermatogenic arrest or tubular degeneration. One study found that 30% of control beagles had bilateral segmental degeneration or hypoplasia.
Spermatozoa travel from the seminiferous tubules, or more correctly the convoluted seminiferous tubules to the straight seminiferous tubule into the rete testis. The rete testis is a network of ducts in the mediastinum testis that are lined by cuboidal epithelial cells. The intratesticular rete become the extratesticular rete tubules and these become the efferent ductules. The efferent ductules are lined by cuboidal and ciliated cells. The efferent ductules number between 13 and 15. These are derived from the mesonephric tubules.
The epididymis, one single tube, forms into distinct regions that, for simplicity are referred to as the head, body and tail.
The spermatic cord is composed of the deferent duct (ductus deferens), testicular artery, veins, lymphatics, and nerves, and the viscereal tunica vaginalis. The deferent duct is a continuation of the tail of the epididymis, and has a thick coat of smooth muscle. The pampiniform plexus is composed of the highly coiled testicular artery surrounded by multiple veins.
Penis and Prepuce
The anatomy of the penis and prepuce is an area for which there is considerable confusion. Part of this is because of the anatomical variation between species. I choose to use the Nomina Anatomica Veterinaria (NAV) wherein the penis is the portion from the root of the penis at the ischia through to the tip of the head of the penis. The root of the penis is the proximal part that attaches to the pelvis. The body of the penis is from the root to the head of the penis. The head of the penis (the glans penis) is the distal cylindrical part that includes the bulb, the long part of the head, and the crown (corona glandis). In other species the head of the penis and the free part of the penis are within the preputial cavity, but in the dog, the head of the penis only is in the preputial cavity, The raphe of the penis is basically nonexistent in the dog, as it is the ventral seam in the free part of the penis of other species.
The prepuce is composed of the external layer (haired skin) and the internal layer. The cavity is between the head of the penis and the internal preputial layer. The preputial oriface is the junction between the internal and external layer of the prepuce. It is important to describe which part of the prepuce is affected.
The haired skin of the canine prepuce has hepatoid glands near the oriface. The mucosal portion of the prepuce is composed of nonhaired skin without mucous glands or goblet cells. The mucosa of the penis and prepuce can contain lymphoid follicles that will enlarge to become macroscopically visible.
The penis is composed of the glans penis, body and root. The glans includes the crown, long part and bulb of the penis. There is a groove in the long part where the urethra runs. This groove is visible on the ventral portion of the os penis, the ossified distal part of the fused corpus cavernosa.
Dogs have a normal flora. Allen and Dagnall cultured the prepuce of 51 dogs and found myriad bacteria including Diphtheroids, Staphylococci, Streptcocci and E coli. (Adapted from "Surgical Pathology of the Canine Male Reproductive Tract" by Dr Rob Foster OVC Pathobiology University of Guelph)
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