Functional Anatomy of the Normal male dog

The reproductive system of male dogs is relatively uniform, despite the marked variation in phenotype of the various breeds. This likely represents a lack of selection pressure on the reproductive system, either directly or indirectly. There is certainly not the type of selection pressures that occur in ruminants where one male and many females form the herd or breeding unit. 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.

Histologically, 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.

Figure 1: Normal scrotal skin of a dog - hairless region.

Figure 2: Normal scrotal skin of a dog - hair follicles.

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.

Figure 1. Normal testis and epididymis from a dog.

Figure 2. Normal testis and epididymis from a dog. Sectioned sagitally. The efferent ductules and head of the epididymis are to the right, the body is dorsal and the tail is to the left. The spermatic cord is intact dorsally.

When sectioned sagitally, the testicular parenchyma is visible. The mediastinum testis is central and there are fibrous bands that run radially from the mediastinum. The normal colour is white pink. Spermatozoa should ooze from the tail of the epididymis.

Histologically, 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.

Figure 3: Histology of a normal testis.

Figure 4: Histology of a normal testis.

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 (Rehm 2000) found that 30% of control beagles had bilateral segmental degeneration or hypoplasia. Peters et al (2001) found no difference in the diameter of the seminiferous tubules with age.

The interstitial compartment of the testis has the interstitial endocrine (Leydig) cells, blood vessels, and macrophages, dentritic cells, mast cells, testicular T cells (in some species) and interstitial fibroblastic cells.

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.

Figure 6: Normal efferent ductules (upper left);,these number 13-15; compared to the head of the epididymis (lower left, larger tube).

The efferent ductules join the single epididymal (mesonephric) duct. Some may be blind ended, but this is regarded as an 'anomaly' as it can lead to disease. The volume of fluid entering the epididymis (called rete testis fluid) is large and much is absorbed in the head of the epididymis.

Dym M. (1976) The mammalian rete testis--a morphological examination. Anat Rec 186(4):493-523.

Staining characteristics

 

In normal testes a clear expression of IGF-I, IGF-II, IGF-IR, IGFBP2, IGFBP4 and IGFBP5 was found. Expression of IGFBP1 and IGFBP3 was weak.

The rete testis expresses low molecular weight cytokeratin, desmin and vimentin (Wakui et al 1994).

Wakui S, Furusato M, Ushigome S, Kano Y. Coexpression of different cytokeratins, vimentin and desmin in the rete testis and epididymis in the dog. J Anat. 1994 184 ( Pt 1):147-51.

Interstitial cells

• Calretinin
• Inhibin-alpha, -betaA, and -betaB chains and 3beta-HSD

Sertoli cells

• Calretinin

Germ cells

Radi ZA, Miller DL.(2005) Immunohistochemical expression of calretinin in canine testicular tumours and normal canine testicular tissue. Res Vet Sci. 79(2):125-129.

It is 'normal' to see lymphoid aggregates in the interstitium of the epididymis (see nonspecific epididymitis)

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.

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.

Figure 10: Normal prostates from 2 dogs. A young, pubertal dog on the left and an older dog on the right. The bladder is the lower structure on each.

The normal histology of the prostate is complicated by the changes associated with hyperplasia. Normal is a dynamic best dealt with in the section on diseases.

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.

Figure 12: Normal head of the penis with prepuce reflected.

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.

Figure 12: Lympoid follicles in preputial mucosa.

 

 

 

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.

Allen WE, Dagnall GJR (1982) Some observations on the aerobic bacterial flora of the genital tract of the dog and bitch. J Small Anim Pract 23: 325-335.

Gunzel-Apel AR, Mohrke C, Poulsen Nautrup C. (2001) Colour-coded and pulsed Doppler sonography of the canine testis, epididymis and prostate gland: physiological and pathological findings. Reprod Domest Anim. 36(5):236-340

McEntee K (1990) Reproductive Pathology of Domestic Mammals. Academic Press, Toronto.

Peters MA, de Rooij DG, Teerds KJ, van de Gaag I, van Sluijs FJ. (2001) Spermatogenesis and testicular tumours in ageing dogs. J Reprod Fertil Suppl.57:419-21

Rehm S. (2000) Spontaneous testicular lesions in purpose-bred beagle dogs. Toxicol Pathol 28: 782-787.