General considerations

It is very important to fix reproductive tissues to minimize artefacts. This is particularly the case for the testis when examination is to detect subtle differences in spermatogenesis. The mainstay of histological assessment is Bouin Solution, a mixture of formaldehyde, acetic acid and picric acid. Other fluids for fixation were tested and 70% ethanol and acetic acid and glycerol seemed promising. (Cabrera et al 2017)

Cabrera NC, Espinoza JR, Vargas-Jentzsch P, Sandoval P, Ramos LA, Aponte PM. Alcohol-based solutions for bovine testicular tissue fixation. J Vet Diagn Invest . 2017 Jan;29(1):91-99

White blood cells are normally found in semen, but in very low numbers. Zart et al (2014) reported an average of 4.73 x 109 /ml. Approximately one per high-power field in a smear of semen was considered normal.

AL Zart, VCL Jurgielewicz and CE Fernandes (2014) Seminal Leucocytary Profile in Beef Bulls. Reprod Dom Anim 2014; 49: 719-724

Scrotum

The scrotum of the bull is located between the back legs, and is mostly lacking in hair. 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 vaginal tunic (parietal part). There are smooth muscle bundles dispersed in the dermal collagen.

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

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

Vaginal tunics

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.

Testis and epididymis

The initial phase of descent is the transabdominal phase and is probably influenced by Mullerian inhibitory substance/Antimullerian Hormone. 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 bull.

Figure 2. Normal testis and epididymis from a bull. 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.

The size of the normal testes in bulls is estimated during the breeding soundness evaluation by measuring scrotal circumference. Because this is a selection criterion, over time bull testis size has increased. Measurement at 12 months of age is the norm, and measurement at 24 months is also done. Garcia Guerra et al (2013) reports on the increase in size over time for different breeds of bulls.

There is variation in the shape of testes of bulls. See heading below.

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, thus 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. Amann detailed seminiferous tubule parameters and spermatogenesis in bulls.

Inhibin alpha is expressed by Sertoli cells of prepubertal dogs, but the interstitial endocrine cells express it after puberty (Greico et al 2011).

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 extratesticular 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.

Amann RP. Reproductive capacity of dairy bulls. III. The effect of ejaculation frequency, unilateral vasectomy, and age on spermatogenesis. Am J Anat 1962; 110: 49-67.

The mammalian rete testis--a morphological examination. Anat Rec 1976; 186: 493-523.

García Guerra A, Hendrick S, Barth AD (2013) Increase in average testis size of Canadian beef bulls. Canadian Vet J 2013; 54: 485-490

Scrotal circumference

Hormonal control

Inhibin is mostly produced by the interstitial endocrine cells, although it can secreted by both Sertoli and interstitial endocrine cells. Inhibin einhibits the release of follicle stimulating hormone from the pituitary gland and it also down regulates the activity of activin. Normal Sertoli cells in adult dogs do not appear to produce inhibin, however most Sertoli cell tumours do.

Activin is also produced by the interstitial endocrine cells and it modulates spermatogenesis and Sertoli cell function.

Shape of testis

There are variations in testicular shape and commensorate changes in blood flow. Age differences are present too. Longer testes have higher resistance and better thermoregulation (?)

Claus LAM, Barca Junior FA, Junior CK, Pereira GR, Fávaro PDC, Ferreira FP, Galdioli VHG, Seneda MM, Ribeiro ELA. Testicular shape, scrotal skin thickness and testicular artery blood flow changes in bulls of different ages. Reprod Domest Anim. 2021; 56: 1034-1039.

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).

Interstitial cells

Sertoli cells

Germ cells

Spermatogonia are cKIT positive

Immune cells

There are several studies detailing the immune cells of the bull testis and epididymis.

Güney Saruhan et al (2018) reported on the presence and location of cells in the testis and epididymis that were detected by the CD68, CD8, MHC 1 and MHC II antibody.

Güney Saruhan B, Sağsöz H, Akbalık E, Ketani MA, Erdoğan S. Distribution of CD68-, CD8-, MHCI- and MHCII-positive cells in the bull and ram testis and epididymis. Anat Histol Embryol 2018; 47(4): 313-321.

Epididymis

The epididymis, one single tube, forms into distinct regions that, for simplicity are referred to as the head, body and tail.

Figure 7: Head of the epididymis.

Figure 8: Body of the epididymis

Figure 9: Tail of the epididymis . The tube is packed with spermatozoa.

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

Spermatic cord

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.

Accessory Genital Glands

Ampulla of deferent duct

Bulbourethral gland

Vesicular glands

Prostate gland

Penis

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 glans penis).

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

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.

Normal flora

References

Prepuce

The prepuce is the epitheliual tissue from the ostium of the prepuce to the penis. There is an external layer and the internal layer. The cavity is formed by the external layer, the internal layer and the intrpreputial portion of the penis. The preputial ostium is the junction between the external layer of the prepuce and the haired skin. It is important to describe which part of the prepuce is affected.

The prepuce is encased in haired skin that can be very pendulous in Bos indicus breeds. This part, composed of haired skin, is called the sheath.

There are preputial muscles that assist in maintaining the prepuce in its normal arrangement. These include the cranial and caudal preputial muscles. The cranial preputial muscles are bilaterally symetrical and originate on the exterior abdomen and act as a sling around the ostium of the prepuce. They protract the sheath. The caudal preputial muscles originate at the external inguinal ring and insert on the prepuce at the fornix. These retract the prepuce. They are particularly small in polled animals.

Irons PC. Applied anatomy, pathophysiology and a revised scoring system of bull sheaths. Aust Vet J. 2019