The internal reproductive tract in a normal stallion consists of the pelvic urethra, the two vasa deferentia with their glandular portions (ampullae), the paired vesicular glands (seminal vesicles), the bilobed prostate, and the paired bulbourethral glands (Figure 11.1).

Normal Anatomy

The vasa deferentia run from the epididymal tails, through the inguinal canals, and turn backwards towards the pelvic cavity, where their diameter increases to form the glandular portions of the vasa deferentia, called ampullae. The ampullae run over the dorsal surface of the urinary bladder, dive under the isthmus of the prostate, where they come very close together, often holding the uterus masculinus between them [1]. The vasa deferentia narrow down again, and continue their course within the urethral wall, beyond the prostatic isthmus, to join the excretory ducts of the vesicular glands, and to form the short ejaculatory ducts [2]. The ejaculatory ducts open on the colliculus seminalis, a summit of the urethral mucosa, as the ejaculatory orifices. In approximately 15% of individuals the vasa deferentia do not fuse with the excretory ducts of the vesicular glands, and open separately [1].

A rudimentary remnant of the uterus masculinus and the urogenital sinus, called utriculus masculinus, is often present within the colliculus seminalis and has its opening in the middle of this structure, between the ejaculatory orifices [3,4]. If the utriculus masculinus has a blind ending, it has a tendency to form cysts, which can enlarge with age, and may affect the ejaculatory process [5].

The vesicular glands have a shape of pyriform sacs, which lie on both sides of the bladder, and are partially enclosed in the urogenital fold. Each gland consists of the fundus, the body, and the neck or the excretory duct, which runs under the prostate before it unites with the ipsilateral vas deferens [6]. The mucous membrane of the vesicular glands has a columnar epithelium and forms a network of numerous folds [1].

The prostate gland lies on the neck of the bladder and the beginning of the urethra, and has two lobes, connected by the isthmus. The lobes have prismatic shapes, while the isthmus is a thin transverse band lying on the junction of the bladder neck and the urethra. The prostatic isthmus covers terminal parts of the ampullae, the necks of the vesicular glands, and the distal portion of the uterus masculinus. The prostate is completely enclosed in the musculo-glandular capsule, and has numerous spheroid or ovoid lobules separated by trabeculae [1]. The secretion is collected in central spaces of the lobules, called tubular diverticula, and is excreted via prostatic ducts to the urethra on both sides of the colliculus seminalis [1].

Just behind the prostate the lumen of the pelvic urethra dilates, and it then narrows again at the level of the ischial arch, between the bulbourethral glands. The caudal part of the prostate and the pelvic urethra are covered by the urethralis muscle, which consists of dorsal and ventral layers of transverse fibers, forming an elliptical sphincter around the urethra [1].

The bulbourethral glands are ovoid in shape, and are located on the both sides of the pelvic urethra [1]. These glands have a similar structure to the prostate and have numerous excretory ducts. The excretory ducts open to the urethra in two parallel rows of small papillae, just behind the prostatic ducts. Each bulbourethral gland is covered by the bulboglandularis muscle, which forms a capsule around the gland [6].

The blood supply to the stallion internal genitalia is derived mainly from the prostatic artery, which gives off the numerous branches to all accessory sex glands and the terminal portion of the vasa deferentia [7].

Palpation Per Rectum

Transrectal ultrasonography (TRUS) is the method of choice for evaluating the internal genitalia in stallions [8], but palpation per rectum is always done first. The pelvic urethra is most prominent and easy to find. It lies directly on the midline, and is detectable just before it curves around the ischial arch as a firm tube. Since the entire length of the pelvic urethra in a mature stallion is only 10–13 cm (4–5 inches) [1], the examiner does not need to introduce his/her hand deeper than up to the wrist in order to detect this structure. Palpation of the prostate gland per rectum is unrewarding in the horse [9]: the capsule is thick and its surface is flat, making detection of this gland difficult.

The next structures of interest are the ampullae of the vasa deferentia. Both ampullae are palpable on the neck of the bladder, just before they dive under the isthmus of the prostate. They are rigid small tubes, often lying very close together, and are easily palpable using fingertips. If the urinary bladder is large, the ampullae are more spread apart, and are found on the lateral aspects of the bladder.

Narrow parts of the vasa deferentia are too small to palpate. However, the internal vaginal rings should be located during palpation per rectum. In order to find one of these rings, the examiner’s hand moves over the pelvic brim, turns laterally and sweeps the caudoventral aspect of the body wall on each side. The internal vaginal ring can be recognized as an elliptical slit, easily accommodating one to two fingertips. Palpation of the vas deferens or testicular vessels passing through the ring is difficult, but detection of a retained testis or herniating intestines is possible.

Palpation of the vesicular glands is often challenging in the stallion. They are detectable only when filled with secretion as fluctuant sacs lateral to the ampullae, or more cranially, occasionally hanging over the pelvic brim. The bulbourethral glands are not palpable, due to the thick muscular capsules that completely cover these glands.

Ultrasonography of the Normal Internal Reproductive Tract

Transrectal ultrasound evaluation (TRUS) of the internal genitalia is performed immediately after palpation per rectum. A linear transducer with a high frequency (7.5–10 MHz), and high resolution is needed to detect the fine structure of the internal genitalia. Small, micro-convex, transverse transducers are also very useful in evaluating the area of the colliculus seminalis. The easiest structure to detect is the pelvic urethra with its urethralis muscle (Figures 11.1, 11.2). The dorsal and ventral layers of this muscle appear as thick hypoechoic lines, when the linear transducer is positioned on the midline, parallel to the long axis of the animal. The urethra itself is an echogenic tube, but contains a few structures with varied echogenicity. The colliculus seminalis can be visualized in the most caudal aspect of the pelvic urethra (Figures 11.2A,C, 11.3, 11.4 (focus on 11.4J)). It appears as a roundish, echogenic or hypoechoic protrusion from the dorsal wall of the urethra, often with an anechoic outline due to the small accumulation of urine, prostatic secretion, or semen in this area. Single or multiple anechoic cysts of the colliculus seminalis may be also found (Figure 11.3). Most often, these cysts have an oval or tear shape, but can also be spindle shaped or rectangular. Occasionally, echogenic or hyperechoic contents may be observed within a cyst.

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