Stallion reproductive tract anatomy; some major aspects
| Note: the term testicle and testes are used interchangeably in peer reviewed literature but testes is about four times more commonly used than the word testicle in the English language. However, the term testicle has been used almost exclusively in LORI. Although the author defers to that term in this entry, future entries in LORI will employ the terms testis and testes as some viewers have suggested. |
In the main image below this paragraph, the relationships between the intra-pelvic and inguinal sections of the tract have been maintained by removing these sections intact and photographing them as they floated under water. The image was then transposed into a "ghost" pelvis, showing the relative position of a hand during per rectum palpation. Although it is possible to palpate all of these structures per rectum, including the internal inguinal openings (rings), the accessory glands are so flaccid that they can barely be defined in most cases. During sexual excitation however, they fill with accessory fluid and enlarge greatly. With the exception of the bulbourethral glands, this allows them to be become palpable.
The inset image shows some of the same structures, well defined in a neonate.
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Notes:
1. The umbilical arteries descend from the aorta on either side of the bladder. In the inset image, the arteries have kinked the lateral aspects of the bladder giving the appearance of two separate vesicular (bladder) structures.
2. In the inset image, the paper-thin cranial margin of the genital fold (the membrane between the ampullae) has been drawn cranially by the author to stabilize structures within the fetal pelvis. The same structure in a mature stallion is shown in the main image.
Recent data on the histology and function of the ampullae of stallions are lacking but findings in Jacks (male donkeys) suggest that the ampullae of stallions have both secretory and smooth muscle functions. Although ampullae produce or store alkaline phosphatase in stallions, the ampullae (at least in donkeys) also secrete various unidentified mucopolysaccharides. That secretory function qualify the ampullae as accessory glands and not merely as storage units for sperm. Interestingly, the ampullae are large structures in donkeys and clearly have the capacity to store sperm but as shown below, the ampullae of stallions have narrow lumens at rest. However, if their histology is similar to that of donkeys, their walls contain elastic fibers, allowing them to stretch and indeed, to act as sperm reservoirs as is commonly asserted.
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Although the function of seminal plasma have been fairly well studied, relatively little information is available on the functions of the individual secretions of the accessory glands.
The sponge-like (see above) lobes of the prostate gland crosses the pelvic urethra just caudal to the neck of the bladder and widens out on either side, rather like a bow-tie. As a student, the author used this mental image to remember the comparative anatomy of prostate glands between species. The stallion is a regal animal, befitting a bow tie. By comparison, the appearance of the (body) of the prostate is rather unspectacular in bulls! Silly perhaps, but useful..
The prostate secretion is carried through through to the colliculus seminalis via 15 to 20 prostatic
ducts. These are shown in the LORI entry on the colliculus seminalis. The prostate produces a watery, alkaline secretion that neutralizes the acidity of fluid from the ampullae and testicles.
The bulbourethral glands are partially covered especially by the bulboglandularis muscle (part of the ischiourethralis muscle that runs between the ischium and the base of the bladder). They produce a thin watery secretion that is thought to be important in flushing urine and bacteria from the urethra before ejaculation. The bulbourethral glands are usually not palpable per rectum because of their muscular covering. That muscular covering is evident in this old, but still valuable specimen:
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The image below shows the cremaster muscle (from the recent dissection) mainly attached to the lateral surface of the parietal vaginal tunic. The cremaster is a slip of the internal oblique muscle on each side yet is is different from the rest of that muscle bed; unique in both its physiology and histology. The cremaster muscle contains smooth muscle fibers as well as striated muscle and and responds to environmental temperature changes autonomously, drawing the testicles towards the warmth of the body or releasing them to a cooler environment. This allows the temperature of the testicles to remain between 30 and 33 deg C; optimal for spermatogenesis. At the body temperature of 37.8 to 38.4 deg. C. spermatogenesis would be completely disrupted.
Note: some anatomists include the cremaster muscle as part of the spermatic cord, others not.
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In the image below, a testicle at left is shown covered by its (parietal) vaginal tunic. On the right side of the image, the parietal vaginal tunic has been removed completely showing the testicle in its visceral vaginal tunic, a tunic that is tightly adherent to the testicle. Functionally therefore, the visceral vaginal tunic is the outermost covering of the testicle itself.
The word vagina originates from the Latin term for sheath; as in the sheath of a sword. In this context however, the vagina is a not a sheath as commonly conceived but a pouch that originates as a ventral extension of the peritoneum itself. This is the parietal vaginal tunic. Within this pouch, lies another pouch; attached firmly to the outside of the testicle. It is none other than the visceral vaginal tunic mentioned in the previous paragraph.. The visceral and parietal tunics are separated from one another by a space i.e. the vaginal cavity. It is important to realize that the vaginal cavity is continuous with the peritoneal cavity. In fact, intestines can move through an inguinal canal and into a vaginal cavity, resulting in an inguinal hernia. Also, septic peritoneal fluid can extend into the vaginal cavities as well. This fluid may cause orchitis or insulate the testicles, thereby disrupting spermatogenesis.
Although the vaginal cavity separates the testicle from the parietal vaginal tunic, the testicle (within the visceral vaginal tunic) is attached to the parietal tunic along a margin that runs distal to proximal on the dorso-caudal aspect of the testicle in horses. In other species, the same connection exists but as shown in another LORI entry its orientation varies (see bottom of that entry). The double walled membrane that connects the visceral and parietal tunics is the mesotestes and within or just adjacent to the mesotestes, run blood vessels, the epididymis and the ductus deferens.
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Although the cremaster plays an indispensable role in thermoregulation in spermatogenesis, the vascularization of the testicle is important as well. Both the arterial supply and venous drainage are highly contorted, providing increased area for heat exchange between the cooler venous blood and arterial blood which is initially at body temperature. This cooling mechanism results in a lower overall temperature of blood supply to the testicles.
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When the function of the gubernaculum has been fulfilled and the testicle has descended into the scrotum, it matures into two ligamentous sections. They are the proper ligament of the testes (the proximal ligament) and the ligament of the tail of the epididymis (the distal ligament of the testes). The author finds these terms confusing because they both relate to the epididymis. Both ligaments arise from the same structure (the gubernaculum) so it would seem logical to simplify terminology by renaming them the proximal and distal ligaments of the epididymis (the testicle being taken as the most proximal object). At risk of invoking the ire of anatomists, the author has labeled these structures according to that assertion.
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As the urethra curves over the ischial arch, its covering changes from the urethralis muscle to the bulbospongiousis muscle and it adopts a name from human anatomy where the area is known as the bulb of the penis. That is because in humans, the corpus spongiosum penis resembles a narrow bulb in cross section. Whether this is also the case in horses, or any other animal for that matter, is a subject for fertile imaginations. Suffice to say, it is that area of the penis that is immediately below the anus.
In the image below, one is reminded that the retractor penis muscles also enter the picture here because they emerge just ventral to the anus, a little distance from where they originate, on the first few coccygeal vertebrae. They join together ventral to the anus and run along the ventral aspect of the penis for most of its length. When the author inspected this specimen closely, it was apparent that the two main bellies of the retractor muscles split into several smaller bellies towards the distal half of the penis.
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As the bulbospongiosus muscle contracts, large volumes of blood are forced down the corpus spongiosum penis (corpus spongiousum urethra in some texts) causing a "tooth paste tube" effect on the urethra, expelling its contents as the ejaculate. Unlike the corpus cavernosum penis, the corpus spongiosum is not a dead-end structure and the substantial volume of blood forced down the sides of the urethra must have a large reservoir into which blood can flow before it is finally drained by the external pudendal veins. This task is fulfilled by a group of venous sinuses dorsal to the penis. As shown in another LORI entry, those sinuses can be quite large. The venous sinus in this stallion was not spectacular.
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In the image above, one can also see the two main dorsal nerves of the penis. Injury to the same nerves (sensory in function) is of major importance in bulls with hematoma penis. Less emphasis is placed on that problem in stallions because stallions do not sustain injury to the dorsal nerves as a result of rupture of the corpus cavernosum. Nevertheless, stallions do sustain kicks to the dorsal surface of the penis, so the potential for damage to those nerves exists in stallions as well.
The image below shows the erectile tissue of the penis i.e. the corpus cavernosum penis with its internal supporting trabecular framework. The control of erection and ejaculation is complex but generally speaking, the corpus cavernosum fills with blood under parasympathetic stimulation, causing erection. Then, under sympathetic stimulation, ejaculation occurs through the mechanism described above. However, immediately after ejaculation, the sympathetic system (certainly in some animals) has an anti-erectile effect on the penis. Why? Perhaps it is essential to retract the penis as soon as possible to prevent injuries after copulation.
Although it is sometimes stated that the pudendal nerves only carry sympathetic fibers, it has been shown that parasympathetic fibers travel in the pudendal nerves as well.
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During ejaculation, the blood pressure wave in the corpus spongiosum caused by contraction of the bulbospongiosus muscles approaches the glans penis. From the image below it can be seen that the corpus spongiosum around the urethra is continuous within the glans, both above and below the urethral process. This causes the glans to swell into a massive mushroom shaped structure containing perhaps ten times the volume that is does at rest. The function of this expanded structure is probably to pump the ejaculate into the uterus and to prevent its back flow into the vagina. Blood begins to leave the glans rapidly, even as the stallion is dismounting, draining into the dorsal venous sinuses.
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Selected references:
Abou-Elhamd et al. 2012. Histological and Histochemical Studies on the Ampulla of the Deferent Duct of donkey (Equus asinus). J. Advanced Veterinary Research. 2:261-270
Auer J and Stick J. 2006. Eds: Equine surgery Third edition.ISBN 13: 978-1-4160-0123-2
Budras, K-D et al 2003. Anatomy of the Horse: An Illustrated Text. Fourth edition. Hannover : Schlüter, Anat Rec (Hoboken). 2008 Jul;291(7):790-6. doi: 10.1002/ar.20711.
Davies Morel, M.C.G Equine Reproductive Physiology, Breeding and Stud Management. 3rd Edition. Cabi. ISBN-10: 1845934504
Kayalioglu, G.1. et al 2008 Morphology and innervation of the human cremaster muscle in relation to its function. Anat Rec (Hoboken). 291:790-796
McDonnell, S. M. 1992 Physiology and Dysfunction. Stallion management. Ejaculation. Physiology and dysfunction. Vet.Clinics of North America. 8: 57-70
Pozora, M.A. and McDonnell, S.M. 2004. Color Doppler ultrasound evaluation of testicular blood flow in stallions. Theriogenology 61: 799–810
Samper, J.C. 2009. Equine Breeding Management and Artificial Insemination. Second edition. Saunders. ISBN: 9781416052340
Smith, J.A. 1974 Biopsy and the testicular artery of the horse. Equine Vet J. 6: 81-83
Szekeres,L. and Papp, J.G. 1994, Pharmacology of smooth muscle. Springer-Verlag ISBN-13: 978-3-642-78922-9
Turner, R.M.O. and McDonnell, S.M. 2003. Alkaline phosphatase in stallion semen: characterization and clinical applications Theriogenology. 60: 1–10
Voermans, M.I. 2006.The complex blood supply to the equine testis as a cause of failure in laparoscopic castration. Equine Vet J. 2006 Jan;38(1):35-9.
