Physical Abnormalities

The major physical abnormalities that affect spermatogenesis are those that involve the testis and its function. It is beyond the scope of this book to discuss the numerous conditions in detail and so a summary of the major problems will be given. Firstly, the classification of a 'normal' stallion should be considered. It has been reported that up to one in five stallions can be considered to be abnormal in some aspect of the testes. The range is vast, from the minor to the life-threatening condition. In general, testes abnormalities can be divided into congenital (inherited) or acquired (disease and accidental damage).

Cryptorchidism. A cryptorchid stallion, or a rig, is an animal in which either one or both of the testes have failed to descend into the scrotal sack. The passage of the testes from a position just ventral to the kidneys should occur, as a gradual process, in utero or during the first few months of life (Fig. 4.24). A cryptorchid stallion may be further classified as illustrated in Figs 4.25-4.27 (Cox, 1993a,b; Davies Morel, 1993).

The failure of testes to descend may be temporary (most will descend within 3 years of birth) or permanent. Evidence suggests that more than 75% of cases involve retention of the right testis (Bishop et al., 1964, 1966). Circumstantial evidence and work reported by Cox (1993a) would suggest that the incidence of cryptorchidism is higher in ponies. The retention of one or both testes results in a significant decline in testicular weight even if it does subsequently descend. The size may be reduced by up to 20-fold in the

Descent The Testes
Fig. 4.24. The normal passage of descent of the testes in the stallion.

abdominally retained testis; the reduction in size of the inguinally retained testis is not as great, but a difference of up to sevenfold has been reported (Bishop et al., 1964).

Palpation Inguinal Rings Horses

Fig. 4.25. An inguinal cryptorchid stallion is characterized by the testes having only partly descended and remaining associated with the inguinal ring. In a unilateral abdominal cryptorchid, one testis has failed to descend; in a bilateral cryptorchid, both remain in the region of the inguinal ring.

Fig. 4.25. An inguinal cryptorchid stallion is characterized by the testes having only partly descended and remaining associated with the inguinal ring. In a unilateral abdominal cryptorchid, one testis has failed to descend; in a bilateral cryptorchid, both remain in the region of the inguinal ring.

Horse Cryptorchid Testis
Fig. 4.26. The testes from an inguinal cryptorchid stallion. Note the relative difference in size between the retained testis (upper right) and the normally descended testis (lower left).

The morphological appearance of retained testes varies with their position. The nearer they are to the scrotum, the more morphologically normal they appear. Abdominally retained testes are flabby in nature, with parenchyma consisting of some seminiferous tubules plus interstitial tissue held within a loose connective tissue. Spermatogenic activity is absent and germ cells are rarely developed beyond primary spermatocytes. As the stallion gets older, the seminiferous tubules appear as isolated groups and the proportion of fibrous tissue increases. In general, the testes appear similar to those of a normal 4-month-old foal (Arighi et al., 1987; Cox, 1993a). Testes which have partly descended, though failed in the last descent into the scrotal sack, show greater testicular development, with most of the testis being made up of seminiferous tubules and fewer interstitial cells. Sertoli cells are evident, as are spermatogonia-like cells. Indeed testes lying in the inguinal region may produce viable spermatogonia and so fertilization is possible with spermatozoa from such testes. The testes in such cases resemble those of a 12-month-old foal (Arighi et al., 1987).

inguinal canal bladder kidney testis inguinal canal bladder kidney testis

Descent The Testes
Fig. 4.27. An abdominal cryptorchid stallion is characterized by the testes lying up within the body cavity. Again, failure of descent may be seen in either one (unilateral) or both (bilateral) testes.

Hernias. Stallion hernias may be classified in a number of ways (Figs 4.28 and 4.29). All have the potential to affect spermatozoan production due to an elevation in testicular temperature from the close proximity of the herniated part of the gastrointestinal tract, mesentry or omentum (Cox, 1988).

Testicular hernias may be aquired, usually due to accident or strain (Varner and Schumacher, 1991), or are congenital, due to inherited abnormality (Schneider et al., 1982; Cox, 1993a). They may be indirect or direct, depending upon whether the tunica vaginalis has been ruptured (Ashdown, 1963; Van der Velden, 1988a,b; Cox, 1993a). The most common form of hernia is the indirect, especially in young foals, where large inguinal rings are the prime cause (Wright, 1963). Spontaneous recovery normally occurs within 3-6 months and no long-term detrimental effects have been reported (Varner and Schumacher,

Testicular hernias may be further classified as inguinal or scrotal, depending on the extent of herniation. Inguinal hernias result from intestinal tissue passing solely through the internal or deep inguinal ring (Fig. 4.28). Scrotal hernias result from further herniation, where the intestine extends beyond the superficial or external inguinal ring (Varner and Schumacher, 1991) (Fig.

Apart from the mortal risk of intestinal strangulation, the biggest problem associated with testicular hernias is the effect on testicular function due to elevated testicular temperature from the close proximity of the intestine. As discussed previously, an increase in testicular temperature has a direct effect on function (earlier in this section). If the condition persists, testicular

Inguinoscrotal Hernia Stallion
Fig. 4.28. An inguinal hernia in the stallion in which a loop of intestine folds through the inguinal ring.

hypoplasia or atrophy may result, along with the possibility (especially in the case of aquired hernias) of the development of adhesions between the intestine and the overlying tissue and the development of a fibrous sack (Varner and Schumacher, 1991; Cox, 1993a).

Ruptured Inguinal Hernia Stallion
Fig. 4.29. A scrotal hernia in the stallion is a more extreme case of inguinal hernia: a loop of intestine has entered the scrotum and there is significant danger of complete ligation of the intestine.

Testicular hypoplasia or degeneration. Testicular hypoplasia or degeneration is often a secondary condition, resulting from another primary abnormality (often cryptorchidism or herniation). Both hypoplasia and degeneration are terms given to an underdeveloped and, therefore, underfunctioning organ. In general, hypoplasia is the term given to a condition present from birth. In the case of testicular hypoplasia, the testes, for some reason, have never developed beyond an immature stage, resulting in inadequate development of primitive germinal cells in the fetus along with retardation of the normal accelerated germinal cell development at puberty (Roberts, 1986a; Varner and Schumacher, 1991; Varner et al., 1991b). Its causes are many, including cryptorchidism and hernias, but also malnutrition, endocrine malfunction, infections, irradiation or toxins, and it may also be an inherited condition (Roberts, 1986).

The extent of the problem varies considerably. In mild cases, the testes appear normal (though possibly slightly small) during external examination; however, on examination of the testicular tissue it is apparent that most seminiferous tubules exhibit spermatogenesis up to primary spermatocytes, but interspersed between these are completely hypoplastic tubules. In more severe cases, testes are significantly smaller than normal and, if the condition is advanced, the testes may have become hard due to the overdevelopment of connective tissue; on examination of the tissue it is apparent that most of the seminiferous tubules are hypoplastic (Ladd, 1985). Spermatozoan production depends on the severity of the condition, varying from slight impairment to azoospermic samples. Any spermatozoa that are ejaculated have a higher incidence of abnormalities. In such cases the libido of the stallion is often not affected (Varner and Schumacher, 1991; Varner et al., 1991b).

Testicular degeneration refers to the condition of underdeveloped testes which has developed post birth. That is, testicular development did originally occur to some extent but some subsequent problem has resulted in a degeneration of the tissue. The germinal epithelium of the testis is highly sensitive to extrinsic factors, as discussed previously. As such, testicular degeneration is a major cause of infertility in the stallion. Unlike hypoplasia, degeneration is an acquired condition, but stallions suffering from hypoplasia do have an increased chance of also suffering from degeneration and reducing still further their chances of producing viable spermatozoa (Varner and Schumacher, 1991). Degeneration may be temporary or permanent, its severity depending upon the severity and duration of the causative agent. It may be unilateral (the cause being localized in origin) or bilateral (a systemic cause) (McEntee, 1970). The condition is also evident as a shrinking of the testes, often showing small epididymides with a reduced number of spermatozoa within (Watson et al., 1994). Spermatozoa counts are depressed (Roberts, 1986a) and a decline in output of spermatozoa is observed with an increase in the percentage of morphologically abnormal spermatozoa (Friedman et al., 1991a; Blanchard and Varner, 1993). Microscopically, degeneration is evident primarily in mid-generation spermatogonia. Undifferentiated germinal cells are relatively resistant, as are mature spermatozoa. Once the causative agent has been removed, the stem cell population provides a pool from which a new generation of spermatogonia can develop (Huckins, 1971, 1978).

The causes of testicular degeneration are many and varied, the prime ones being elevated testicular temperature, hydrocoel, scrotal haemorrhage, increased scrotal insulation due to scrotal oedema, scrotal dermatitis and cryptorchidism (McEntee, 1970; Varner and Schumacher, 1991; Blanchard and Varner, 1993). Minor affectors include hormonal disturbances, radiation, malnutrition, production of antispermatozoa antibodies, toxins, tumours, obstructions of the vas deferens, testicular torsion and old age (Rossdale and Ricketts, 1980; Pickett et al., 1989; Varner and Schmacher, 1991; Varner et al., 1991b).

In most cases, testicular degeneration is reversible, provided that the duration of the problem is limited and the causative condition can be alleviated. Infective and traumatic degeneration is more likely to be permanent (Burns and Douglas, 1985; Blanchard and Varner, 1993).

Testicular torsion. The extent to which the testes of a stallion may twist or turn is variable, as is the resultant effect. Torsion occurs most commonly in younger stallions. The twist may occur through an angle of up to 360°, a condition difficult to detect immediately, as the testes, on cursory examination, would appear to be positioned correctly. More commonly, the angle of twist is 180°, resulting in the epididymis being in a cranial or anterior presentation at palpation. This condition is evident during testes descent (Hurtgen, 1987). A minor torsion may be transient and may present just a little pain and a slight decrease in spermatozoan concentration of ejaculates. Such torsions may correct themselves (Threlfall et al., 1990). Major torsion can result in symptoms similar to orchitis (discussed later in this section), including acute colic pain, scrotal swelling and obstruction of the blood supply, which if present in a chronic case may lead to degeneration (Kenney, 1975; Threlfall et al., 1990). There is some dispute as to the effect of the condition on semen quality. It is evident that if degeneration does result, then semen quality will suffer. However, Hurtgen (1987) stated that the condition per se does not affect libido and semen quality.

Testicular neoplasms. Testicular neoplasms are rare in horses, though their exact incidence rate is difficult to ascertain as the majority of stallions are gelded at a young age. Most neoplasms can be divided into germinal neoplasms and non-germinal neoplasms (Caron et al., 1985). Germinal neoplasms include epididymal, dermoid cysts (especially prevalent in crypt orchids), teratomas (Fig. 4.30) and seminomas (malignant germinal cells of seminiferous tubules). Non-germinal neoplasms include Sertoli and Leidig cell tumours (Bostock and Owen, 1975; Morse and Whitmore, 1986; Schumacher and Varner, 1993). Such conditions normally occur in older stallions and, in the case of seminomas, are more prevalent in cryptorchid stallions (Vaillencourt et al., 1979). Often the condition is not associated with pain or elevated temperatures but a firm swelling may be felt in the testicular tissue and the affected testis may be enlarged. Neoplasms are causes of testicular degeneration and, therefore, associated with depressed spermatozoan counts and high incidence of morphological abnormalities (Hurtgen, 1987).

Fig. 4.30. A teratoma, containing mainly hairs, found within the testis of a stallion post mortem.

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