CHAPTER 32Spermatogenic Arrest (Testicular Degeneration)
Testicular degeneration is the most common cause of male infertility across species.1 Because the seminiferous epithelium of the equine testis is highly susceptible to insult and injury, the resulting damage is at times reversible but is often irreversible.2 Within the seminiferous tubules the dividing primary spermatocytes appear to be most sensitive to insult, followed in sensitivity by other differentiating germinal cells all the way to spermatids. Stem cells (type A spermatogonia), however, and Sertoli cells appear relatively resistant to injury and in many conditions will regenerate.3 Therefore, the Sertoli-spermatic cell index can function as a useful measure of testicular degeneration.4
Testicular degeneration, which is acquired and potentially reversible, must be differentiated from testicular hypoplasia, which is irreversible and either congenital or, less likely, acquired.5 Testicular degeneration can be further classified to differentiate between hypospermatogenesis (germ cell hypoplasia), which is characterized by a reduction in the number of germ cells per seminiferous tubule, and a maturation abnormality or arrest, which is manifested as a failure to complete spermatogenesis beyond a certain stage. Finally, testicular degeneration can also be associated with germ cell aplasia—a condition in which only Sertoli cells line the tubules.
CAUSES
Thermoregulation of the testicle has received a great deal of attention with respect to testicular degeneration in the stallion. A temperature differential exists between the core body temperature and the intrascrotal testes. Thermal degeneration of the testicle occurs if temperature in the scrotum equals or exceeds core body temperature. This temperature derangement can occur in conditions of herniation, cryptorchid testicles, scrotal edema, heavy working conditions, scrotal dermatoses, and any cause of inflammation or orchitis (trauma, infectious agent, vascular disturbance) or periorchitis (secondary to peritonitis).6
An elevation of scrotal temperature, even as little as 2°C to 3°C, results in decreased concentration, motility, and total number of spermatozoa and an increase in morphologic abnormalities.6 The damage is most apparent for 10 to 40 days following the insult in the stallion. Furthermore, adhesion formation following an acute episode of thermal damage can inhibit future movement of the testicles within scrotum, leading to a chronic condition.
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