Fine structure of mule testes: light and electron microscopy study.

Testes of 3 mules, 3 stallions, and 3 donkeys were studied by light and electron microscopic techniques. Mule testes were firm, with an average weight less than that of stallion or donkey testes. As expected, mule seminiferous tubules had few cell elements. There were 2 cell populations within the seminiferous tubules: (a) Only basal spermatogonia and Sertoli's cells in 70% of the tubules and, (b) the cells in (a) plus leptotene and pachytene primary spermatocytes in 30% of the tubules. Degenerative changes occurred at the pachytene stage and began with the appearance of nuclear vesicles and chromatin condensation. The last stages disclosed pyknotic nuclei and cytoplasmic vesicles. The interstitia were filled by connective tissue elements and few Leydig's cells. Interstitial cells had well-developed, smooth endoplasmic reticula and mitochondria, indicating a normal steroidogenic activity. Epididymides in the mule had morphologic features similar to those of the stallion or donkey, except for the presence of intertubular fibrosis. Degenerative changes in primary spermatocytes indicate metabolic alterations which could be explained by the blockage of meiosis.