Among mammals prolonged survival of spermatozoa in the reproductive tract of the female (circa 7 ± months) is known in only a few species of hibernating bats, but its physiological basis remains obscure. Presumably survival involves special physiological adaptations of the spermatozoa, the uterus, or both. We have initiated studies of sperm survival mechanisms in the hibernating bat Myotis lucifugus involving analysis of the cytology and fine structure of intra-uterine sperm, the morphology and histochemistry of sperm-uterus interactions, and selected biochemical parameters of the reproductive tract. This paper summarizes our findings concerning the morphology of epididymal and intra-uterine spermatozoa, and emphasizes those aspects not detailed by others. The microscopic organization of epididymal sperm is described in detail with particular attention to peculiarities of the head, middle piece and cytoplasmic droplet. The latter contains amylase-resistant PAS+ granules and acid phosphatase, both possibly of lysosomal origin. Most ultrastructural features elucidated by Fawcett and collaborators are confirmed, but additional details concerning the head, post-nuclear sheath, origin of axial filament complex, and the outer coarse fibrils are provided. A functional interrelationship between coarse fibrils 3 and 8 and the longitudinal columns of the fibrous sheath is also postulated.
Structural differences between epididymal and uterine sperm are minor, suggesting that uterine environmental factors are most important for sperm survival.
A new basis is suggested for the apparent helical arrangement of mitochondria in the middle piece; in Myotis sperm the configuration is illusory, being the result of an orderly alternation in the staining of the serially arranged mitochondrial pairs. We suggest that the “patterned” staining of mitochondria may reflect sequential, cyclic functional alterations of significance in the initiation and/or coordination of the geometrically oriented tail movements.