Ultrastructure of Spermatogenesis and Sperm of Bats

Spermatogenesis is a highly regulated and synchronized process of cellular division and differentiation whereby spermatogonia proliferate, meiotically divide, and gradually differentiate into highly specialized haploid cells, known as spermatozoa. Its main events are generally similar in most mammals, and it may be divided into three phases: the proliferation phase, the meiotic phase, and spermiogenesis. The proliferation phase corresponds to the process of stem cell renewal, where spermatogonia mitotically divide to both replenish the stem cell pool and to originate cells that are subject to further differentiation. The meiotic phase is the process in which spermatocytes undergo meiotic divisions that give rise to haploid spermatids. Moreover, spermiogenesis is the process in which a round spermatid differentiates into a mature spermatozoon that is capable of motility and fertilization. The process of spermatogonial differentiation in bats is relatively similar to that found in primates, with three main types of spermatogonia: Ad, Ap, and B. Meiotic divisions proceed similarly to those of most mammals, and spermiogenesis can be divided into 9 to 16 steps. Despite these similarities, some species-specific variations are observed. Bats present three different processes for the formation of the acrosome, and the ultrastructure of spermatozoa has been found to have unique characteristics, including many wavy acrosomal projections on the acrosome; surface of the family Molossidae, an extraordinarily large head with accordion-like folds of the Noctilionidae, differences in the degree of development of the outer dense fibers, and the presence/absence of a perforatorium.

Keywords: Acrosome, Bat, Chiroptera, Golgi complex, Molossidae, Perforatorium, Phyllostomidae, Sperm, Spermatogenesis, Spermiogenesis, Testis, Ultrastructure, Vespertilionidae.