Comparative ultrastructural studies of spermiogenesis and spermatozoa in some species of Polyplacophora (Mollusca)

Summary

Comparative data on the ultrastructure of spermiogenesis and spermatozoa of the Polyplacophora Acanthochitona crinita, Chaetopleura angulata and Callochiton septemvalvis are presented in this study. In contrast to what has been described for this and other classes of Mollusca, no acrosome is present in the spermatozoa of these Polyplacophora. The nucleus is extended by a long, thin apical point. In A. crinita and C. angulata the mitochondria are situated at the basal and lateral regions of the nucleus. They do not present a typical middle piece. These species present a pericentriolar process. In C. septemvalvis the mitochondria are situated at the base of the nucleus, surrounding the centrioles, which are orthogonally positioned in all species. The ultrastructural development during spermiogenesis is similar. In middle spermatids of A. crinita, the chromatin is arranged in fine filaments. In C. septemvalvis and C. angulata the chromatin filaments are thicker, forming coarse bands. In late spermatids elongation of the nucleus continues, it becomes rather electron-dense and the chromatin filaments are more condensed. Finally, the nucleus has a uniformly electron-dense appearance, with no signs of filamentous organization. Considering the ultrastructural modifications observed, the Polyplacophora spermatozoa could be included in a modified type.