Immunocytochemical studies on the acroframosome during spermiogenesis of the caridean shrimp Macrobrachium nipponense (Crustacea, Natantia)

Spermiogenesis is a highly ordered process in male germ cell differentiation. KIFC1, a C-terminal kinesin, participates in acrosome formation and nuclear shaping during spermiogenesis of mammals. In this paper we used Western Blot and confocal microscopy to identify the existence and distribution of KIFC1 and microtubules during spermiogenesis in the caridean shrimp Macrobrachium nipponense. KIFC1 first appears in the cytoplasm of early spermatids, and then co-localizes with the acrosome in a punctate pattern. KIFC1 is also diffusely present in the nucleus of late spermatids. In mid- and late spermatids, microtubules have a specific polarized organization shaped like an everted umbrella, and we name this structure the acroframosome (AFS). Such a distinct structure formed by microtubules which frames the acrosome is described here for the first time in an invertebrate. We found that the AFS is a special microtubular structure which is present at the proacrosomal pole in mid- and late spermatids; KIFC1 but not kinesin I was found to be co-localized with the AFS. Our conclusion is that the AFS is a framework for acrosome biogenesis and the motor KIFC1 participates in these processes. We propose that the AFS may function in the following ways: (1) as a pathway to transport vesicles to the proacrosome; (2) as a framework for acrosome biogenesis; (3) as an anchor to tether the acrosome to the nucleus.