Strange vesicles with a homogeneous content in spermatocytes and spermatids of a click beetle, Adelocera murina (Elateridae). A fine structure study

Abstract

The restructuring of primary spermatocytes of Adelocera murina, a click beetle, is described using electron microscopy of ultrathin sections. Emphasis is on spherical or rod-shaped cytoplasmic inclusions, invested by a unit membrane. The content of the inclusions is slightly more electron-dense than that of the surrounding cytoplasm and homogeneously textured in most cases. The inclusions are missing in spermatogonia but are abundant in prophase I through anaphase I spermatocytes. Their number declines in telophase I. Very similar elements are associated with the distal ends of the outgrowing flagella in metaphase I through telophase I spermatocytes and form the so-called flagellar vesicles. The biological significance of the cytoplasmic vesicles is a matter of speculation. It can be ruled out, on account of their structure, that they are parasites, endosymbionts or chromatoid bodies. The suggestion is ventured that the cytoplasmic inclusions represent unused flagellar vesicles. In order to determine the fate of these putative unused flagellar vesicles beyond meiosis, young spermatids were also examined. In the tail portion of the spermatids, membrane-invested inclusions were detected. Their structure is reminiscent of the cytoplasmic vesicles found in spermatocytes in that they posses a homogeneous content which is, however, of lesser electron density. This could mean that the vesicles serve also as containers for substances produced in the cytoplasm of prophase I spermatocytes. The material may be used up throughout the remainder of meiosis and at the onset of spermiogenesis.