Ultrastructual studies on spematids and sertoli cells during early spermogenesis in the bandicoot Peramelea nasuta geoffroy (Marsupialia)

Abstract

The present paper deals with spermiogenesis up to and including the attachment of spermatids to Sertoli cells. The first observed step in spermatid differentiation was the development of the anlage of the middle piece and principal piece. This anlage, called the axial filament complex, has the structure of a cilium and arises from the future longitudinal centriole, while the latter, together with the future transverse centriole, lies in the vicinity of the Golgi complex. The definitive acrosomal vacuole, which ultimately becomes attached to and invaginates the nuclear envelope, is formed by the enlargement and coalescence of Golgi vacuoles. While this definitive vacuole is developing, the centrioles and attached axial filament complex migrate to the opposite pole of the nucleus. Before and during migration a number of accessory structures are developed in association with the centrioles, and one of these structures, the proximal junctional body, invaginates the nuclear envelope when the centrioles reach their definitive abacrosomal position. During this period, a cytoplasmic canal forms around the intraspermatid part of the axial filament complex. The definitive acrosomal vacuole ultimately extends out to make close contact with the plasma membrane of the spermatid. This stage of development is followed by a process of nuclear protrusion, initiated by the migration of the nucleus towards the region of contact between acrosomal vacuole and spermatid plasma membrane. During the migratory phase, that part of the nuclear envelope previously invaginated by the acrosomal vacuole becomes everted and the latter collapses, finally becoming sandwiched in between the nucleus and the plasma membrane of the spermatid. The nucleus subsequently projects from the surface of the spermatid, its acrosome-covered apex becoming coneshaped. During these phases of development the accessory structures elaborated in association with the centrioles, and which now lie in the neck region of the spermatid, have become more highly organized. The manchette begins to develop in spermatids at the stage at which the acrosome has become sandwiched in between the nucleus and the plasma membrane of the spermatid. Concurrently the spermatids become surrounded on all sides by Sertoli cell cytoplasm. In the later stages of nuclear protrusion, the manchette elongates and its walls become thicker. The protruding nuclei become orientated with their acrosome-covered apices facing towards the basement membrane of the tubules. Aggregations of finely granular material appear in Sertoli cell cytoplasm in the region of contact with the acrosomal vacuole. The possible role of the manchette and of Sertoli cell cytoplasm in the phenomenon of nuclear protrusion and orientation is discussed.