Nuclear reorganization in ram spermatids

Abstract

Nuclear changes as a function of ram spermatid differentiation have been studied using electron microscopy, cytochemistry, and treatment of isolated nuclei with chemical and mechanical agents. In round nuclei, chromatin consists of intertwined knobby fibers about 210 and 120 A in diameter. In flattening nuclei, these fibers progressively change into smooth filaments at least 25–33 A thick. This change is interpreted as reflecting somatic histone removal known to occur in these nuclei (M. Loir and M. Lanneau, Exp. Cell. Res. 115 , 231, 1978). The packaging of chromatin is carried out by aggregation of the smooth filaments into large (about 300 A) contorted threads which finally coalesce to form a homogeneous mass. In flattened nuclei a small fraction of chromatin (basal knobs) does not undergo complete packaging and retains both cytochemical properties and resistance to disruption similar to those of chromatin in flattening nuclei. Correlations of changes in nuclear morphology, ultrastructure, and resistance to chemical and physical degradation with changes in nucleoproteins suggest that the cystine-containing spermatid-specific proteins could play an important role in chromatin-structure reorganization, shaping and increasing stabilization of the nucleus. The sperm-specific protein promotes terminal packaging and stabilization of chromatin. Other nuclear components considered in this study are: the RNP-containing structures, the nuclear posterior space, and the redundant nuclear envelope; the last two structures are possibly related to the transfer of proteins leaving the chromatin for the cytoplasm.