Fine structure of spermiogenesis in the elasmobranch fish Squalus suckleyi. II. Late stages of differentiation and structure of the mature spermatozoon

Abstract

The fine structure of late aspects of spermatid differentiation and the mature spermatozoon of Squalus suckleyi is described. After the major part of nuclear elongation is completed, the nucleus assumes helical form beginning at the posterior end and progressing anteriorly. Accompanying nuclear spiralization are dissolution of the fibrous nuclear sheath, segregation of parachromatin material, and sequential changes in position of a band of nuclear pores. The two fibrous elements of the axial midpiece rod fuse and finally lose evidence of crossstriations. Elongate mitochondria fragment into smaller, spherical forms that aggregate around the axial midpiece rod to delineate the midpiece. A helical sheath of filaments of doubtful origin closely surrounds the mitochondrial sheath. The helical form of the head, midpiece, and tail elements is obviously correlated with the observed pattern of motility, that is rotation of the helical sperm about its long axis.