The Ultrastructure of Spermatozoa with a Note on the Formation of the Acrosomal Filament in the Lamprey, Lampetra japonica

Abstract

The mature spermatozoon of Lampetra japonica is about 130μm long.The head is rodshaped, measuring about 8μm long and 1μm wide. It bears a membrane-bounded acrosomal vesicle at its anterior end.The anterior surface of the nucleus in the head forms a depression, where a subacrosomal ring is found.The bottom of the depression leads to the nuclear canal.The central fiber, which starts from the posterior surface of the acrosomal vesicle, perforates the subacro-somal ring and runs the entire length of the nuclear canal.The elongated nucleus forms a pouch laterally near the posterior end of the head, where two cylindrical centrioles are visible;the axes of both centrioles coincide with a long axis of the spermatozoon.The tail flagellum shows the 9 +9 +2 structure known in that of animals with internal fertilization.There is no strictly defined middle piece, but several mitochondria are detected near the centrioles or in regions far from the snerm head.
An acrosomal filament is formed in some spermatozoa when they are subjected to stimulation at the time of fixation: the central fiber is discharged from the nuclear canal and pushes against the posterior membrane of the acrosomal vesicle;this induces the fusion of the anterior membrane of the acrosomal vesicle with the plasma membrane.The acrosomal filament thus formed consists of the central fiber and its surrounding membrane.No changes in the morphology of the subacrosomal ring are detected during the filament formation.We conclude that the acrosome reaction closely resembles that reported in certain molluscs and annelids which have an axial rod in the head.