The Acrosome Reaction In

Suspensions of Mytilus edulis eggs were fixed with osmium tetroxide at various intervals between 1 and l0 seconds after heavy insemination, and sectioned for electron microscopy to follow the natural process of acrosome reaction in the spermatozoa around the eggs. Sperm suspensions were also fixed after the addition of I0 per cent by volume of M/3 calcium chloride. Within the first second after the acrosolne is stimulated to react, an opening appears at its apex, around which the plasma and acrosomal membranes fuse to each other, and the resulting membrane complex is reflected backward, presumably by the swelling of material lining it. At the same time the other material within the now open vesicle disappears, and the rudiment of the acrosomal process, consisting of a short axial rod loosely surrounded by the invaginatcd part of the acrosomal membrane, is exposed at the anterior side of the sperm head. Within another second this rudiment is extended by elongation of the axial rod and expansion of the surrounding membrane. If the spermatozoon has reacted close to the egg surface, the elongation may be very slight, whereas in suspended spermatozoa the process may reach a length of 13 #. Possible mechanisms underlying these changes are suggested. As the previous paper has demonstrated (10), the intact acrosome of the Mytilus edulis spermatozoon has a conspicuous conical acrosomal vesicle applied to the anterior face of the sperm nucleus, directly under the plasma membrane. The membrane of this vesicle is invaginated at the side facing the nucleus, to form a spacious pocket; a slender axial rod originates in a tube passing through the nucleus and extends through the center of the invagination to a length of about 2 #. The present report will show that the acrosomal vesicle opens when the spermatozoon comes into contact with an egg, exposing the invaginated portion of the acrosomal membrane, within which the axial rod elongates. Whereas the acrosomal process formed in this way resembles those of the echinoderms (6-8) in being single and having a central core, the intact acrosome of Mytilus is much more like that of the annelid Hydroides (1, 3) in its general structure and in its mode of reaction, the details of which will be described in this paper. M A T E R I A L AND METHODS Eggs of Mytilus edulis, obtained by electrically stimulated spawning (9), were heavily inseminated and 249 on July 3, 2017 jcb.rress.org D ow nladed fom