CHANGES IN THE SPERMATOZOON DURING FERTILIZATION IN HYDROIDES HEXAGONUS (ANNELIDA)

Abstract

In the previous paper the structure of the acrosomal region of the spermatozoon was described. The present paper describes the changes which this region undergoes during passage through the vitelline membrane. The material used consisted of moderately polyspermic eggs of Hydroides hexagonus, osmium-fixed usually 9 seconds after insemination. There are essentially four major changes in the acrosome during passage of the sperm head through the vitelline membrane. First, the acrosome breaks open apically by a kind of dehiscence which results in the formation of a well defined orifice. Around the lips of the orifice the edges of the plasma and acrosoma] membranes are then found to be fused to form a continuous membranous sheet. Second, the wails of the acrosomal vesicle are completely everted, and this appears to be the means by which the apex of the sperm head is moved through the vitelline membrane. The lip of the orifice comes to lie deeper and deeper within the vitelline membrane. At the same time the lip itself is made up of constantly changing material as first the material of the outer zone and then that of the intermediate zone everts. One is reminded of the lip of an amphibian blastopore, which during gastrulation maintains its morphological identity as a lip but is nevertheless made up of constantly changing cells, with constantly changing outline and even constantly changing position. Third, the large acrosomal granule rapidly disappears. This disappearance is closely correlated with a corresponding disappearance of a part of the principal material of the vitelline membrane from before it, and the suggestion is made that the acrosomal granule is the source of the lysin which dissolves this part of the vitelline membrane. Fourth, in the inner zone the fifteen or so short tubular invaginations of the acrosomal membrane, present in the normal unreacted spermatozoon, lengthen considerably to become a tuft of acrosomal tubules. These tubules arc the first structures of the advancing sperm head to touch the plasma membrane of the egg. It is notable that the surface of the acrosomal tubules which once faced into the closed acrosomal cavity becomes the first part of the sperm plasma membrane to meet the plasma membrane of the egg. The acrosomal tubules of Hydroides, which arise simply by lengthening of already existing shorter tubules, are considered to represent the acrosome filaments of other species. 231 on

ROLE OF THE GAMETE MEMBRANES IN FERTILIZATION IN SACCOGLOSSUS KOWALEVSKII (ENTEROPNEUSTA)

“…In the light of the present observations, together sperm cell to the egg plasma membrane and in doing so is itself expended. In the second place, when the acrosomal tubule(s) fuse(s) with the egg plasma membrane and a funnel-like connection between the two gametes is established, the con- with previous observations in Saccoglossus and Hydroides (8,9,13), Bowen's views are no longer tenable. In the first place, the spermatozoon is not "swallowed," that is, it does not "enter" the egg intact.…”

“…But in Hydroides (8,13) and also in Saccoglossus, detailed series of stages show that at no time is there any engulfment of the spermatozoon with its intact cell membrane by the egg plasma membrane, and the observations in the rat (24) and in the alga, Prasiola (15, engulfment. There is no evidence for such a role in Hydroides in which, it is true, microvilli do project into the egg envelope.…”

“…It will be recalled that the sperm plasma membrane at this time is a mosaic and that, until activation, its mosaically inserted apical segment faced into the closed acrosomal cavity. Thus the surface of this segment is one which is newly exposed to the exterior (8,9,13). It is the apex of this part of the membrane which meets and fuses with the egg plasma membrane (6,8).…”

Acrosomal Reaction ofThyoneSperm I: Changes in the Sperm Head Visualized by High Resolution Video Microscopy

“…The membrane permeability changes and, according to Dan et al (10,12), water enters the acrosome. Unless the pH of the medium is too low, the membrane bounded by the annulus is cast off (6,11). At this time the outer face of the sperm membrane has become contiguous with the inner face of the membrane surrounding the acrosomal vacuole .…”

“…The new acrosome membrane is in turn covered by the contents of the acrosomal vacuole which is believed to contain the lysins needed to penetrate the jelly coat and vitelline envelope (for review, see references 6,9), and bindin, a protein thought to attach sperm to the egg (see references 19) . The whole acrosomal reaction, including the extension of the process, is completed in a few seconds .…”