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

Abstract

This, the last of a series of three papers, deals with the final events which lead to the incorporation of the spermatozoon with the egg. The material used consisted of moderately polyspermic eggs of Hydroides hexagonus, osmium-fixed at various times up to five minutes after insemination. The first direct contact of sperm head with egg proper is by means of the acrosomal tubules. These deeply indent the egg plasma membrane, and consequently at the apex of the sperm head the surfaces of the two gametes become interdigitated. But at first the sperm and egg plasma membranes maintain their identity and a cross-section through the region of interdigitation shows these two membranes as a number of sets of two closely concentric rings. The egg plasma membrane rises to form a cone which starts to project into the hole which the spermatozoon earlier had produced in the vitelline membrane by means of lysis. But the cone does not literally engulf the sperm head. Instead, where they come into contact, sperm plasma membrane and egg plasma membrane fuse to form one continuous membranous sheet. At this juncture the two gametes have in effect become mutually incorporated and have formed a single fertilized cell with one continuous bounding membrane. At this time, at least, the membrane is a mosaic of mostly egg plasma membrane and a patch of sperm plasma membrane. The evidence indicates that the fusion of the two membranes results from vesiculation of the sperm and egg plasma membranes in the region at which they come to adjoin. Once this fusion of membranes is accomplished, the egg cytoplasm intrudes between the now common membrane and the internal sperm structures, such as the nucleus, and even extends into the flagellum;finally these sperm structures come to lie in the main body of the egg. The vesiculation suggested above appears possibly to resemble pinocytosis, with the difference that the vesicles are formed from the plasma membranes of two cells. At no time, however, is the sperm as a whole engulfed and brought to the interior of the egg within a large vesicle.

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.…”

“…In Hydroides, following sperm activation the sperm and egg plasma membranes fuse to form the zygote plasma membrane (6,8). Other investigators have found evidence for zygote formation by gamete membrane fusion in the rat (24), a green alga (16), and, in a sense, in the paramecium 0,21).…”

“…Thus, in Hydroides (8), a sperm profile which in the light microscope might seem to be just attached apically to the egg, can actually be a profile of a sperm cell into which the egg cytoplasm has penetrated even as far as into the flagellum. In the light microscope, in Saccoglossus, projections of the cone appear to rise up around the sperm apex.…”

“…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).…”