Laura Hunter Colwin

Membrane Fusion in Relation to Sperm-Egg Association

Publisher Summary This chapter describes the relationship between membrane fusion and sperm–egg association. At the inception of the zygote, the external rather than the cytoplasmic sides of the plasma membranes are the first to be involved. In species with vitelline envelopes, the meeting of plasma membranes takes place beneath the envelope so that a part of the egg plasma membrane that meets the sperm plasma membrane is in a sense also a newly exposed surface. However, in algae, the gametes are recently shed cells so that both membranes are newly exposed ones. The parts of the plasma membrane that begin the fusion are newly exposed exterior surfaces. As a result of the fusion, the content of the vesicle is conducted out of the cell without coming in contact with the cytoplasm.

THE ACROSOME FILAMENT AND SPERM ENTRY IN THYONE BRIAREUS (HOLOTHURIA) AND ASTERIAS

1. As shown by quantitative data, spermatozoa of Thyone briareus treated with egg water or alkaline (ammoniated) sea water undergo an acrosome reaction resulting in the production of acrosome filaments in a percentage significantly greater than found in untreated controls. Acrosome filaments are also found in inseminated cultures of eggs, either associated or unassociated with the eggs.2. The acrosome filaments of Thyone briareus are exceptionally long and may even exceed the thickness of the jelly hull (55 µ) or even the length of the flagellum (60 µ).3. Reacted spermatozoa of Asterias forbesii and A. vulgaris seen in inseminated cultures of eggs have acrosome filaments measuring about 15-22 µ.4. The general nature of the acrosome filament is much the same in both species of Asterias and in Thyone, regardless of the stimulating agent. Though thread-like in dimensions it is usually straight, or curved only in a wide arc. However, it is capable of curving sharply, bending and perhaps even breaking. The tip...

Role of the Gamete Membranes in Fertilization

An earlier paper showed that in Saccoglossus the acrosomal tubule makes contact with the egg plasma membrane. The present paper includes evidence that the sperm and egg plasma membranes fuse to establish the single continuous zygote membrane which, consequently, is a mosaic. Contrary to the general hypothesis of Tyler, pinocytosis or phagocytosis plays no role in zygote formation. Contact between the gametes is actually between two newly exposed surfaces: in the spermatozoon, the surface was formerly the interior of the acrosomal vesicle; in the egg, it was membrane previously covered by the egg envelopes. The concept that all the events of fertilization are mediated by a fertilizin-antifertilizin reaction seems an oversimplification of events actually observed: rather, the evidence indicates that a series of specific biochemical interactions probably would be involved. Gamete membrane fusion permits sperm periacrosomal material to meet the egg cytoplasm; if an activating substance exists in the spermatozoon it probably is periacrosomal rather than acrosomal in origin. The contents of the acrosome are expended in the process of delivering the sperm plasma membrane to the egg plasma membrane. After these membranes coalesce, the sperm nucleus and other internal sperm structures move into the egg cytoplasm. I N T R O D U C T I O N This is the second of two papers concerning the fine structure of fertilization in Saccoglossus kowalevskii, a member of the phylum Hemichordata. The previous paper (9) showed that the acrosomal structure and the events of sperm activation in Saccoglossus follow the same basic pattern as in the annelid, Hydroides (6, 7, 13), a pattern for which there is some evidence from four other phyla as well. 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 , 2 1 ) . In the present paper the method of zygote formation in Saccoglossus will be described. On the basis of electron microscopic observations, the hypothesis that pinocytosis might be a method of zygote formation (26) will be examined, and the role of the acrosomal region in egg activation (2) will be discussed. M A T E R I A L S A N D M E T H O D S The materials used and the methods of preparation were the same as those described in the preceding paper (9). 501 on O cber 0, 2017 jcb.rress.org D ow nladed fom

Behavior of the spermatozoon during sperm-blastomere fusion and its significance for fertilization (Saccoglossus kowalevskii: hemichordata)

SummaryAssociation of spermatozoa with blastomeres removed from the egg envelope was studied in order to investigate sperm membrane behavior under circumstances unlike those attending fertilization of normal envelope-enclosed eggs. Under the altered circumstances all parts of the surface of the sperm cell were afforded the opportunity of meeting blastomere plasma membrane. Nevertheless, in each case the spermatozoon first became activated, an acrosomal tubule formed, and it was this organelle alone which fused and established continuity with the blastomere plasma membrane. This behavior of the spermatozoon with respect to the denuded blastomere parallels the behavior of the sperm cell with respect to the envelope-enclosed egg at fertilization. Since the sperm cell behaves so similarly in two situations which are so different with respect to what the spermatozoon encounters, this behavior is considered to reflect the inherent nature of the spermatozoon, and analysis of this behavior in relation to a blastomere is considered to be valid also in relation to an egg during fertilization. It is concluded that membrane fusion, involving the acrosomal tubule of an activated spermatozoon, is the only means by which gametic union is established and that these are obligatory rather than fortuitous features of sperm-egg association in Saccoglossus. It is suggested that these features are probably obligatory also in other species which exhibit this pattern of sperm-egg association.