Acrosomal Reaction of Thyone Sperm

Structural changes inside the head of Thyone sperm undergoing the acrosomal reaction were followed with a high-resolution, differential interference contrast (DIC) video microscope . The beating sperm, adhering by their midpiece to the cover slip of a wedge perfusion chamber, were activated by a calcium ionophore (201uM A23187) suspended in sea water containing 50 mM excess CaC12. Before activation of the sperm, the acrosomal region appears as a 1 .1-,um diameter sphere, slightly less dense than the rest of the sperm head. Upon activation, the acrosome pops; the acrosomal region suddenly swells and its refractive index drops. After "̂1 s, a crescent-shaped periacrosomal cup appears behind the acrosomal vacuole. In the next several seconds, the cup loses more refractive index and expands forward as the acrosomal process extends. The acrosomal vacuole becomes smaller, but without appreciable drop in refractive index. These observations, coupled with the behavior of the extending acrosomal process reported in the companion paper, and in electron microscopy (EM) and early physiological studies, suggest that the acrosomal process is extended by a combination of the explosive polymerization of actin and the osmotic swelling of the periacrosomal cup material . In this paper, we also consider the meaning of the enhanced DIC image seen in the highresolution video microscope, and discuss the reliability of measurements on small linear dimensions made with the DIC microscope . For successful fertilization, a sperm encountering the egg must first traverse the egg's protective jelly and other coats . Within seconds after the arrival of sperm, a delicate thread is observed in thejelly of some echinoderm eggs, extending from the sperm head to the cytoplasmic surface of the egg. At the base of the thread, the surface of the egg may protrude, or display a "fertilization-" or "entrance-cone." Some earlier observers believed that the fertilization cone, and the thread that was believed to be spun out of the cone, were the egg's way of lassoing the sperm into the ooplasm (4, 14) . Others did not find the thread and also argued that the cone formed only after the sperm came into contact with the egg or its envelope (e.g ., reference 27, p . 409) . In the 1950's, however, J . C . Dan (8) and the Colwins (5) showed decisively that the fine thread that had been seen in Dedicated to Jean Clark Dan, humanist, friend, pioneer in research on the acrosomal reaction . 812 the egg jelly was produced by the sperm rather than the egg. Even in the absence ofthe egg, they could induce the thread to grow from the sperm, specifically from its acrosome, a small bior tripartite body found at the apex of the sperm (1) . The sperm was said to go through an acrosomal reaction, and the fine thread produced at the tip of the sperm was named the acrosomal process . Thus, in the normal process of fertilization, the sperm encountering the egg jelly undergoes an acrosomal reaction and the acrosomal process of the sperm extends and penetrates the jelly and vitelline envelope . After the sperm and egg plasma membrane have united at the tip of the acrosomal process, the nucleus and cytoplasm ofthe sperm are drawn into the egg. In the acrosomal reaction, the sperm first reacts by a change in the cell membrane that lies in contact with the "acrosomal granule," or acrosomal vacuole (Fig . 1) . In a reaction requiring the presence of Ca", the membranes ofthe acrosomal vacuole and the sperm fuse in a ring-shaped annulus at the apex of the THE JOURNAL Of CELL BIOLOGY " VOLUME 93 JUNE 1982 812-819 © The Rockefeller University Press " 0021-9525/82/06/0812/08 $1 .00 on Jne 4, 2017 D ow nladed fom Published June 1, 1982