Acrosomal triggers

The acrosome reaction as observed in marine invertebrates is consistently found to begin with an apical opening where the acrosome lies in immediate contact with the plasma membrane. In an effort to elucidate the fine-structural properties of the acrosomal component that triggers this breaking of the plasma membrane, sea urchin and Mytilus edulis spermatozoa were fixed with osmium tetroxide, potassium permanganate and glutaraldehyde, and observed at high magnifications with the electron microscope.The special feature which was found to characterize both types of acrosome is an extremely close contact between the plasma membrane and the underlying acrosomal trigger component. This relation is limited to the part of the plasma membrane which is observed to be removed as the first visible step in the acrosome reaction.