Mechanisms of Fertilization in Fishes

Abstract

SYNOPSIS. Sperm motility in fishes is, in general, initiated by simple dilution of the epididymal suspension. A few simple organic compounds and poorly characterized proteincontaining “factors” associated with fish eggs seem to enhance sperm motility. True chemotaxis in fish sperm has not been demonstrated. Ultimately, sperm-egg interaction leads to a large, transient increase in the concentration of free calcium in the egg's cytoplasm. This calcium increase is responsible for lifting the egg's developmental block; some mechanisms by which the calcium flux may accomplish this are discussed. A pH change is apparently not involved in turning on synthetic activity in fish eggs. Electrophysiological events and the fusion of cortical vesicles with the plasma membrane, which are among the earliest changes in the egg resulting from fertilization, are apparently not necessary for development of fish eggs. Synthesis of RNA and protein required for normal cleavage, axiation and organogenesis occurs during and soon after the calcium transient. Completion of the egg's second maturation division occurs within minutes after fertilization. Ooplasmic segregation (by some totally unknown mechanism) and opposition of male and female pronuclei complete the transformation of the egg into a functional zygote.