A CYTOLOGICAL ANALYSIS OF FERTILIZATION INCHAETOPTERUS PERGAMENTACEUS

Abstract

We have examined sperm-egg interaction in Chaetopterus pergamentaceus by electron microscopy. The initial contact between sperm and egg involved the membrane of the unreacted acrosome and either the tips of egg microvilli which penetrated the vitelline layer or jelly emanating from the tips of the microvilli. This resulted in an acrosome reaction and fusion between the inner acrosomal membrane and the tip of the microvillus. Sperm did not produce acrosomal processes like those of many other invertebrates, and no part of the sperm penetrated the vitelline layer until the sperm was incorporated into the fertilization cone. The fertilization cone was very small and was composed of egg microvilli. The sperm nucleus and mitochondrion were incorporated into the fertilization cone, but a recognizable sperm mitochondrion could not subsequently be seen in the egg cytoplasm. Although the axoneme of the sperm tail was present in the fertilization cone at early stages of sperm penetration, the sperm tail evidently detached in the later stages of incorporation because it could not be seen in the zygote cytoplasm after sperm incorporation. The sperm chromatin decondensed uniformly and became surrounded by a typical nuclear envelope. The results indicate that Chaetopterus provides an example of a previously undescribed model for sperm penetration of egg vestments in which the sperm needs neither to produce an acrosomal process nor to liberate vitelline layer lysins because it penetrates the vitelline layer passively after incorporation into the egg cytoplasm.