Fertilization and early development of sea urchins.

Scanning electron microscopy (SEM) has been successfully employed for the study of several surface-mediated events during fertilization and early development in sea urchins. In addition to basic morphological descriptions of the sperm, the extrusion of the acrosomal process has been documented with SEM. During sperm incorporation, short microvilli are found to elongate around the successful sperm. In eggs denuded of vitelline layers, in which the elevation and hardening of this fertilization coat is prevented, numerous long microvilli have been shown to cluster around and elongate over the entering sperm during sperm incorporation. Following sperm incorporation and the elevation of the fertilization coat, scanning electron microscopy has been utilized to study the bursts of elongation of the previously short egg microvilli. These microvilli appear to undergo two bursts in length due primarily to the new assembly of microfilaments in the egg cortex. Cytokinesis occurs shortly after the second burst of microvillar elongation. The morula stage is characterized by loosely attached cells which become more closely apposed in subsequent cell divisions to result in the hollow blastula. The ciliated blastula hatches from the fertilization coat, whereupon gastrulation occurs, resulting in a free-swimming, feeding larval stage. This paper reviews the surface alterations and the contribution of scanning electron microscopy to the study of these surface alterations, from fertilization through early development.