Pinocytosis in fertilized sea urchin (<i>Arbacia punctulata</i>) eggs

Abstract

Abstract Fertilized and unfertilized Arbacia eggs were incubated with extracellular tracers in an effort to: (1) correlate the onset of pinocytosis with egg activation and its possible role in restructuring the zygote's surface and (2) determine the fate of internalized vesicles and their contents. Zygotes incubated with horseradish peroxidase (HRP), native ferritin, or cationized ferritin incorporated tracers within coated vesicles by 4–7 minutes postinsemination. In contrast, endocytosis by unfertilized eggs was not observed. Zygotes taken at intervals up to cleavage (60 minutes postinsemination) exhibited all three tracers within coated and uncoated vesicles and lysosomes. HRP reaction product was identified within Golgi cisternae in only a few zygotes; native and cationized ferritin were never observed within Golgi elements. The sequential appearance of tracers in pinocytotic vesicles and lysosomes of fertilized eggs suggests that surface‐related components are interiorized and then modified or degraded. Because the zygote surface undergoes a reduction in area, temporally correlated with the onset of pinocytosis, it is possible that endocytosis may be instrumental in the accommodation of the plasmalemma. The interiorization of surface membrane in fertilized eggs may represent a mechanism for the alteration of developmentally related cell surface components.