Ultrastructure of a fertilized barnacle egg (Pollicipes polymerus) with peristaltic constrictions

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Abstract
The egg ofPollicipes polymerus, the common intertidal gooseneck barnacle, has been studied by electron microscopy. Constriction rings, similar to the contractile rings of cleaving cells and polar lobes, move unidirectionally from the animal to the vegetal pole of newly fertilized eggs. This is referred to as peristaltic constriction. The present paper describes the fine structure of the egg during first polar body formation and peristalsis. During formation of the polar body, dense bodies are produced by the Golgi and extracellular plaques are observed. Thin microfilaments (40–60 Å) are in the egg adjacent to the polar body. In eggs undergoing peristalsis, the appearance of extracellular spheres, flocculent material and filaments is observed. Intracellularly large numbers of multivesiculate bodies, glycogen granules, mitochondria and protein-carbohydrate and lipid yolk bodies are seen at the level of constriction. Thin microfilaments are found in the cortical area of newly-fertilized eggs exclusively in peristaltic constriction rings. Filaments are oriented primarily in a meshwork, although circumferentially-oriented filaments are also found in rings near the vegetal pole. Microvilli extend into the space created between a constriction and the elevated egg membrane. A model is proposed to explain the peristalsis in this species. It is suggested that information from a pacemaker region activates peristalsis by affecting filament polymerization and orientation. One function of peristalsis may be elongation of the egg from a sphere to an ovoid, although other possibilities such as elevation of the egg membrane, segregation of the lipid yolk to the vegetal pole and predetermination of the first cleavage plane are also discussed.