MORPHOLOGICAL AND BIOCHEMICAL-STUDIES ON YOLK DEGRADATION IN THE SEA-URCHIN, HEMICENTROTUS-PULCHERRIMUS

The yolk granules of sea urchin, Hemicentrotus pulcherrimus, were observed by an electron microscope and classified into 4 types; dense, intermediate, sparse and lysosomal yolk granules, on the basis of their structural characteristics. In unfertilized eggs, most of the yolk granules were dense granules, whereas they were not observed in gastrulae. During the development of the sea urchin embryo, the dense granules decreased, while the sparse granules increased. The yolk granules seem to change from the dense state to the sparse state via the intermediate state. These morphological changes are in accordance with those observed in the yolk granules, which are isolated and incubated in acidic conditions [22]. Some biochemical changes, which underlie these morphological changes, were also investigated. Analyses of yolk proteins by column chromatography and SDS-polyacrylamide gel electrophoresis revealed that overall physico-chemical properties of yolk glycoprotein complex were maintained after proteolytic processing. The lipid composition of the yolk granules was analyzed. In acidic conditions, although triacylglycerol, phosphatidylcholine and phosphatidylethanolamine were lost more than 50%, 70% of cholesterol was retained after the incubation in acidic conditions for 24 hr, when the yolk granules became the sparse state.