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

The yolk granules of sea urchin, Hemicentrotus pulcherrimus, were obse rved by an electron microscope and classified into 4 types; dense, int ermediate, sparse and lysosomal yolk granules, on the basis of their s tructural characteristics. In unfertilized eggs, most of the yolk gran ules were dense granules, whereas they were not observed in gastrulae. During the development of the sea urchin embryo, the dense granules d ecreased, while the sparse granules increased. The yolk granules seem to change from the dense state to the sparse state via the intermediat e state. These morphological changes are in accordance with those obse rved in the yolk granules, which are isolated and incubated in acidic conditions [22]. Some biochemical changes, which underlie these morpho logical changes, were also investigated. Analyses of yolk proteins by column chromatography and SDS-polyacrylamide gel electrophoresis revea led that overall physico-chemical properties of yolk glycoprotein comp lex were maintained after proteolytic processing. The lipid compositio n of the yolk granules was analyzed. In acidic conditions, although tr iacylglycerol, phosphatidylcholine and phosphatidylethanolamine were l ost more than 50%, 70% of cholesterol was retained after the incubatio n in acidic conditions for 24 hr, when the yolk granules became the sp arse state.