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The echinoid perivisceral coelomic plasma contains a prominent glycoprotein particle which has a sedimentation coefficient of 22S in Dendraster excentricus and 24S in Strongylocentrotus purpuratus. The major constituent of the plasma particles is a 200 kDa glycopeptide which is also an abundant constituent of the major yolk glycoprotein precursor particles stored in the ovarian accessory cells prior to vitellogenesis [Ozaki, H. et al.: Wilhelm Roux' Arch. Dev. Biol. 195, 74-79 (1986)]. Through in vitro culture analysis we have found that the high molecular weight plasma glycoprotein is secreted by coelomocytes of the perivisceral coelom. Moreover, the results of radioisotope labeling experiments in D. excentricus have shown that the synthesis of the high molecular weight glycoprotein took place predominantly in the coelom, and that its synthesis was not detectable in the ovary or digestive tract. It appears, therefore, that the yolk glycoprotein precursor when secreted by the coelomocytes reaches the ovary through the circulatory system.

A high-molecular-weight glycoprotein with a sedimentation coefficient of 22.6 has been isolated and characterized from the accessory cells in the previtellogenic ovary of the echinoid Dendraster excentricus. This glycoprotein is similar to the major yolk glycoprotein of the mature egg in its electrophoretic mobility under non-denaturing conditions, high mannose-type glycan, amino acid composition, constitutive glycopeptides, and immunological determinants. Previous histological and electron microscopical analyses led to the hypothesis that vitellogenesis involves a translocation of material from the accessory cell in the ovary to the oocyte. Because of the close similarities of the accessory cell glycoprotein to the yolk glycoprotein of the mature egg, we conclude that the glycoprotein in the accessory cell is a precursor to the major glycoprotein of the egg yolk. This conclusion is further supported by our additional finding that the accessory cell of another echinoid, Strongylocentrotus purpuratus, also contains a high-molecular-weight (24 S) glycoprotein which shows similarities to the yolk glycoprotein of the mature egg in the carbohydrate moiety and the constitutive glycopeptides.