Identification and Partial Characterization of Yolk and Cortical Granule Proteins in Eggs and Embryos of the Starfish, Pisaster ochraceus

Abstract

The unfertilized oocyte contains various granules which serve as storage sites for proteins, the majority which are yolk granules or platelets. Yolk granules are thought to provide the developing embryo with nutrients essential for its survival, while other granules contain proteins such as enzymes and extracellular matrix components that are required for fertilization and perhaps for early development. This study uses immunofluorescence and immunogold techniques with two novel monoclonal antibodies against proteins found in egg yolk and cortical granules to study the localizations of these antigens during early starfish development. Partial biochemical characterizations using the anti-yolk antibody have revealed that there are a family of structurally related proteins in oocyte yolk granules and that while the molecular compositions of the yolk proteins change during embryogenesis, their depletion is not significant until the larval stage, suggesting these proteins are not required for early development. In addition, a large immunoreactive protein has been found in the intestine and coelomic fluid, suggesting that, as in other species, the starfish yolk proteins may be derived from a large precursor, such as vitellogenin. Analysis of the anti-cortical granule antibody has revealed that a 120-kDa antigen is stored in cortical granules of unfertilized eggs. Upon egg activation, the cortical granules located in the peripheral egg cytoplasm undergo exocytosis, and the 120-kDa antigen is released into the perivitelline space. However, other granules, which are also labeled by this antibody, remain dispersed throughout the egg cytoplasm and are still present in the early gastrula, where they appear to contribute to the extracellular matrices of the developing embryo. This suggests that starfish cortical granules play a dual function: At fertilization, where they help create a block to polyspermy, and in embryonic development, where they secrete extracellular matrix components.