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Freeze fracture analysis of the early sea urchin embryo
Plasma membrane molecular topography in the early development of the sea urchin embryo, Strongylocentrotus purpuratus was analyzed using freeze-fracture electron microscopy. Membrane topography was analyzed with respect to intramembranous particle densities and size class frequencies in the 2, 4, and 8-cell embryonic stages. Results show similarities in membrane topography between cells of the 2-cell stage, and between cells of the 4-cell stage. Overall membrane particle size class frequencies between the 2 and 4-cell stages are similar, yet the 8-cell embryo is comprised of cells that differ from each other and the previous embryonic stages topographically in membrane particle densities and size class frequencies. Both the 2 and 4-cell embryonic stages contain cells with homogeneous particle distributions, while some cells of the 8-cell stage show membrane particle heterogeneities. Membrane particle clusters appear in some cells of 8-cell embryos, yet no clusters were apparent in membranes of the 2 and 4-cell stages. Results indicate that membrane particle homogeneity is apparent in blastomeres of the 2 and 4-cell stages, yet massive membrane particle insertion and reorganization occurs during division of the 4-cell embryo. The membrane structural changes observed correlate well with the overall pattern of determination in the early development of the sea urchin embryo.