Freeze-fracture analysis of the 16-cell sea urchin embryo

Freeze-fracture electron microscopy was used to analyze the hydrophobic interior of the plasma membranes of micromeres, mesomeres, and macromeres of the 16-cell sea urchin embryo. A quantitative assessment was made of membrane fracture faces in terms of particle densities and particle size frequency distributions. The average intramembranous particle (IMP) diameters of micromeres, mesomeres, and macromeres were 89 A, 78 A, and 107 A respectively. The micromere plasma membrane (PM) protoplasmic face (PF) was characterized by a heterogeneous and polar distribution of IMP’s, whereas both the mesomere and macromere PM-PF had a homogeneous particle distribution. Additionally, the macromere plasma membrane had particle clusters set in bumps or hollows. Cluster distribution was 2 per um2 of membrane and was the same for glycerinated as well as unglycerinated materials. The polar distribution of IMP’s in micromere membranes was interpreted as a form of membrane patterning which correlated well with a lectin-induced “capping” response and also with the migratory tendencies of these cells. Macromere particle clusters were interpreted as membrane structural specializations which may determine the bidirectional fate of these cells in the adult. The data suggest that the overall membrane topography and patterning differences of the IMP’s for the three cell types correlate rather well with the diverse functional and behavioral characteristics of these cells, such as “capping”, migration, cell-cell associations, and cell destiny in the adult.