The spatial and temporal distribution of imp clusters in sea urchin embryos

Sea urchin embryos of Strongylocentrotus purpuratus, Lytechinus pictus and Lytechinus variegatus were investigated with respect to appearance, disappearance, and distribution of clusters of larger than average intramembraneous particles and associated cytoplasmic vesicles using thin section and freeze-fracture TEM as well as experimental procedures. This study identified a stable pattern of IMP clusters that becomes established during the eight-cell stage in S. purpuratus embryos. The pattern essentially distinguishes apical surface and lateral surface compartments in blastomere P-face plasma membranes and is spatially the same as the distribution of cytoplasmic (pigment) vesicles which occupy the apical cortices of the blastomeres. Analysis of post 16-cell stages revealed the IMP pattern persists through the early stages of blastulation. Largely due to a process called the vegetal cortical rearrangement, the micromeres are formed without the typical IMP pattern and without the pigment vesicles. Thus, the micromeres have a significantly different plasma membrane organization, and probably, also, a significantly different degree of stability in their cortex at the time of their formation. The implications of these findings are discussed in relation to the development of radial polarity and micromere determination. (See more in text.)

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