Changes in cell surface and cortical cytoplasmic organization during early embryogenesis in the preimplantation mouse embryo

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Abstract
Membrane topography and organization of cortical cytoskeletal elements and organelles during early embryogenesis of the mouse were studied by transmission and scanning electron microscopy [SEM] with improved cellular preservation. At the 4- and early 8-cell stages, blastomeres were round and SEM showed a uniform distribution of microvilli over the cell surface. At the onset of morphogenesis, a reorganization of the blastomere surface was observed in which microvilli became restricted to an apical region and the basal zone of intercellular contact. As the blastomeres spread on each other during compaction, many microvilli remained in the basal region of imminent cell-cell contact, but few were present where the cells had completed spreading on each other. Microvilli on the surface of these embryos contained linear arrays of microfilaments with lateral cross bridges. Microtubules and mitochondria became localized beneath the apposed cell membranes during compaction. Arrays of cortical microtubules were aligned parallel to regions of apposed membranes. During cytokinesis, microtubules became redistributed in the region of the mitotic spindle and fewer microvilli were present on most of the cell surface. The cell surface and cortical changes initiated during compaction were the 1st manifestations of cell polarity in embryogenesis. Cell surface changes associated with trophoblast development appeared as early as the 8-cell stage. Morphogenesis involved the activation of a developmental program which coordinately controlled cortical cytoplasmic and cell surface organization.