The meroblastic egg of the teleost, Fundulus heteroclitus, was studied electrophysiologically from cleavage to mid-gastrula stages. The yolk is an intracellular inclusion surrounded by a membrane of high resistivity (50 kΩcm2). This membrane generates a cytoplasm-negative resting potential in later stages. Cells of all stages studied are coupled electrically. In gastrulae, coupling is both by way of specialized junctions between cells and by way of intra-embryonic extracellular space, the segmentation cavity. The latter mode is present because the segmentation cavity is sealed off from the exterior by a high resistance barrier, and the outer membrane of surface cells is of high resistance (50–100 kΩcm2) compared to the inner membrane. It can be inferred that clefts between surface cells are occluded by circumferential junctions. Isolated cells from late cleavage stages develop coupling in vitro, confirming the existence of coupling by way of intercellular junctions. Both modes of coupling could mediate communication between cells that is important in embryonic development.

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