Properties of electrotonic junctions between embryonic cells of Fundulus

Cells isolated from blastulae of the teleost Fundulus can be reaggregated in pairs in physiological salt solution. The cell pairs usually become electrically coupled. Unlike many coupled cells of adult tissues, injected fluorescein does not pass between these cells in detectable amounts, although it can enter the cells when externally applied. Glutaraldehyde fixation rapidly and greatly increases coupling resistance, but causes only moderate increases in cell resistance exclusive of the coupling pathway; thus fixation decouples cells. Electrical measurements demonstrate that fixation does not disrupt zonulae occludentes in blastulae or gastrulae. Electron microscopy reveals that the appositional region between reaggregated cell pairs is penetrated by La(OH)₃ applied with the fixative. There are also extensive regions of close apposition between the cell membranes. The combined data indicate that these embryonic cells are coupled by way of specialized, electrotonic junctions rather than a region of extracellular space between them that is walled off by a zonula occludens. A morphological substrate for these junctions is provided by the close appositions seen electron microscopically. The relatively restrictive permeability of the electrotonic junctions between embryonic cells may allow small ions to function as intercellular signaling molecules while restricting movement of larger molecules involved in intracellular controls.