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In electron micrographs, the plasma membrane at the surface of a cell is about 8 m # thick (16) and has a triple-layered structure consisting of two dark layers separated by a light layer. Recent studies suggest that, in the majority of tissues, the plasma membranes of adjacent cells arc separated by a distance of not less than 15 to 20 m # (15), this intercellular space being distensible, e.g. by fat globules (12). In some situations, notably during the formation of a myelin sheath, this separation is lost and the adjacent plasma membranes come together to form a structure about 15 m # thick. Robcrtson (17) has called this structure an external compound membrane, while Karrer (7, 8) prefers the term quintuple-layered cell interconnection
Muir et al. (Thu,) studied this question.
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