The transfer of 6-carboxyfluorescein between islet cells in monolayer culture was observed by fluorescence microscopy, and the endocrine cells involved in this transfer were identified by immunohistochemistry and electron microscopy. The results show that carboxyfluorescein was directly exchanged between homologous B-cells and also between B- and A- or D-cells. Successive microinjections of the probe into different cells of the same cluster showed the existence of separate territories, each formed by 2-8 communicating cells. Intercellular communication was not observed after every dye microinjection, and communicating and noncommunicating islet cells were found to coexist within the same cluster. The data indicate that the exchange of exogenous cytoplasmic molecules occurs between different types of endocrine islet cells. However, within a single cluster, all islet cells are not metabolically coupled to one another, at a given time.

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