[Gap junctions in the thyroid gland: distribution, regulation, function].

As in most organized tissues, cells of the thyroid gland, thyrocytes, are connected by various types of intercellular junctions, among which gap junctions. Gap junctions are composed of channels that allow the direct cell-to-cell exchange of small cytoplasmic molecules (Mr < 1000). Proteins forming gap junction channels are the connexins (Cx). Thyrocytes coexpress two Cx: Cx32 and Cx43 that form distinct channels localized in different regions of the lateral plasma membrane domain; gap junctions formed of Cx43 are localized in tight junctions. The tissue-specific hormone, TSH, increases the synthesis of these two Cx and induces the opening of gap junction channels. Gap junction-mediated cell-to-cell communication is likely involved in the control of thyroid cell proliferation since the reinduction of cell-to-cell coupling in communication-deficient thyroid cell lines, by stable transfection of the Cx32 cDNA, induces a decrease of the proliferation rate of these cells.