Gap junction and its cytoskeletal undercoats as involved in invagination-endocytosis.

Plasmamembrane and its cytoskeletal undercoat were characterized by electron microscopy in gap junctions (GJs) of steroidogenic cells of the guinea pig and bullfrog adrenal glands. In both species GJs varied in shape considerably and measured 0.1-4 microns in diameter. Planar GJs were not provided with any distinct form of the undercoat. In contrast, variably invaginating GJs had a network of actin-containing microfilaments located in the protruding cytoplasm arising from either one of adjoining cells. In the deeper invaginations, on the contrary, parallel arrays of actin-containing microfilaments formed a submembranous sheath in the withdrawing cytoplasm. The microfilaments were arranged at right angles for the long axis of the invagination. In completely internalized GJs, the network and sheath became less organized or obscured. A mechanical force driving the invagination-endocytosis involving GJ areas is presumably generated by the microfilament network and sheath, organized differently in forms, but working in concert together. It is also likely that there is another dissolving process for GJs via clathrin-coated vesicles.