Characterization of actin microfilaments at the apical pole of thyroid cells.

In thyroid cells (rat or hog), actin has been detected by immunofluorescence with an antiactin antibody and, in electron microscopy by decoration "in situ" with heavy meromyosin. The antibody as the heavy meromyosin method have shown that actin microfilaments are especially localized at the apical pole of the cells, in a region where thin filaments are usually observed by conventional methods of electron microscopy. These microfilaments are attached to the apical membrane at the ends of the microvilli and form dense bundles at their cores. They are polarized towards the interior of the cell. Decorated filaments are also organized in a clear network, parallel to the apical membrane; they are associated with microvillar bundles, but also with small apical vesicles and lateral membranes, in tight or gap junctions.