ABSTRACT
Studies on plasma membrane receptors are usually performed in vitro on cultured cells. Biochemical methods have been the mainstay for the assessment of the presence of receptors in a given cell population. However, biochemical and other averaging techniques lack the resolution for visual localization of individual receptors on the plasma membrane of a single cell. Therefore, attempts have been made to develop methods that allow the direct visualization of membrane receptors. Many such investigations have been carried out using light microscopy and fluorescent probes. In principle, many existing cytochemical techniques can be combined with surface replication. Coated pits appear in electron micrographs of thin sections as indentations in the plasma membrane about 200 nm in diameter. The cytoplasmic side of the membrane of the coated vesicles is differentiated as a lattice composed primarily of a protein called clathrin. The combination of lysis-squirting and immunocytochemical labeling procedures allows the visualization of the protoplasmic plasma membrane surface and its constituents.
