Physiologically Significant Specializations of the Cell Surface

This paper presents a brief review of the ultrastructure of some of the structural elaborations of the plasma membrane that may be regarded as adaptations for specific cell functions. Among the relatively stable specializations of the free surface, it considers the striated border of the intestinal mucosa, the brush border in the nephron, and filamentous coatings on the membranes of certain cells of the gastric mucosa. It then turns to those transient configurations of cell surfaces involved in the dynamic processes of pinocytosis, phagocytosis, and liberation of secretory products and considers the turnover of membranes associated with these phenomena. Discussion of specializations of the contact surfaces includes the desmosomes and terminal bars and the present status of intercellular cement and interfacial canals. A section on specializations of the cell base considers the basement membrane and basal infoldings of the plasma membrane in cells engaged in active transport. And finally, there is a description of the terminal web and the marginal band of nucleated erythrocytes—specializations of the superficial cytoplasm concerned with maintenance of cell shape.