Cells are surrounded by the plasma membrane, which forms the boundary between their cytoplasm and the environment. The principal components of the plasma membrane and of all other cellular membranes are (glyco)lipids and (glyco)proteins.

In ultrathin sections, the plasma membrane appears quite simple in structure (panel A). It consists of two electron-dense leaflets and a lucent space in between, together reaching a thickness of about 75 nm. In the electron micrograph shown, the trilamellar plasma membranes of two adjacent enterocytes and the narrow intercellular space are visible. This fine structural monotony does not reflect the asymmetric and complex composition and the dynamic nature of the plasma membrane, which differs between cell types.

Freeze-fracture electron microscopy is highly suitable for the study of membranes and has provided proof of the presence of membrane-spanning proteins. The fracture plane preferentially passes through the hydrophobic membrane interior and produces two membrane halves: the P-face, which is the cytosolic membrane half, and the E-face, which corresponds to the external membrane half. In panel B, the E- and the P-faces of two neighboring red blood cells are shown. Both membrane faces are studded with intramembranous particles, which are related to fractured transmembrane proteins. The smooth parts of the fracture faces principally correspond to membrane lipids. As seen in panel B, the P-face usually contains a higher density of intramembranous particles. Variants of the freeze-fracture technique applied to cell cultures permitted the preparation of plasma membrane fracture faces of enormous size, as shown in panel C. In contrast to the uniform distribution of intramembranous particles in the erythrocyte plasma membrane, those of cultured hepatocytes are irregularly arranged. The clusters of intramembranous particles correspond to coated pits involved in receptor-mediated endocytosis (cf. Fig. 58). The numerous elevations correspond to plasma membrane processes (cf. Fig. 92).

The plasma membrane performs two basic functions. On the one side, the lipid bilayer constitutes an impermeable barrier for most water-soluble molecules. On the other, its membrane-spanning proteins make it porous for bidirectional transmembrane transport and diffusion, communication and signaling, and cell-cell and cell-matrix interactions. The lipid bilayer represents a two-dimensional fluid in which both lipids and proteins are relatively mobile in the plane of the membrane. However, lipids and proteins may be confined to specific membrane regions, the microdomains. Hence, the name “fluid mosaic model of membranes.”

The lipid bilayer consists of phospholipids, cholesterol, and glycolipids, which are differentially distributed in the two membrane leaflets. The oligosaccharide side chains of glycolipids are exclusively found on the outer plasma membrane surface together with the oligosaccharides of glycoproteins and form the glycocalyx (cf. Fig. 94).

The plasma membrane asymmetry is not only confined to the two lipid layers. The apical and the basolateral plasma membrane in polarized cells can differ in their protein and lipid composition in relation to their specific functions.

Observation of living cells by light microscopy and of fixed cells by scanning electron microscopy has provided a wealth of information on cell spreading and locomotion under culture conditions. Spreading of fibroblasts from a cell suspension occurs through thin, thread-like protrusions, the filopodia, which establish initial contacts with the substrate (arrowhead in panel A) that finally result in a well-attached, flattened cell. Attached fibroblasts and other cell types can crawl over the substratum. This represents a directional movement associated with the formation of lamellipodia, which are flat, two-dimensional protrusions formed at the leading edge of the cells. Thus, moving cells are distinctly polarized (arrows in panel A). Both types of cell protrusions contain actin: filopodia have long, bundled filaments and lamellipodia orthogonally cross-linked meshworks essentially arranged parallel to the substratum. The actin filaments of the lamellipodia in concert with myosin and microtubules and accessory cytoskeletal proteins are the active principle for the cell movement. The cytoskeleton is actively reorganized during cellular locomotion and includes the formation, contraction, and disassembly of actin networks in lamellipodia.

Panel B shows a group of rat hepatocytes attached to a plastic support, which form a coherent, monolayered sheet. Their surface is covered by microvilli-like membrane extensions that can be clearly seen in an ultrathin section cut perpendicularly to the plane of the cell monolayer (panel C). The ultrathin section shown in panel C reveals that the microvilli-like extensions are restricted to the free cell surface and that the basal cell surface is rather flat and focally attached to the plastic support. Epithelial cells grown on a solid plastic or glass support have a discoid shape, as seen in panels B and C, and form adherens junctions and desmosomes at sites of lateral cell-cell contacts. As mentioned, when grown on permeable, porous membranes, epithelial cells such as kidney epithelial cells form a highly polarized cell monolayer. This represents a most useful system to analyze aspects of polarity of cellular traffic and cytoarchitecture.

Cell crawling is a basic phenomenon in living organisms during embryogenesis and in adult organs. It is important for the function of cells involved in inflammation and immune defense, wound healing, and tissue remodeling as well as the spread of malignant cells.