Plasmalemmal undercoat: the cytoskeleton supporting the plasmalemma.

The plasmalemmal undercoat can be defined as the electron-dense material of layered organization closely applied to the cytoplasmic surface of the plasma membrane (plasmalemma) as revealed by thin-section electron microscopy. Though the structures which fulfill this criterion occur widely, most of them have not received the attention they may deserve. The undercoat is a special form of the cytoskeleton-membrane interaction, though it constitutes a part of the cytoskeleton. The significance of the plasmalemmal undercoat may be primarily to provide a structural support for the membrane. With this support, cells can perform many important functions on their limited or whole surfaces. The undercoat may provide a mechanical support to the plasmalemma so that the membrane may acquire rigidity, strength and elasticity. Through association with the membrane, the undercoat may regulate the distribution of integral membrane proteins to form and maintain various functional domains on the plasmalemma. The undercoat may further provide attachment sites for cytoskeletal fibrous components such as actin filaments, intermediate filaments and microtubules.