The preceding volumes of Cell and Muscle Motility have focused on various aspects of motile systems in both muscle and non muscle cells. These essays have been critical reviews on topics of current interest and, hopefully, have provided a base from which future investigations may develop. During the past decade, however, much attention in the fields of biochemistry and cell biology has focused on motile systems in non muscle cells. Our current under- standing of the three-dimensional organization of the cytoplasm involve three major fibrous proteins which are collectively known as the cytoskeletal system. These polymorphic cytoskeletal proteins are microtubules (25-nm diameter), microfilaments (6-nm diameter), and intermediate filaments (lO-nm diame- ter). Microtubules consist of tubulin and several well-characterized micro- tubule associated proteins (MAPs) including MAP , MAP , tau, and others. l 2 Microfilaments consist of actin and associate with actin-binding proteins in- cluding a-actinin, filamin, myosin, tropomyosin, vinculin, and others. Inter- mediate filaments (lO-nm filaments) consist of at least five different tissue- specific classes, including desmin or skeletin (muscle), prekeratin (epithelial), vimentin (mesenchymal), neurofilament (nerve), and glial acidic fibrillary protein (astrocytes). These major fibrous proteins apparently interact with each other as well as other cytoplasmic components and appear to be inti- mately associated with such biological processes as cell shape changes, growth, motility, secretion, cell division, and uptake of materials from the exterior of the cell.

Motility is a fundamental property of living systems, from the cytoplasmic streaming of unicellular organisms to the most highly differentiated and de veloped contractile system of higher organisms, striated muscle. Although of motility have a long and in scientific investigations into the mechanisms teresting history, the knowledge of molecular processes, especially in the area of regulation of control of motility, has been developing at an ever more rapid pace with the utilization of multidisciplinary approaches from physiology, cell biology, genetics, biochemistry, pharmacology, and biophysics. In Volume 3, Cell and Muscle Motility continues the same philosophy as that of the preceding volumes. The essays are meant to focus on topics of current interest, to be critical rather than exhaustive, and to indicate the current trends of research efforts. The series is intended to foster an in terchange of concepts among various workers in a variety of disciplines and to serve as a reference for students and workers who wish to familiarize them selves with the most current progress in motility. Robert M. Dowben Jerry W. Shay Dallas vii Contents Chapter 1 Intermediate Filaments in Striated Muscle: A Review of Structural Studies in Embryonic and Adult Skelttal and Cardiac Muscle Maureen C. Price and Joseph W. Sanger 1. Introduction ................................................ ."

The term cytoskeleton in muscle and nonmuscle cells refers to fibrous proteins that are directly or indirectly associated with cell shape, contractility, and/or motility. These cytoskeletal elements include microtubules and associated proteins, microfilaments and their binding proteins, and lastly, intermediate filaments. Even though the structure and function of myosin and tro- pomyosin have been extensively investigated in muscle ceIls, considerably less is known about their functions in nonmuscle ceIls; they are often referred to generically as actin-binding proteins, as are a-actinin, filamin, vinculin, and others. Indeed, most nonmuscle researchers are unaware that the term intermedi- ate filament was derived from the observation that the diameter of intermedi- ate filaments is intermediate between those of actin (5 nm) and myosin (15 nm) and not intermediate between those of actin (5 nm) and microtubules (25 nm). It has therefore been the purpose of this series to attempt to bridge the communication gap that appears to exist between muscle and nonmuscle researchers. For those students and active scientists who are faced with the task of absorbing new data and concepts at an ever-accelerating rate, review essays are an excellent way to keep up. The essays in this volume, as weIl as in the previous ones, focus on topics of current interest. They are intended to be critical rather than exhaustive.