SUBSTRUCTURE OF CYTOPLASMIC DENSE BODIES AND CHANGES IN DISTRIBUTION OF DESMIN AND ALPHA-ACTININ IN DEVELOPING SMOOTH-MUSCLE CELLS

The substructure of assembling cytoplasmic dense bodies (CDBs) and cha nges in the distribution of desmin and alpha-actinin during developmen t of smooth muscle were studied in gizzard samples from 10- and 16-day embryos and from 1- and 7-day post-hatch chickens. CDBs in these cell s lack the density of CDBs in mature or adult smooth muscle cells and, thus, allow observations of the changes inside CDBs. The random filam ent orientation seen in younger embryonic cells is first modified to i nclude relatively small patches of Ifs that are somewhat straighter an d are approaching a side-by-side arrangement. As development proceeds, the Ifs in these arrays become straighter, are parallel over longer l engths of the Ifs and later acquire the density characteristic of matu re CDBs. Antidesmin labeling in embryonic 10- and 16-day cells showed that desmin intermediate filaments (Ifs) were located in the myofilame nt compartment but were concentrated in or near assembling CDBs. Anti- desmin labeling shifted to the perimeter of CDBs after hatching. Cross sections, longitudinal sections, and stereo pairs all show that IF pr ofiles are present inside unlabeled assembling CDBs. Anti-alpha-actini n labeling was directly on CDBs and was often associated with the cros s-connecting filaments (CCFs) (average diameter of 2-3nm) inside CDBs. We propose, based on these data, that desmin IFs, alpha-actinin-conta ining CCFs, and actin filaments are the principal components of the su bstructure of assembling CDBs. We also present a proposed model for CD B assembly. (C) 1994 Wiley-Liss, Inc.