[Structure of the cytoskeleton of embryonic muscle fibroblasts. Electron microscopic study of platinum replicas].

Cultured mouse embryo fibroblasts were extracted with 1% Triton X-100 at pH 6.7 in buffer containing EGTA and stabilizing supplements. Exposed during this extraction cytoskeletons were fixed, dried with the critical point technique, and shadowed with platinum. The platinum replicas of cytoskeleton were used for electron microscopic characterization of the three-dimensional distribution of cytoskeletal fibrils in cytoplasm. Four cytoplasmic regions are revealed with different cytoskeletal structure; these being a "mesh-work zone", a "loose zone", which together formed an active edge of cell lamelloplasm, the "lamella proper" and endoplasm. The two former zones are occupied with dense three-dimensional or loose planar actin network, respectively. The "lamella proper" contains two-dimensional network of different fibrils: microfilaments, microtubules, intermediate filaments and thin connective filaments. The central perinuclear area has a thick microfilament sheath at dorsal cell surface. The most important cytoplasmic elements of fibroblasts are actin microfilament bundles. The fine structure of these bundles, their junctions with each other ("organizing centers") and their terminal parts corresponding to cell-substrate focal contacts are described.