Cell to substratum contacts of chick fibroblasts and their relation to the microfilament system. a correlated interference-reflexion and high-voltage electron-microscope study

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Abstract
The interference reflexion microscope may be used to examine the pattern of cell-substratum separation of cultured cells. Earlier studies on chick fibroblasts using this technique identified 3 types of separation: (1) focal contacts, representing approx. 10—14 nm separation and which are sites of cell-substratum adhesion and form at the leading edge of a cell during locomotion; (2) close contacts, representing approx. 30 nm separation; and (3) regions of greater separation, generally 100 – 140 nm. Using light microscopy these studies described the association of the focal contacts with the distal ends of cytoplasmic fibres and noted their similarity to the adhesion plaques described in EM studies of thin vertical sections of fibroblasts. In this investigation we have directly confirmed these earlier observations by using a correlated interference-reflexion and high-voltage stereo electron-microscopic study of cultured chick heart fibroblasts. The results show that focal contacts are always associated with the distal ends of linear bundles of microfilaments which pass centripetally and often obliquely through the cytoplasm. At their proximal ends the bundles insert into a loose matrix of microfilaments and organelles surrounding the nucleus. The close contacts are often associated with a meshwork of microfilaments. The evidence supports the hypothesis that the microfilament bundles of a fibroblast form synchronously with the focal adhesions to the substratum. Our results are discussed with reference to the current evidence for the role of contractile proteins in cell locomotion.