The force generated in granulation tissue during wound contraction is thought to be cell mediated; however, it is unclear whether contractile forces are generated by fibroblast locomotion or contraction of myofibroblasts. To help clarify this question the force of this contraction can now be determined accurately in a human dermal fibroblast collagen lattice system using a novel instrument known as a Culture Force Monitor. Three distinct phases of contraction of such collagen gels could be identified over the first 24 hours. Most of the force generated by human dermal fibroblasts was produced during the first stage in parallel with cell attachment and associated changes in cell shape, and the appearance of cell processes. During this initial 24 hours no evidence could be found for the presence of myofibroblasts, but stereoscopic and electron microscopic analysis at a range of time points indicated that migratory fibroblasts were present in the system. Comparison of the contraction profiles of cells extracted from other tissues (tendon and articular cartilage), and extracted by different means from the same tissue specimen, indicated that different populations of fibroblasts can be distinguished on the basis of their pattern of contractions. It would seem that most of the force generated in this model is a result of fibroblast attachment and movement within the collagen lattice. Furthermore, different groups of fibroblasts, even within the same tissue, may vary in their contraction (hence locomotory) activity.

Contractile processes in non-muscle cells mimic those occurring in smooth muscle tissues, indicating the value of using non-muscle cells to investigate the biochemical pathways that regulate contraction (4 -8). Phosphorylation of myosin light chains (MLC) in non-muscle cells causes the formation of myosin-containing stress fibers, which are contractile bundles of actin filaments associated with myosin II (4 -8). Phosphorylation of MLC similarly increases actin-myosin interactions in smooth muscle cells, resulting in smooth muscle contraction (reviewed in Refs. 9 -12). In addition to smooth muscle contraction, many fundamental cellular processes such as adhesion, migration, and division depend upon the interaction of myosin with actin in contractile filaments (reviewed in Ref. 12). Activation of mAChR may affect these fundamental processes by altering myosin activity in non-muscle cells as it does in smooth muscle. Although mAChR activation induces MLC phosphorylation and subsequent contraction in smooth muscle cells (13)(14)(15)(16)(17), the ability of mAChR to regulate MLC phosphorylation and myosin organization in non-muscle cells has not been reported.We investigated the ability of transfected human mAChR subtypes to regulate myosin organization in Chinese hamster ovary (CHO) cells. Activation of transfected M 3 mAChR induces MLC phosphorylation and causes myosin-containing stress fibers to form in CHO cells. The involvement of PKC in these events is indicated by our findings that 1) direct activation of PKC with phorbol esters induces MLC phosphorylation and myosin reorganization in CHO cells, 2) specific PKC antagonists inhibit M 3 mAChR-mediated myosin reorganization, and 3) activation of transfected M 1 but not M 2 mAChR subtypes also induces the formation of myosin-containing stress fibers, demonstrating that only mAChR subtypes that stimulate PKC activity induce myosin reorganization. The participation of myosin light chain kinase (MLCK) and RhoA in mAChRmediated myosin reorganization was also investigated, since these proteins regulate contractile processes in other systems

Fibroblast locomotion is thought to generate tractional forces which lead to contraction and reorganisation of collagen in tissue development and repair. A culture force monitor device (CFM) was used to measure changes in force in fibroblast populated collagen lattices, which resulted from cytoskeletal reorganisation by cytochalasin B, colchicine, vinblastine, and taxol. Microfilament disruption abolished contraction forces, microtubule disruption elicited a new peak of contraction, while taxol stabilisation of microtubules produced a gradual fall in measured force across the collagen gel. Based on these measurements, it is suggested that the cell can be viewed as an engineering structure in which residual intracellular forces, from contractile microfilaments, exert compressive loading on microtubular elements. This microtubular structure appears to act as a "balanced space frame" (analogous to an aeroplane chassis), maintaining cell shape and consequently storing a residual internal tension (RIT). In dermal fibroblasts this hidden RIT was up to 33% of the measurable force exerted on the collagen gel. Phenotypic differences between space frame organisation and RIT levels could explain site and pathological variations in fibroblast contraction.

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