Morphologic examination of mesenchymal cells in healing wounds of normal and tight skin mice.

The healing process of an open wound as effected by wound contraction is complete by 3 weeks in the normal mouse. In contrast, its onset is delayed by 3 weeks and complete healing requires 6 weeks in the tight skin mouse (TSM), a mutant mouse strain with the autosomal dominant gene for tight skin. Possible mechanisms for this delay were evaluated. The frequency and distribution of myofibroblasts were studied during the 3-week delay in wound contraction by actin staining and electron microscopy. It was determined, by electron microscopy and phalloidin staining, that myofibroblasts were found in high density in noncontracting TSM wounds. Electron microscopy showed, however, that these myofibroblasts were surrounded by a pericellular matrix that separated their surface from adjacent collagen fibers. No pericellular matrix was found around cells in granulation tissue of normal mice. At 3 weeks, as TSM wounds began to contract, the number and intensity of cells stained by phalloidin in this tissue was less than that seen earlier. The pericellular matrix was fragmented at this time, and cell surface and collagen fiber associations were apparent. Finally, at 5 weeks, when wound contraction was well developed in the TSM, only a small area in the center of the healing wound beneath the epidermis contained phalloidin-positive myofibroblasts. Electron-microscopic examination of the residual granulation tissue at this time revealed the complete absence of the pericellular matrix. It is postulated that during the 3-week delay in wound closure, the presence of a localized pericellular matrix prevents the interaction between cells and collagen fibers necessary for the reorganization of collagen. It is also thought that the tightly adherent uninjured skin surrounding the healing wound may cause delayed wound closure. There was no evidence that the absence of myofibroblasts is responsible for delayed wound contraction.