In mammals, wound healing is thought to result in the formation of scar tissue, with the exception of bony healing after fractures. Here we describe a previously unknown pattern of wound healing in which adipose rather than scar tissue is formed. Adipogenesis is normally confined to the embryo, although there are several experimental models for adipogenesis with highly specific dietary, cytokine, matrix, sex, or age requirements. The adipogenic healing described in this work provides a simple and reproducible experimental mouse model for adipogenesis without these limitations. Mice received intramuscular implants of nylon mesh material. Fibrinous material impregnated implants and within 4 weeks was replaced with highly vascular granulation tissue, typical of wound healing. Also consistent with wound healing was a reduction in vascularity of the newly formed tissue over time (P < 0.05). Lipoblasts were prevalent in granulation tissue, reaching a maximum in week 2 (P < 0.001) but falling to very low levels by week 9. These cells matured to adipocytes, with intermediate forms being seen. The identity of lipoblasts and adipocytes was confirmed by Oil Red O staining and electron microscopy. Control experiments confirmed that adipogenesis was independent of the materials used as well as of the sex and age of the animals. Rather, adipogenesis appeared to be due to replacement of fibrinous material in a space created within muscle. It is possible that adipogenic healing represents an adaptation for limiting the formation of restrictive scar tissues within muscle, and that this is the basis for the formation of traumatic lipomas in humans. Also, muscle tissue is replaced by adipose cells, seemingly derived from pluripotential satellite cells, in several degenerative muscle conditions, suggesting a role for adipogenic healing in these conditions.
Copyright 2002 Wiley-Liss, Inc.