The myofibroblast is a mesenchymal cell with functional and structural characteristics in common with fibroblasts and smooth muscle cells. These cells play a critical role in wound closure and in the pathologic sequelae of healing. It has been shown in adult humans and experimental animals that the myofibroblast expresses α-smooth muscle actin (ASMA) temporarily during wound contraction and more persistently during fibrocontractive diseases; however, it is unclear whether this cell makes any contribution to tissue repair in utero. Experimental work in fetal animal models has demonstrated that wound repair in fetal skin occurs by reconstitution of epidermal appendages and organized restoration of the dermal collagen network. Fetal lamb wound healing studies have shown that a transition from scarless tissue repair to healing with scar formation occurs late in gestation.
In this study we examined the ontogeny of myofibroblasts in fetal lamb wounds at early through late gestation, using transmission electron microscopy (TEM) and ASMA immunohistochemistry.
Dramatic differences were observed in ASMA content of early as compared to late gestation fetal wound granulation tissue: ASMA was absent in wounds made at 75 days gestation but was present in progressively greater amounts in wounds made at 100 and 120 days gestation (term=145 days). TEM studies also demonstrated progressive development and organization of microfilament bundles. Early in development microfilament bundles were sparse and disorganized, but as gestation progressed the bundles became more prevalent and formed tightly parallel arrangements. The organization of microfilament bundles was also accompanied by fibronexus formation. Thus, the ASMA expression in granulation tissue myofibroblasts appears in relation to healing with scar formation.
Additional in vitro experiments demonstrated that the majority of fibroblasts cultured from granulation tissue of 75, 100 and 120 days gestation lambs expressed ASMA, suggesting that factors present in fetal calf serum can induce ASMA expression even in fibroblasts not expressing this protein in vivo.
Our observations demonstrate an ontogenic change in the fetal myofibroblast in response to injury in vivo and suggest that local factors may regulate the variable phenotype of myofibroblasts during gestation. These results have implications for the therapy of pathologic scarring.