A myofibroblast is a cell phenotype that was first described as being in a state between a fibroblast and a smooth muscle cell.

Myofibroblasts are contractile web-like fusiform cells that are identifiable by their expression of α-smooth muscle actin within their cytoplasmic stress fibers.

In the gastrointestinal and genitourinary tracts, myofibroblasts are found subepithelially in mucosal surfaces. Here they not only act as a regulator of the shape of the crypts and villi, but also act as stem-niche cells in the intestinal crypts and as parts of atypical antigen-presenting cells. They have both support as well as paracrine function in most places.

Myofibroblasts were first identified in granulation tissue during skin wound healing. Typically, these cells are found in granulation tissue, scar tissue (fibrosis) and the stroma of tumours. They also line the gastrointestinal tract, wherein they regulate the shapes of crypts and villi.

Myofibroblasts usually stain for the intermediate filament vimentin, which is a general mesenchymal marker, α-smooth muscle actin (human gene = ACTA2), and for palladin, which is a cytoskeletal actin scaffold protein. They are positive for other smooth muscle markers, such as intermediate filament type desmin in some tissues, but may be negative for desmin in other tissues. Similar heterogeneous positivity may exist for almost every smooth muscle marker except probably a few which are positive only in contractile smooth muscles like metavinculin and smoothelin.

Myofibroblasts upregulate the expression of fibronectin, collagens, and hyaluronic acid during and after their differentiation from fibroblasts. Among these, the EDA isoform of fibronectin (EDA-FN), and collagen type I (COL1A1/COL1A2) are typical markers of myofibroblast-dependent synthesis of pro-fibrotic extracellular matrix.

Some myofibroblasts (especially if they have a stellate form) may also be positive for GFAP.

There are many possible ways of myofibroblast development: