Fibroblast

A fibroblast is the resident mesenchymal cell of connective tissue, responsible for synthesising collagens, proteoglycans and glycoproteins and for producing the enzymes that degrade them. They are not a single cell type. Single-cell transcriptomics has shown fibroblasts differ substantially by anatomical site and by position within a tissue, and specialised lineages include tenocytes and the fibroblast-like synoviocytes of joint lining. That heterogeneity is why a dermal fibroblast is a doubtful stand-in for a tendon cell.

During repair a subset becomes myofibroblasts, acquiring alpha-smooth muscle actin stress fibres and generating contractile force that draws wound margins together. The switch requires TGF-beta signalling, mechanical tension, and the ED-A splice variant of fibronectin together, a rare example of a differentiation step gated jointly by a soluble factor and by substrate stiffness. In normal healing myofibroblasts disappear by apoptosis once tension falls; their failure to disappear is the cellular definition of hypertrophic scar and of most organ fibrosis.

That two-sided role determines how to read fibroblast assays. Proliferation is not a virtue. A compound that makes fibroblasts divide faster or deposit more collagen in culture has demonstrated fibrogenic potential as readily as healing, and the distinction depends entirely on whether the response resolves.

The specific error to watch for is the scratch assay. Scraping a line through a confluent monolayer and measuring gap closure is a migration and proliferation readout in one purified cell type, with no immune cells, no vasculature, no three-dimensional matrix and no mechanical load. Most repair and cosmetic peptides are supported by exactly this experiment, usually at concentrations far above anything reachable in interstitial fluid.