Mesenchymal cells and growth factors in bunions.

Bunion formation in adults is an example of bone growth that occurs after physis closure. Bone is laid down secondary to mechanical irritation caused by foot deformity. It is a mechanism of ectopic bone formation unrelated to physeal growth. In this article, bone formation is analyzed using immunohistochemical and cell culture techniques. Using markers specific for mesenchymal cells (collagen type IIa and fibroblast growth factor receptor 3), a cell population is defined in the soft tissues that overlie the bunion and is isolated from explant cultures. The cells do not produce bone matrix in culture, and they do not express osteoblast-related antigens. Stimulation of the cells by fibroblast growth factor (FGF) 2 leads to rapid cell proliferation and phenotype change. The cells start to form humps and at the same time express alkaline phosphatase and collagen type I. Expression of collagen type IIa and fibroblast growth factor receptor 3 ceases. These series of experiments indicate that a specific population of mesenchymal cells occurs in the soft tissues that overlie the bunion. This population is capable of bone formation when stimulated by FGF, a common mediator of inflammatory processes. Thus, FGF stimulation of mesenchymal cells in soft tissues that overlie the head of the first metatarsal is a potential link between the biomechanical forces that cause hallux valgus and bunion formation.