Extracellular matrix and cell surface as determinants of connective tissue differentiation.

This paper reviews in vitro studies, largely from the author's laboratory, concerning the conditions that are permissive for the differentiation of limb bud mesenchymal cells into chondrocytes. In high-density cell culture, even in a defined medium, the same normal sequence of events that is found in vivo in developing cartilage is also observed. This system can be used to study heritable disorders in model systems such as in mutant mouse embryos. In addition, single mesenchymal cells can differentiate into hypertrophic chondrocytes in hydrated collagen gel or agarose cultures. A rounded cell shape promotes chondrogenesis, while a flattened cell shape promotes fibroblast differentiation. The actin cytoskeleton is shown to play a central role in regulating connective tissue cell differentiation. By use of such cell culture manipulations, it is now possible to grow large numbers of fibroblastic cells from human biopsy material for storage and to carry out experimental studies after re-expression of chondrogenesis in gel cultures. It is suggested that cytoskeletal-extracellular matrix interactions play a fundamental role in connective tissue differentiation. Matrix receptors might be developmentally regulated and modify epithelial effects on mesenchymal cells. In this way mesenchymal cells differentiate in a highly organized manner in spatial and temporal terms.