The differentiation of chondrocytes and of several other cell types is associated with a switch from the ␣ 6B to the ␣ 6A isoform of the laminin ␣ 6  1 integrin receptor. To define whether this event plays a functional role in cell differentiation, we used an in vitro model system that allows chick chondrogenic cells to remain undifferentiated when cultured in monolayer and to differentiate into chondrocytes when grown in suspension culture. We report that: (i) upon over-expression of the human ␣ 6B , adherent chondrogenic cells differentiate to stage I chondrocytes (i.e. increased type II collagen, reduced type I collagen, fibronectin, ␣ 5  1 and growth rate, loss of fibroblast morphology); (ii) the expression of type II collagen requires the activation of p38 MAP kinase; (iii) the over-expression of ␣ 6A induces an incomplete differentiation to stage I chondrocytes, whereas no differentiation was observed in ␣ 5 and mock-transfected control cells; (iv) a prevalence of the ␣ 6A subunit is necessary to stabilize the differentiated phenotype when cells are transferred to suspension culture. Altogether, these results indicate a functional role for the ␣ 6B to ␣ 6A switch in chondrocyte differentiation; the former promotes chondrocyte differentiation, and the latter is necessary in stabilizing the differentiated phenotype.Growth factors, cell-extracellular matrix (ECM), 1 and cellcell interactions are the primary determinants of lineage decisions and differentiation events in embryogenesis. These regulatory events involve different types of receptors and may lead to activation of several signaling pathways, such as those mediated by MAP kinases (1-3). Changes in the expression pattern of most of these receptors, including the integrins (heterodimeric ␣ receptors involved in cell-ECM and cell-cell interactions), are able to modulate several events associated with cell differentiation (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20).The onset of chondrogenesis in developing long bones is characterized by the reduction of intercellular spaces and establishment of extensive cell-cell contacts between mesenchymal chondrogenic cells, i.e. cell condensation (21). Several factors have been shown to play a role in this process, including cell-cell interactions (22-27), composition of the ECM (25, 28 -30), changes in cell shape (31), and response to cytokines (32). Following cell condensation, chondrogenic cells that produce type I collagen, fibronectin (FN), and its integrin receptor ␣ 5  1 , differentiate to stage I chondrocytes that express type II collagen and eventually to stage II hypertrophic chondrocytes, characterized by type II and X collagen production (12).Most of these events can be reproduced in vitro in a tissue culture model system that allows condensation and differentiation of chick embryo tibiae chondrogenic cells (12,(33)(34)(35). These cells, which adhere to tissue culture dishes and display a fibroblast-like phenotype (pre-chondrogenic cells), proliferate and secrete type I collagen...