Cartilage differentiation is a multistep process starting from mesench
ymal cells and eventually resulting in mature hypertrophic chondrocyte
s, a stage reach only in cartilage undergoing endochondral ossificatio
n. This differentiation is characterized by a switch of gene activatio
n. Mesenchymal cells produce collagens I and III, whereas chondrocytes
synthesize the large proteoglycan agrecan and collagens II, IX, and X
I. Hypertrophic chondrocytes produce collagen X, a biochemical marker
for this stage of cell maturation. This critical-developmental pathway
of the skeleton is under environmental control of growth factors, hor
mones, cytokines, extracellular matrix, which been extensively studied
these past years. However, the understanding of the molecular mechani
sms of chondrocyte gene regulation or of the precise function of carti
lage matrix components has been hindered because of the instability of
primary chondrocytes In traditional culture. Suspension culture metho
ds and immortalization strategies have been developed and have allowed
a better insight into the mechanisms of synthesis and assembly of car
tilage macromolecules. The use of immortalizing oncogenes have also sh
own that growth and maturation are mutually exclusive phenomena for ch
ondrocytes. These chondrocyte cell lines could be used to clone genes
controling the pathway of cartilage differentiation. Furthermore, immo
rtalization of normal human chondrocytes represent a valuable tool to
test pharmacological factors, and the potential exists for immortalizi
ng chondrocytes from diseased cartilage. This approach should be usefu
l to identify genes involved in maintenance or disruption of the carti
lage phenotype. Taken together, in vitro chondrocyte studies supported
by the recent developments of cell and molecular biology should help
to better understand the mechanisms of skeletogenesis.