THYROXINE IS THE SERUM FACTOR THAT REGULATES MORPHOGENESIS OF COLUMNAR CARTILAGE FROM ISOLATED CHONDROCYTES IN CHEMICALLY-DEFINED MEDIUM

Epiphyseal chondrocytes cultured in a medium containing 10% serum may be maintained as three dimensional aggregates and differentiate termin ally into hypertrophic cells. There is an attendant expression of gene s encoding type X collagen and high levels of alkaline phosphatase act ivity. Manipulation of the serum concentration to optimal levels of 0. 1 or 0.01% in this chondrocyte pellet culture system results in format ion of features of developing cartilage architecture which have been o bserved exclusively in growth cartilage in vivo. Cells are arranged in columns radiating out from the center of the tissue, and can be divid ed into distinct zones corresponding to the recognized stages of chond rocyte differentiation. Elimination of the optimal serum concentration in a chemically defined medium containing insulin eliminates the even ts of terminal differentiation of defined cartilage architecture. Chon drocytes continue to enlarge into hypertrophic cells and synthesize ty pe X collagen mRNA and protein, but in the absence of the optimal seru m concentration, alkaline phosphatase activity does not increase and t he cells retain a random orientation. Addition of thyroxine to the che mically defined medium containing insulin and growth hormone results i n dose-dependent increases in both type X collagen synthesis and alkal ine phosphatase activity, and reproduces the optimal serum-induced mor phogenesis of chondrocytes into a columnar pattern. These experiments demonstrate the critical role of thyroxine in cartilage morphogenesis.