Conversion to an elastogenic phenotype by fetal hyaline chondrocytes is accompanied by altered expression of elastin-related macromolecules.

Elastic fibers are produced during fetal and neonatal periods, and by maturity elastin expression has ceased. Fetal bovine hyaline chondrocytes acquired an elastogenic phenotype within 24 hr after isolation from the tissue, even though the tissue does not produce elastic fibers or tropoelastin mRNA in vivo. By multiple parameters, hyaline chondrocytes produced elastic fibers that were indistinguishable from those made by elastic chondrocytes derived from fetal elastic ear cartilage. The levels of tropoelastin mRNA, secreted protein, and elastic fiber crosslinks as well as the immunostaining and ultrastructural appearance of elastic fibers produced by cultured hyaline chondrocytes were similar if not identical to those of cultured elastic chondrocytes. We also examined the expression of elastin-associated microfibrillar proteins. In intact hyaline cartilage, we did not detect mRNA for fibrillin 5 mRNA and saw only a relatively weak signal for fibrillin 15 mRNA. These microfibrillar products were expressed with tropoelastin in cultured hyaline chondrocytes as well as in intact elastic cartilage and cultured elastic chondrocytes, suggesting that fibrillin 5 and fibrillin 15 are required for elastic fiber formation. In contrast, the levels of microfibrillar-associated glycoprotein mRNA were decreased in both cell types relative to the high expression seen in vivo. These data indicate that conversion to the elastin phenotype includes induction or modulation of all known components of elastic fibers.