Maintenance of regional histodifferentiation patterns and a spatially restricted expression of type X collagen in rat Meckel's cartilage explants in vitro

The major, central portion of Meckel's cartilage undergoes fibrous transfor mation and contributes to the sphenomandibular ligament, whereas its distal end undergoes endochondral ossification ultimately giving rise to inner-ea r ossicles. This regional histodifferentiation of Meckel's cartilage is kno wn to be associated with the spatially restricted expression of type X coll agen. The objective of this study was to determine if this unique histodiff erentiation is regulated by local environmental factors or by a preprogramm ed genetic mechanism. Meckel's cartilage, and condylar cartilage used for c omparison, were isolated from 17-day-old rat embryos and from newborn rats, respectively. The cartilage explants were maintained in vitro for 50 days with or without supplementation with 10% fetal bovine serum. When the expla nts were cultured under serum-free conditions, well-regulated cartilage dev elopment was observed. Expression of type X collagen, a differentiation mar ker for hypertrophic cartilage, was restricted to the distal end of Meckel' s cartilage, whereas type II and IX collagens were found uniformly along th e entire explant. Matrix calcification was examined histochemically using a lizarin red S staining and found to be restricted to the distal end of Meck el's cartilage. Both Meckel's and condylar cartilage cultured with 10% feta l bovine serum developed unregulated dysmorphogenesis. These data suggest t hat, although Meckel's cartilage has an intrinsic potential to differentiat e to its terminal stage, external regulatory factors can significantly infl uence its normal development at the molecular level. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.