The fate of Meckel's cartilage chondrocytes in ocular culture

Modulation of the chondrocyte phenotype was observed in an organ culture system using Meckel's cartilage. First branchial arch cartilage was dissected from fetal rats of 16- and 17-day gestation. Perichondrium was mechanically removed, cartilage was split at the rostral process, and each half was grafted into the anterior chamber of an adult rat eye. The observed pattern of development in nonirradiated specimens was the following: hypertrophy of the rostral process and endochondral-type ossification, fibrous atrophy in the midsection, and mineralization of the malleus and incus. A change in matrix composition of the implanted cartilage was demonstrated with immunofluorescence staining for cartilage-specific proteoglycan (CSPG). After 15 days of culture, CSPG was found in the auricular process but not in the midsection or rostral process. In order to mark the implanted cells and follow their fate, cartilage was labeled in vitro with [3H]thymidine [3H]TdR). Immediately after labeling 20% of the chondrocytes contained [3H]TdR. After culturing for 5 days, 20% of the chondrocytes were still labeled and 10% of the osteogenic cells also contained radioactive label. The labeling index decreased in both cell types with increased duration of culture. Multinucleated clast-type cells did not contain label. Additional cartilages not labeled with [3H]TdR were exposed to between 20000 and 6000 rad of gamma irradiation before ocular implantation. Irradiated cartilage did not hypertrophy or form bone but a fibrous region developed in the midsection. Cells of the host animal were not induced to form bone around the irradiated cartilage. Our studies suggest that fully differentiated chondrocytes of Meckel's cartilage have the capacity to become osteocytes, osteoblasts, and fibroblasts.