EVIDENCE FOR INSUFFICIENT CHONDROCYTIC DIFFERENTIATION DURING REPAIR OF FULL-THICKNESS DEFECTS OF ARTICULAR-CARTILAGE

The main objective of this study was to characterize the cellular phen otypes in the repair tissue of full-thickness defects of articular car tilage by histologic and molecular biologic techniques. Healing of the defects in the articular cartilage of the knee joints of 12 rabbits w as analyzed at days 3, 7, 14, 28 and 50 using histology and Northern a nalysis of mRNA levels for type I, II and III collagens and osteonecti n. The cellular source of each mRNA was determined by in situ hybridiz ation. Two novel cDNA clones for rabbit type II and III collagen mRNAs were constructed to obtain species-specific hybridization probes. The repair tissue of full-thickness defects consisted of two types of tis sue. At the bottom of the defect, bone-derived cells with high levels of type I collagen and osteonectin mRNA were actively producing new os teoid, while superficially a slow transition from a fibrin clot into u ndifferentiated mesenchyme with cells containing type III collagen mRN A was observed. This tissue subsequently became fibrocartilaginous, wi th small groups of cells turning on the transcription of the type II c ollagen gene and acquiring a phenotype typical for hyaline cartilage. The data suggest that small clusters of cells in the repair tissue of full-thickness articular cartilage defects are capable of turning on a n apparently correct chondrocytic phenotype. The low transcription lev el of the type II collagen gene suggests, however, that insufficient a mounts of fundamentally important regulatory factors or progenitor cel ls are present in the repair tissue. In the future, such factors shoul d be administrable into the joint by novel therapeutic means.