ESTABLISHMENT AND CHARACTERIZATION OF 2 CLONAL CELL-LINES DERIVED FROM MURINE MANDIBULAR CONDYLES

We have established two clonal cell lines, designated SM1/9 and SM25/3 from the mandibular condyles of newborn BALB/c mice by immortalizatio n with the SV40 large T antigen, These cells have a high proliferative activity and have been maintained in culture for over 50 passages, Th ey are polygonal in shape. Electron microscopic studies indicate an im mature phenotype for both clones and a lack of prominent intracellular filaments typical of fibroblasts. SM25/3 demonstrates different biolo gical properties as compared to SM1/9, it is tumourigenic in nude mice , has a faster growth rate and exhibits less differentiatied features, Both cell lines have low constitutive levels of alkaline phosphatase, and the activity of this enzyme is increased significantly in a dose and confluency dependent manner by retinoic acid and 1,25 (OH)(2) vita min D-3. The cells express transcripts for retinoic acid receptors mRA R-alpha and mRAR-gamma but not for mRAR-beta. They also express mRNA f or the 1,25 (OH)(2) vitamin D-3 receptor. They co-express transcripts for collagen types I, II, III, Expression of mRNA for extracellular ma trix proteins such as biglycan, osteopontin, PAI-1 is detected, Cultur ed cells do not express mRNA for osteocalcin and this transcript is no t inducible with 1,25 (OH)(2) vitamin D-3 or retinoic acid. Chondrocyt e markers such as link protein and aggrecan are not detected. In vitro assays indicate that the cell lines have a limited capacity for osteo genic or chondrogenic differentiation, Similarly agarose culture exper iments and extended treatment with retinoic acid indicate that they do not resemble dedifferentiated chondrocytes. Both the cell lines appea r to express a phenotype intermediate to osteoblasts and chondroblasts and possibly represent transitional differentiation stages of the pro genitor cells of the mandibular condyle. These cells could serve as us eful models in elucidating the pathways of early mesenchymal cell diff erentiation.