DEVELOPMENTAL RESTRICTION OF EMBRYONIC CALVARIAL CELL-POPULATIONS AS CHARACTERIZED BY THEIR IN-VITRO POTENTIAL FOR CHONDROGENIC DIFFERENTIATION

The mechanism(s) by which the cells within the calvaria tissue are res tricted into the osteogenic versus the chondrogenic lineage during int ramembranous bone formation were examined, Cells were obtained from 12 -day chicken embryo calvariae after tissue condensation, but before ex tensive osteogenic differentiation, and from 17-day embryo calvariae w hen osteogenesis is web progressed, Only cell populations from the you nger embryos showed chondrogenic differentiation as characterized by t he expression of collagen type II, The chondrocytes underwent a tempor al progression of maturation and endochondral development, demonstrate d by the expression of collagen type II B transcript and expression of collagen type X mRNA, Cell populations from both ages of embryos show ed progressive osteogenic differentiation, based on the expression of osteopontin, bone sialoprotein, and osteocalcin mRNAs. Analysis using lineage markers for either chondrocytes or osteoblasts demonstrated th at when the younger embryonic cultures were grown in conditions that w ere permissive for chondrogenesis, the number of chondrogenic cells in creased from similar to 15 to similar to 50% of the population, while the number of osteogenic cells remained almost constant at similar to 35-40%, Pulse labeling of the cultures with BrdU showed selective labe ling of the chondrogenic cells in comparison with the osteogenic cells , These data indicate that the developmental restriction of skeletal c ells of the calvaria is not a result of positive selection for osteoge nic differentiation but a negative selection against the progressive g rowth of chondrogenic cells in the absence of a permissive or inductiv e environment, These results further demonstrate that while extrinsic environmental factors can modulate the lineage progression of skeletal cells within the calvariae, there is a progressive restriction during embryogenesis in the number of cells within the calvaria with a chond rogenic potential, Finally, these data suggest that the loss of cells with chondrogenic potential from the calvaria may be related to the pr ogressive limitation of the reparative capacity of the cranial bones.