Distraction osteogenesis has proven to be of great value for the treatment of a variety of musculoskeletal problems. Little is still known, however, about the phenotypic changes in the cells participating in the bone formation process, induced by the procedure. Recent findings of the expression of a contractile muscle actin isoform, a-smooth muscle actin (SMA), in musculoskeletal connective tissue cells prompted this immunohistochemical study of the expression of SMA in cells participating in distraction osteogenesis in a rat model. The tissues within and adjacent to the distraction site could be distinguished histologically on the basis of cell morphology, density, and extracellular matrix make-up. The percentage of SMA-containing cells within each tissue zone was graded from 0 to 4. The majority of the cells in each of the zones stained positive for SMA within five days of the distraction period. The SMA-containing cells included those with elongated morphology in the center of the distraction site and the active osteoblasts on the surfaces of the newly forming bone.These finding warrant further investigation of the role of this contractile actin isoform in distraction osteogenesis and investigation of the effects of modulation of this actin isoform on the procedure.

1. Sheep-, rabbit-and ox-muscle minces prepared soon after slaughter were diluted with 1 vol. of 0-16 M-potassium chloride in the absence (potassium chloride mince) and presence of added cofactor or glycolysable substrate, and the effects on the ultimate pH were examined. 2. Changes in the concentrations of glycogen and lactate and the concentrations of some phosphorus-containing fractions were determined in ox-muscle preparations. 3. Glycolysis ceased at appreciably higherpH in the potassium chloride mince than in undiluted mince. The inclusion of glycogen, ATP, ADP, NAD or magnesium chloride in the diluent had little effect on the ultimate pH of the diluted mince. 4. Lactic acid production continued at lower pH values in diluted mince containing added glucose 1-phosphate, fructose 1,6-diphosphate or glucose plus hexokinase than in potassium chloride mince. 5. The evidence points to failure of the phosphorylase step being responsible for the dilution effect.