Fracture callus formation in young and old mice observed with polarized light microscopy

The organized cartilaginous fibrous system which appears during growth of the fracture callus in traumatized femora was investigated in mice of five, 52 and 78 weeks of age. Histological preparations were stained with toluidine blue O and observed under polarized light using a first order red retardation plate. Development and organization of the cartilaginous fibrous system was similar in animals of all age groups studied, however, its time of appearance was dependent upon the time of appearance of cartilage cells and in association with these cells. Alignment of cartilage cells followed very closely after appearance of the fibrous system. Initial orientation of the fibrous system was almost perpendicular to the bone surface. As the system became more extensive the slope of fiber direction became less acute, pointing towards the fractured end of the shaft. Orientation of the fibrous system is believed to be a consequence of the forces set up during growth of the fracture callus, e. g., the force of the growing cell mass due to postfracture, active cell proliferation of the osteogenic layer of the periosteum and hypertrophy of cartilage cells and the tension placed on the fibrous layer of the periosteum in response to the events taking place below this tissue layer. The importance of the cartilaginous fibrous system is believed to lie in the early temporary support of the fracture fragments and the proper alignment of the cellular elements of the fracture, necessary for proper architectural reconstruction. Formation of the cartilaginous fibrous system was independent of the existing periosteal fibrous system.