Comparison of whole calvarial bones and long bones during early growth in rats. Histology and collagen composition

The distribution of ossified collagen (bone) and uncalcified collagen (fibrous tissue and cartilage) was compared histologically for rat and dog calvaria at birth. The relative amount of bone and uncalcified collagen was quantitated morphologically for rat calvaria during the first four weeks of rapid growth. Whereas dog calvaria are essentially ossified at birth, rat calvaria at birth consist mostly of fibrous tissue but rapidly become ossified with growth. Bacterial collagenase was used to separate uncalcified collagen from calcified collagen of whole membranous bones (frontal and parietal) and long bones (femur and humerus) at birth from man, monkey, dog, guinea pig, rabbit and rat. By this means quantitative changes in the relative fractions of the two forms of collagen were determined during the first eight weeks of postnatal growth for each type of rat bone. Quantitative biochemical data on whole rat bones (calvarium, femur, humerus) confirmed measurements based on histology which showed that at birth rat calvaria are mostly uncalcified as compared to other species whose bones are mostly ossified at birth. With growth rat membranous bones ossify more rapidly than long bones.