Maturation Process of Secondary Ossification Centers in the Rat and Assessment of Bone Age

The maturation process from the appearance to the fusion of the secondary ossification centers of extremities was studied in Wistar rats aged 0 to 134 weeks. The examination of the secondary ossification centers made by radiography. The assessment of the stage of development was made in accordance with the criteria proposed by Ohwada and Sutow. The secondary ossification center was found to be take one of the following three types of maturation processes : (1) the acute ossification, (2) the delayed ossification, and (3) the incomplete ossification. No fusion was observed up to 134 weeks in certain epiphyses of the rat. This type of ossification designated as the incomplete ossification may be specific to the mouse and rat. The relative lengths of time required for appearance and fusion in the average prospective life were obtained for the rat. They were compared with those of the mouse and man. The relative length of time necessary for maturity of the secondary ossification centers was shown to be the shortest in the rat and the longest in man. The results suggested that the rat may reach maturity in the bone age at 17 to 21 weeks of age. The rat at this age may be regarded as being adult corresponding to age 17 weeks in mice and 18 to 24 years in man.