Comparative studies on maturation process of secondary ossification centers of long bones in the mouse, rat, dog and monkey.

Comparative studies was made on the whole processes of maturation of the secondary ossification centers in the extremity in the mouse, rat, dog and monkey in order to determine points of similarity and difference among these species. The appearance and the union of the secondary ossification centers were observed to follow a common sequence and order in all the species examined. The total bone score was defined as the sum of the bone scores of respective secondary ossification centers over the observation period according to a set criteria. By plotting these indices against the chronological age it was shown in all of these species that the whole process of maturation of the secondary ossification centers in the long bone consisted of three biological stages as indicated by the three straight lines with different slopes. The first stage is considered to account for the appearance of the secondary ossification centers and their accelerated development at early period, while the second stage corresponds to the subsequent gradual development. The third stage represents a period where the ossification centers reach a complete or almost complete unmion to diaphysis. It is concluded that in the mouse, rat, dog and monkey the maturation process of the secondary ossification centers follow the same growth consisting of three developmental stages and that each of these three stages provides a measure of comparison among these species.