Surface structures and osteoclasts of mouse parietal bones: a light and scanning electron microscopic study.

The resorption surface caused by osteoclasts are identified by scanning electron microscopy (SEM), but the morphological interrelation between these resorption surfaces and Howship's lacunae observed by light microscopy remains obscure, and little evidence showing that the distribution and structures of the resorption surface change according to the function of osteoclasts is available. To understand the relationship between the function of osteoclasts and the morphological evidence of bone resorption, surface structures of the parietal bones in mice, newborn to adult, were studied by SEM. The osteoclasts on the inner surface of the calvaria are considered to be involved in the growth of the skull. The outer surface of the parietal bone was smooth, whereas the inner surface consisted of both smooth and rough areas. The rough areas were usually much larger than Howship's lacunae, which are generally thought to be of osteoclast size. These areas were composed of small and shallow concavities with oval or polygonal outlines. The borders between adjacent concavities appeared as ridges. The rough areas were very wide in the growing skull and osteoclasts were scattered on these wide rough areas. The osteoclasts were much larger than the concavities in the rough areas. During the growth of the skull, the proportion of rough areas occupying the inner surface changed parallel to the number of osteoclasts, which varied in correspondence with the growth rate of the skull. The maximum value of the proportion was ab out 60% at 1 week of age. The findings suggest that the osteoclasts resorb the bone by moving along its surface, forming small concavities and leaving rough areas larger than Howship's lacunae of osteoclast size. Furthermore, they suggest that the size and distribution of the rough areas and the morphological features of the concavities in the rough areas vary depending upon the activities of the osteoclasts.