Sex differences in bone resorption: a scanning electron microscopic study of mouse parietal bones.

Bone resorption surfaces formed by osteoclasts can be identified as rough areas by scanning electron microscopy (SEM). In this study, the endocranial surfaces of mouse parietal bones were examined by SEM at 1, 4, 6, 8, 14 and 20 weeks of age in an attempt to understand the bone resorbing activity after the cessation of bone growth. On the inner surface of the parietal bones, the rough areas, composed of a group of 8 to 30 microns wide concavities, can be divided into two distinct groups (type I and type II). The surface of type I areas appeared irregularly rough and those of type II revealed feather, fire-flame, or wave-like patterns due to characteristic arrangements of the concavities. The rough areas occupying the inner surface of the parietal bones made up about 60% of the surface area at 1 week and about 5% at 4 weeks in both sexes. The rough areas were all type I at this age. After puberty, the rough areas in males occupied about 10% of the inner surface until 20 weeks and consisted of large type I and small type II areas. In females, the proportion of the rough areas increased after puberty and occupied about 40% of the endocranial surface at 14 and 20 weeks. The type I areas were much larger, increasing in size with age and eventually occupying a large proportion of the rough areas. The sex differences in the rough areas were reversed by gonadectomy. The results suggest that male and female gonadal products, possibly sex hormones, have respective inhibitory and stimulatory effects on the formation of bone resorptive surfaces, and may be responsible for the sex differences in the size and morphological features of the areas on the bone surfaces.