The architecture and material properties of bone have evolved to enable each bone to fulfill its physiological role in the most efficient way. Bone is a dynamic tissue in which resorption and formation occur during the modelling and remodelling processes. Itis the balance between the milieu of osteotropic influences which determines whether the net effect of these processes results in bone formation or bone resorption.

For load-bearing bones there is a close relationship between the bone's architecture and its mechanical strain environment. For other bones, in which load-bearing is not important, strain may not represent an important influence on the structural form of these bones. It is likely that there are differences in the hierarchy of osteotropic control of bone mass in different bones of the skeleton. The relative importance of strain, hormones and other osteotropic influences will depend on the function of each bone. In certain instances, bone form may be determined phylogenetically and osteotropic influences may not affect greatly, the final form of the bone. As such, the bone would be less sensitive to these external influences or would respond to them slowly.

The experiments in this thesis investigate several aspects of the response of bone to external stimuli. The specific studies determine the cellular response of mechanical loading on a resorbing surface of the modelling ulna of a growing rat, the role of parathyroid hormone (PTH) on ovariectomy (OVX) induced osteopenia in the adult rat by investigating the mechanical and structural properties of various long bones, vertebrae and the parietal bone of the skull. The final study investigates regional differences in human bone strain in vivo by bonding rosette strain gauges to the tibia and parietal bone.

The results of mechanically loading the ulna demonstrated a highly significant increase in fluorochrome label incorporation on the medial periosteal surface compared with controls (P< 0.005). An enzymatic marker of osteoclast activity (tartrate resistant acid phosphatase) was reduced significantly in loaded bones compared with controls (P< 0.005), while the histological appearance of the bone surface in controls was altered from one with characteristic features of resorption to those of new mineralised bone formation.

The study investigating the possible role of PTH on OVX induced osteopenia demonstrated a significant interaction between the effects of thyroparathyroidectomy and OVX on the development of trabecular osteopenia in the proximal tibia (P< 0.05). However, there were no significant differences between the experimental groups and the controls in terms of parietal thickness or porosity. The study investigating human tibial and parietal strain in vivo demonstrated strain rates and magnitudes that were up to ten times greater in the tibia compared with the calvaria.

From the results, it can be concluded that:​

1. There are regional differences in the mechanical strain environment to which different bones of the skeleton are exposed.
2. There are regional differences in the response of different bones to hormonal influences.