Introduction

It is widely accepted that bone is a highly adaptive tissue that modulates its architecture (mass and structure) in response to its mechanical environment. The adaptive process is regulated through homeostatic pathways as well as epigenetic regulatory processes (Turner, 1992). In bone, as in all connective tissues, the main constituents are the cells and the extracellular matrix. The latter is composed of collagen fibres and ground substance rich in glycosaminoglycans which undergo mineralisation. The surfaces of bone are lined with osteogenic cells and layers of precursors which are at interface with bone marrow on the endosteal surface and with fibrous connective tissue and muscle tissue at the periosteum. The calcified bone matrix surrounds the osteocytes, which have numerous long cell processes in contact with those of other osteocytes, or with processes from the lining cells on the endosteum or on the periosteum. This kind of morphology creates a very potent communication system between the matrix and the cells, as well as, between the lining cells on the surface and their neighbouring tissues (marrow and ligaments) and between the lining cells and osteocyte network. It is most probable that this unusual combination of mineralised collagenous fibres and the cellular system determine bone's unique mechanical properties and its remodelling capacity.

The general anatomical form of a bone is inherent in the skeletogenic tissue. Muscular and gravitational stresses determine the mass and distribution of bone tissue.