Collagen fibre patterns in mammalian bone.

From its first appearance in the skeleton of the embryo until death occurs, mammalian bone is subject to two developmental processes. First there are those mechanisms taking place in relation to the periosteum, the endosteum and the epiphyseal cartilages by which the bone grows in length and girth. The greater part of this development is completed by the time the epiphyseal plates are ossified in late adolescence or early adult life, but, even after this time, minor intermittent changes in girth are frequently produced by reactivation of the periosteal and endosteal mechanisms. Associated with this growth process, but not necessarily contributing to it, is a continual change in the internal structure of the tissue. In this process of internal development, bone passes through a series of stages, each of which is characterized by a specific arrangement of the intra-osseous collagen fibres, both in relation to one another and in relation to the vascular spaces of the tissue. It is these differences in the pattern of the collagenous fibres that allows bone to be classified into a number of clearly defined types. The standard classification is that of Weidenreich (1930) which depends largely on the size of the collagen bundles and on the relationship which they bear to neighbouring bundles. In this investigation, however, a somewhat different classification has been found to be advantageous. This is based on three factors, namely, the relationship of neighbouring fibres to one another, their relationship to vascular channels, and the place occupied by the tissue in the developmental history of the skeleton as a whole. The types of bone recognized in this classification are enumerated below in the chrono-logical order in which they appear in the development of mammalian bone: The fibre patterns of secondary osteones, and of interstitial bone derived from secondary osteones, have been considered in a previous publication (Smith,