THE normal sequences encountered in endochondral bone formation can be divided into three main categories: (1) growth of cartilage, (2) maintenance of a fine balance between osteoblastic and osteolytic activities of the bone cells and (3) deposition of inorganic materials in cartilage matrix and/or bone matrix (osteoid). Derangements in each of these three categories may be observed in bone disease naturally occurring in the growing child or in the adult. I n the experimental animal one can produce a t will changes which precisely duplicate many of the alterations seen in the human. One can sometimes go even farther and create changes which have no counterpart in human disease.
We should like t o present some interesting, though diversified, examples of experimental bone disease which fit into these categories. The changes which we wish t o discuss will indicate a few of the large number of unsolved problems which concern the growth of normal cartilage and bone. This presentation might better be entitled " A miscellany of unrelated and peculiar changes which may be produced experimentally in cartilage and bone and which may some day throw light on the biochemical activities of these structures."
When the dietary of an animal, or a human, too, for that matter, is restricted calorically or by the removal of one or more of any of the essential elements, amino acids, vitamins or fatty acids, growth of cartilage stops, sometimes very abruptly. The cells cease proliferating; in particular, the zone of hypertrophic cells becomes narrower and narrower (Fig. ). Of course, the influence of the dietary restriction is 240 Bone Structure and Metabolism