Role of epiphyseal cartilage in endochondral bone formation.

The avian epiphyseal growth plate plays an important role in skeletal development. Compared with the mammalian species, the avian growth plate contains more cells, is less well organized and appears more susceptible to malformation. Abnormal cartilage development is associated with chondrodystrophy, tibial dyschondroplasia and rickets. Many nutrient deficiencies result in chondrodystrophy, which is characterized by shortened, thickened bones and a narrowing of the epiphyseal growth plate. Manganese, the most extensively studied of these deficiencies, is necessary for proteoglycan biosynthesis. Tibial dyschondroplasia is a condition in which the prehypertrophic cells fail to hypertrophy and vascularization is aborted. This abnormality is found in genetically predisposed animals and its occurrence is altered by subtle changes in calcium, phosphorus and electrolyte content of the diet. Calcium and vitamin D deficiencies cause rickets, which is characterized by an increase in the width of the prehypertrophic zone of the epiphyseal growth plate. Most of the biochemical and histological changes can be related to the need for calcium for chondrocyte hypertrophy and maturation, although there is some evidence that chondrocytes require specific vitamin D metabolites. Phosphorus deficiency increases the zones of hypertrophy and metaphyseal primary spongiosa.