Mechanisms and nutritional influences in skeletal development: Influence of macro- and microelements on bone formation

Bone is composed of crystals of hydroxyapatite, a calcium phosphate, within an organic matrix composed mainly of collagen. Long bones grow longitudinally by endochondral ossification and bones thicken by intramembranous ossification. Vitamin D, after conversion to its active metabolite 1,25-dihydroxyvitamin D (1,25-D), is an important regulatory factor for the function of genes involved in calcium metabolism and bone growth. Calcium and phosphorus homeostasis is maintained by a regulatory feedback system involving 1,25-D and parathyroid hormone. Deficiencies and/or imbalances in dietary intake of calcium, phosphorus and/or vitamin D in young birds can result in problems of bone formation and quality characterised by rickets or tibial dyschondroplasia. There is evidence that genetic improvements in broiler performance are resulting in increased requirements for these nutrients. In laying hens, the problem of osteoporosis are not caused primarily by lack of nutrients but can be partially alleviated by providing a particulate source of calcium. Bone composition Bone is composed of a bone mineral deposited in and around an organic matrix. The matrix, secreted by osteoblasts, is composed mainly of crosslinked microfibrils of type I collagen. The bone mineral is hydroxyapatite, deposited as small hexagonal rods, 200x50 A. It is a calcium phosphate containing calcium and phosphorus in the approximate ratio of 2.15:1 but other cations and anions can also be present in small quantities. Normal bone contains about 30% matrix and 70% mineral. These proportions are optimal for the biomechanical properties of bone; a lower proportion of matrix results in bones being brittle, a lower proportion of mineral gives softer bones. The biomechanical properties of bone change with age as a result of continued collagen crosslinking as bones mature. In poultry, there are three types of bone whose physical properties are related to their collagen matrix. Cortical bone is lamellar in nature, with concentric rings of bone based a highly regular arrangement of collagen microfibrils. Trabecular or cancellous bone is another form of structural bone but is termed a woven bone because the collagen structure is less organised on an extended scale. The third bone type is found only in laying hens. This is medullary bone, also a woven bone but with a more disorganised collagen structure and much less intrinsic strength than the other two types. Mechanisms of bone growth and remodelling Bone metabolism in the growing bird Bone growth occurs in poultry in two ways. Longitudinal growth in long bones of the appendicular skeleton is brought about by a process called endochondral ossification. This is based on the epiphyseal growth plate where some of the resting chondrocytes with fibroblastic phenotype in the germinal layer become committed to differentiate into proliferative chondrocytes. These cells multiply and form columns of flattish cells closely packed within an extracellular matrix, secreted by the chondrocytes, that contains a high content of type II collagen. This zone of proliferative chondrocytes is nourished by the epiphyseal blood capillaries. These cells gradually become more separated within their columns as more matrix is secreted, then they start to differentiate into a hypertrophic state. They become enlarged and more rounded and start secreting a new matrix component, type X collagen. The hypertrophic zone receives its blood supply from metaphyseal blood vessels, so there is a narrow avascular zone of prehypertrophic chondrocytes between the proliferative and hypertrophic zones. Apart