FINE-STRUCTURE OF THE PROXIMAL GROWTH PLATE OF THE AVIAN TIBIA

Abstract

The ultrastructure of the proximal growth plate of 7 wk old normal white Leghorn and broiler chickens is described. Little ultrastructural difference exists between these 2 strains of bird. The growth plate is divided into 5 zones: interstitial, proliferating, prehypertrophic, hypertrophic and degenerating hypertrophic. They reflect a maturation of chondrocytes, beginning at the proximal face of the growth plate, with a stage of high mitotic and cytoplasmic activity (proliferating zone) passing through a stage of active secretion of matrical components (prehypertrophic and hypertrophic zones) and ending with cell degeneration and matrix calcification. Matrix mineralization appears to be initiated within matrical dense bodies. Single hydroxylapatite crystals are first encountered about 0.1 mm proximal to the limit of metaphyseal blood vessel ingrowth, while dense calcification is observed 0.1 mm distal to the tips of these vessels. The diameter of microfibrillary collagen in the growth plate matrix ranges from approximately 9 nm in the proximal zones-19 nm in the distal zones. The fibrils in the distal zones have a relatively distinct periodicity. Major elements of the growth plate matrix are ruthenium red stained syncytial aggregates of mucopolysaccharides, which probably are derived from granules within the large intracellular Golgi vesicles. Although light microscopy reveals considerable structural differences between avian and mammalian growth plates, detailed EM observations demonstrated many similarities, suggesting that the physiological control mechanisms in these 2 vertebrate classes have much in common.