The fine structure of the proximal growth plate of the avian tibia.

The ultrastructure of the proximal tibial growth plate of the 7 weeks old chicken has been described. Little ultrastructural difference could be ascertained between growth plates examined from normal white leghorn and commercial broiler chickens. The growth plate may be divided into five zones: interstitial, proliferating, prehypertrophic, hypertrophic, and degenerating hypertrophic. These zones reflect a maturation of chondrocytes, beginning with a stage of high mitotic and cytoplasmic activity passing through a stage of active secretion of matrical components (prehypertrophic and hypertrophic) and ending with degeneration of the cells and calcification of the matrix. Mineralization of the matrix appears to be initiated within matrical dense bodies, as in the mammal. Single hydroxyapatite crystals are first encountered about 0.1 mm proximal (i.e. towards the knee) to the limit of metaphyseal blood vessel ingrowth, while dense calcification is observed 0.1 mm distal to the tips of these metaphyseal vessels. The diameter of microfibrillary collagen in the growth plate matrix ranges from approximately 9 nm in the proximal zones to 19 nm in the distal zones. Many of the fibrils in the distal zones have a more or less distinct periodicity. Other major elements of the growth plate matrix are the ruthenium red-stained syncytial aggregates of mucopolysaccharides which are probably derived from the granules within the large intracellular Golgi vesicles. These findings have led the author to conclude that, while light microscopy indicates that avian and mammalian growth plates have very different structures, electron microscopy finds many similarities, suggesting that the physiological control mechanisms in these two vertebrate classes have much in common.