Ultrastructural observations of amorphous bone mineral in avian bone

Mineral from medullary bone of three avian species was examined with the electron microscope in order to clarify the ultrastructure of amorphous bone mineral (ABM) in a mineralized tissue. Powders from freeze-dried bone revealed bone mineral with morphology and behavior identical to synthetic amorphous calcium phosphate (ACP). These powders exhibited spherically shaped particles 180--400 A in diameter with uniform electron density when viewed at low beam intensity. Thin sections of embedded freeze-dried bone also revealed spherically shaped particles 100--350 A in diameter with electron beam sensitivity characteristic of ACP. Regions of bone mineral with irregular outline and morphology were observed which closely resemble the discoidal form of synthetic ACP. More electron-dense spherical particles (150 A in diameter) could be seen budding from these regions. Some of these buds exhibited electronlucent centers characteristic of AMB. The inorganic nature of these features of bone mineral was confirmed using ultramicroincineration. Preliminary exploration of a freeze-substitution technique showed spherical bone mineral particles which were similar in morphology to those observed in freeze-dried samples. A limited degree of preservation of cellular material was observed using this freeze-substitution technique.