Ultrastructure and composition of asteroid bodies

PURPOSE. Asteroid hyalosis is a disease of the vitreous, characterized by b rilliant reflecting particles, termed asteroid bodies, which are surrounded by a tightly adhering network of fibrils, The composition and mode of form ation of asteroid bodies is not yet understood in detail. The purpose of th is study was to investigate the ultrastructure of asteroid bodies and to id entify the intrinsic inorganic and organic components that contribute to th e nature and development of asteroid bodies. METHODS. Electron energy loss spectroscopy and energy-filtered transmission electron microscopy were used for the elemental analysis of asteroid bodie s. The ultrastructural localization of glycosaminoglycans was investigated, using lectin and antibody conjugates in conjunction with transmission elec tron microscopy and epifluorescence microscopy. Anionic sites of glycosamin oglycans were detected with 15 nm cationic colloidal gold at low pH, applie d as a postembedding technique. Ultrastructural details of asteroid bodies were documented using fast Fourier transform analysis of zero-loss filtered images. RESULTS. Element mapping of asteroid bodies by electron spectroscopic imagi ng revealed a homogeneous distribution of calcium, phosphorus, and oxygen. The electron energy loss spectra of these elements showed details similar t o those found for hydroxyapatite. Additionally, high contrast and sensitivi ty against a calcium-specific chelator highlighted the crystalline, apatite -like nature of asteroid bodies. Immunofluorescence microscopy revealed the presence of chondroitin-6-sulfate at the periphery of asteroid bodies, whi ch is in agreement with the ultrastructural colocalization of anionic sites . Fast Fourier trans form analysis revealed that each 7-nm periodicity of a steroid lamellar stacks is divided by a fine, parallel-oriented line, separ ating each 7-nm laver into two halves of 3.5-nm thickness. Carbohydrates sp ecific for hyaluronic acid were observed by lectin-gold labeling to be pan of the inner matrix of asteroid bodies. CONCLUSIONS. The results of this study demonstrate the structural and eleme ntal similarity of asteroid bodies with hydroxyapatite. Proteoglycans and t heir glycosaminoglycan side chains are implicated in playing a role in regu lating the biomineralization process.