The Ultrastructure of the Organic Phase Associated with the Inorganic Substance in Calcified Tissues

Abstract

An organic phase is closely associated with the mineral substance is all calcified matrices, where it can be demonstrated as crystal-bound proteins by biochemical methods and as crystal ghosts by electron microscopy. Interest in crystal ghosts derives chiefly from the observation that they have the same shape, arrangement, and orientation as inorganic crystallites, which suggests they may have a role in their formation. Histochemically, crystal ghosts of epiphyseal cartilage react with colloidal iron (pH 2.0), acidic phosphotungstic acid, ruthenium red, and a number of cations including calcium, barium, magnesium, lanthanum, strontium, and terbium chloride. Their reactivity is removed by methylation and only incompletely restored by saponification. Moreover, the crystal ghosts located at the periphery of the calcified areas contain vic-glycol groups, as shown by their reactivity with periodic acid-silver nitrate and periodic acid-thiosemicarbazide-osmium. All these reactions show that the crystal ghosts of epiphyseal cartilage contain acidic, probably sulfate groups and, at least initially, vic-glycol groups. Their reactivity decreases as the calcification process is completed. Although the available data are not sufficient to allow a full understanding of the nature and function of these structures, they seem to play an important role in calcification. The hypothesis is presented that crystal ghosts are preformed in calcifying matrices and are activated by the unmasking of the reactive groups in their polymeric molecule; the unmasked groups then link up with inorganic ions in such a way to form organic-inorganic structures the inorganic ions of which are arranged in an apatitelike configuration and the filamentlike shape of which is the same as that of the polymeric molecule.