Is there a calcification factor common to all calcifying matrices?

This paper reviews the principal morphological findings pertinent to the early phases of the calcification process and explores the possibility that there may be a calcification factor common to all calcifying matrices. Three structures have a main role in calcification: collagen fibrils, matrix vesicles, and crystal ghosts. Only crystal ghosts are present in all calcified tissues, so that only they can be taken into consideration as a common calcification factor. They are organic molecules which have the same morphology as that of the inorganic structures present in the calcified matrix, which means that they can be considered as templates for those structures. Early calcification might be initiated by the binding of calcium and phosphate ions to the unmasked reactive groups of the crystal ghosts which are probably contained not only in the matrix, but also in the "holes" of the collagen fibrils and in matrix vesicles. The available data suggest that crystal ghosts share many of the properties of "crystal bound proteins". The involvement of alkaline phosphatase in their composition may account for their calcium- and phosphate-binding activity.