Ultrastructural Aspects of Physiological Mineralization: A Comparative Study in Different Hard Tissues

The calcified tissues of vertebrates are essentially represented by bone, cartilage, dentin and calcified tendons. In all these tissues a major hallmark of mineralization is the deposition of the inorganic phase on a pre-existing collagen template, but evident differences exist among these materials and the molecular details of the process are still incompletely understood. In this study, the ultrastructural aspects of the mineral phase of these tissues were investigated by means of high-resolution scanning electron microscopy (HR-SEM) after low-temperature thermal deproteination, a technique allowing a direct, unrestricted visualization of the mineral component. Each tissue showed distinctive features. In most cases, calcification proceeds in a discontinuous way through the formation of clumps or clusters of mineralized tissue; in all cases, except cartilage, the mineral phase shows an evident relationship with the layout and/or the D-period of the collagen fibrils. Our results highlight the peculiar aspect of the mineralization process in the cartilage with respect to the other tissues, all of them containing collagen type I instead of type II, and suggest that a different molecular mechanism may be at work. It is still unclear whether and how this may be related to the content, exclusive of cartilage, of collagen type II. The identification of the tissue-specific features exhibited by cartilage versus those shared by all the other three tissues, although from different species, requires further research on physiological calcification.