Evolution of hard-tissue mineralization: comparison of the inner skeletal system and the outer shell system.

Abstract

polymorphs, calcite, aragonite, and vaterite, of which the former two are most commonly observed as biological polymorphs in marine mollusks. The pearl oyster P. fucata can generate these two polymorphs in vivo; calcite crystals in the outer prismatic layer and aragonite crystals in the inner nacreous layer of the shell, under the same physical conditions. These phenomena are a key to solving the enigma of bio-mineralization. The early research on shells of mollusks was mainly carried out by electron-microscopic observation [2–6]. Shells are mainly composed of two distinct layers. The outer prismatic layer shows the honeycomb-like network structure of a polygonal prism. The inner nacreous layer shows a cemented brick or veneer-like multilayer structure. Figure 1 shows a schematic representation of the inner nacreous layer. In this case, part of the brick corresponds to the aragonite crystal and part of the cement corresponds to the organic matrix. The organic matrix was first discovered in 1855 by Fremy [7], and is designated as “conchiolin”. Biochemical research on the organic matrix has recently developed, and now it has become clear that the organic matrix is mainly composed of proteins. Some parts of the organic matrix are soluble in water containing EDTA, and are called the water-soluble organic matrix (WSM). Most parts of the organic matrix are insoluble in water and are called the water-insoluble organic matrix (WISM). The group of Morse and colleagues [8–12] and that of Weiner and colleagues [13– 17] reported that shell proteins existing in the WSM mainly controlled the polymorphs of calcium carbonate crystals. In contrast, the early work of Watabe and Wilbur [18] and Wilbur and Watabe [19] and more recent reports by Wheeler [20] and by Samata et al. [21] showed that WISM proteins were mainly involved in polymorphic control. At present, it is difficult to determine which view is correct, because the groups with opposing views used different materials. The groups of Morse and colleagues [8–12] and Weiner and colleagues