Diverse hard tissues constituted a tooth-like skeletal element in extinct jawless vertebrates. Today, similar tissues are found in our teeth. These tissues mineralize in the extracellular matrix and involve various macromolecules. Among these molecules are secretory calcium-binding phosphoproteins (SCPPs) coded by genes that arose by duplication. Although the repertoire of SCPPs may vary in different lineages, some SCPPs are unusually acidic and are thought to participate in the mineralization of a collagenous matrix, principally either bone or dentin. Other SCPPs are rich in Pro and Gln (P/Q) and are employed to form the tooth surface. In tetrapods, the tooth surface is usually covered with enamel which develops in a matrix comprised of P/Q-rich SCPPs. By contrast, the tooth surface tissue in teleosts is called enameloid and it forms in a dentin-like collagenous matrix. Despite the difference in their matrix, both enamel and enameloid mature into hypermineralized inorganic tissues. Notably, some P/Q-rich SCPP genes are primarily expressed at this stage and their proteins localize between the tooth surface and overlying dental epithelium. Moreover, an orthologous gene is used for maturation of these 2 different tissues. These findings suggest distinct roles of acidic and P/Q-rich SCPPs during the evolution of hard tissues. Acidic SCPPs initially regulated the mineralization of bone, dentin, or a similar ancient collagenous tissue through interaction with calcium ions. P/Q-rich SCPPs arose next and originally assembled a structure or a space that facilitated the hypermineralization of dentin or a dentin-like tissue. Subsequently, some P/Q-rich SCPPs were coopted for the mineralizing enamel matrix. More recently, however, many SCPP genes were lost in toothless birds and mammals. Thus, it appears that, in vertebrates, the phenotypic complexity of hard tissues correlates with gain and loss of SCPP genes.
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