Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such minerals are typically either calcium-based, such as calcium carbonate or calcium phosphate, or silicon-based, such as silica. Biomineralization occurs in a wide range of taxa and both extra- and intracellularly.
The most common example of biomineralization is the formation of bone mineral; this process can be traced back to early vertebrates (Liem et al., 1980). The role of biomineralization in skeletal tissue engineering has been extensively reviewed (Dittmar & Bühler, 2006). Other well-studied examples include tooth enamel (Beard & Pradyutomnicksharma, 2009) and coral skeletons (Morse et al., 2011), both of which are composed primarily of calcium carbonate.
Recent years have seen an explosion in our understanding of the molecular mechanisms underlying biomineralization, thanks largely to advances in genomic technologies (De Yoreo & Vázquez-Losada, 2010). This article will provide a brief overview of some key findings in this area before turning to a more detailed discussion of two representative systems: mammalian bone and echinoderm spicules.
Bone is a composite material consisting of hydroxyapatite crystals embedded in an organic matrix. The protein component of this matrix is secreted by osteoblasts, cells that also regulate the deposition and organization of the mineral phase (Parish et al., 2000). Key proteins involved in these processes include osteocalcin (OCN), bone sialoprotein (BSP), and collagen type I—the most abundant protein in bone—all of which have been shown to play essential roles in scaffolding hydroxyapatite deposition (Weber et al., 2002; Grynpas & Kohn 2004).
In vitro studies have demonstrated that OCN promotes nucleation and growth of hydroxyapatite crystals through direct interaction with calcium ions (Fukushima et al., 2003; Kitagawa et al., 2005); it has also been shown to bind directly to BSP, thereby providing another link between the organic and mineral phases (Hosoya et al., 2005). Collagen type I serves a similar function: it too binds directly to hydroxyapatite crystals and helps organize their growth into linear arrays parallel to the long axis of the collagen fibril (Weber & Maurer 1993; Chuang et al.). In addition to its structural role, collagen provides sites for cell adhesion during tissue development and regeneration following injury; it also regulates cell proliferation and differentiation via interactions with various signaling molecules (Werner & Grodzinsky 2006).