In vertebrates, bone and dentin have been found in the earliest mineralized
skeletal elements, which consisted of odontodes and osseous plates. For more than 450
million years, the two tissues evolved concomitantly in various skeletal elements, which
were either conserved or subjected to wide changes as illustrated in extinct and extant
lineages: odontodes, teeth, scales, osteoderms, fin rays, bony spines, dermal bones, and
cartilage bones from the endoskeleton. Bone is cellular in most species, with a few
exceptions of bone lacking osteocytes in some early vertebrates and in derived teleost
fishes. Dentin has been classified into eight distinct types and some of them are highly
specialized such as the elasmodin of fish scales. In this chapter, we summarize our
knowledge and propose a scenario for the evolution of these two tissues. We also
review hypotheses proposed for tracing back the origin of the skeletal tissues during the
prechordate-vertebrate transition. Specific molecules and specialized cells were
undoubtedly differentiated prior to the identification of these tissues in the fossil record.
Cartilage, bone and dentin matrices were probably created when typical fibrillar
collagen type II and type I, the major components of these skeletal tissues, were
recruited after the genome duplications that predated the vertebrate differentiation. The
secretory calcium-binding phosphoproteins (SCPPs) are crucial molecules controling
crystallization of calcium phosphate in these skeletal tissues. SCPP genes are thought to
have arisen from SPARCL1 (SPARC-like 1), which originated from SPARC (secreted
protein, acidic, cystein-rich), at about the same period when fibrillar collagen genes
used for vertebrate skeletal tissues were differentiated. Eventually, neural crest-derived
specialized cells employed a fibrillar collagen and an SCPP, among other proteins, and
created the first mineralized dermal skeletal elements, probably in association with
sensory organs located on the body surface. Currently at least five acidic SCPPs (DSPP,
DMP1, IBSP, MEPE, SPP1) are involved in the mineralization of bone and dentin
tissues in mammals, and other SCPPs were identified in teleost fishes. In this chapter
we analyze the evolution of these SCPPs and their relationships within the framework
of vertebrate phylogeny.
Keywords: Odontodes, teeth, scales, dermal bone, endoskeleton, secretory
calcium-binding proteins, SPARC-like 1, SPARC, fibrillar collagen, bone &
dentin mineralization.
