Conservation of signal molecules involved in biomineralisation control in calcifying matrices of bone and shell

Bone and nacre feature highly orchestrated biomineralised structures. They are calcified structures that share an intervening organic matrix laid by specialized cells, bone cells in vertebrates and epithelial mantle cells in molluscs. Organic framework provides a scaffold for crystallisation and directs mineralisation. Bone and shell likely undergo self-reparation when damaged, clues to the presence in these mineralised structures of signal molecules, targeting the cells involved in their renewal. In molluscs, calcium carbonate crystallise under two mineral polymorphs, aragonite or calcite. Sudo et al. [22] reported two exclusive proteins in the shell of a bivalvia molluscan, Pinctada fucata , MSI 60 in aragonite and MSI 31 in calcite. We built up specific antibodies and experimented immunohistochemistry in Haliotis. These tools revealed the presence of Pinctada-like proteins within Haliotis mantle cells, in a zonation consistent with shell matrix mineralisation control involvement. Another facet of matrix-mediated biomineralisation control arises from bone and nacre interactivity. In vivo experiments of nacre implantation in sheep and rabbit established the biocompatibilty of nacre and bone. The nacreous part from the shell of Pinctada, a bivalve mollusc, was implanted as bone device in sheep and showed that nacre passes bone acceptance. Nacre implants were not subjected to intolerance reaction and the recipient bone provided with nacre underwent a sequence of bone regeneration within an osteoprogenitor-rich cell layer. The water-soluble organic matrix was extracted from powdered nacre by a gentle non-decalcifiying process. Three mammalian cell types, fibroblasts (human), bone marrow stromal cells (rat) and pre-osteoblasts (mouse) were used to characterize in vitro the effect of nacre water-soluble matrix on mammal bone cell lineage. In vitro studies provided evidence for the presence, in nacre organic matrix, of signal molecules responsible for the recruitment of mammal cells in the osteogenic pathway and bone cell activation undergoing a complete sequence of mineralisation. Retrieving like-proteins in shell matrix of molluscs from distant taxa and bone nacre interactivity provides convergent data supporting the conservation of molecular signals for biomineralisation control within the organic framework of biomineralised structures.