Muñoz Schuler C., Torres V., Aguilera F.
Most mollusks produce external calcified shells, which are fabricated by specialized epithelial cells on the dorsal mantle. This tissue is underneath the shell and expresses a suite of genes encoding secreted proteins that are delivered into the extrapallial space to finally be embedded into the shell. Recent shell proteomic analyses have revealed that secreted proteins, but also proteins lacking a signal peptide, form part of the shell, raising the question of how these non-secreted proteins reach the mineralization front. Some authors have postulated the hemocytes as possible protein carriers; however, there is not a full understanding of how this could be happening. By performing tough bioinformatics analyses on the genomic resources available for the bivalve Crassostrea gigas, we found the majority of components concerning the biogenesis and secretion of exosomes, suggesting that this species has the molecular repertoire required to produce exosomes. We further evaluated the expression patterns of those exosomal components in different tissues of this oyster and found that hemocytes seems to be the main exosome-producing tissue, followed by the mantle. Indeed, gene expression of exosome-related proteins, which have also been reported as embedded in the shell, display high expression in mantle, followed by hemocytes, digestive gland, and adductor muscle. Furthermore, shell-forming genes are expressed in a similar fashion, with a dominant expression in the mantle tissue, followed by digestive gland, male gonad and hemocytes. Our results indicate that genes encoding components of the shell matrix are not only generated in the mantle but also in other tissues, proposing the participation of non-classical biomineralization pathways in shell formation. Altogether, we suggest that molluscan shell formation is probably aided by diverse tissues, with hemocytes, given their high capacity to produce exosomes and infiltrate into different tissues, being the second most important shell-forming tissue.