Development, structure and composition of the otoliths in vertebrates

Abstract

The vertebrate otoliths are biologically-altered crystals composed of an organic matrix and inorganic constituents. The organic component contains proteins and an acidic mucosubstance with sulphuric and carboxylic groups. The inorganic component of the otoliths in gnathostome vertebrates is almost always composed of polymorphs of calcium carbonate, i.e. calcite, aragonite and rarely, vaterite. This work reviews mainly the morphological aspects (by scanning electron microscopy), and phenomena of seeding and of growth of otolithic biocrystals. Factors essential for seeding and growth of otolithic biocrystals are discussed. Congenital otolithic defects, in fact, can be caused by various environmental factors (carbonic anhydrase inhibitors, manganese and zinc dietary deficiencies) and by genetic factors. The main site of the formation of endogenous otoliths is believed to be the macula and in some cases the endolymphatic sac. Origin of the organic and inorganic constituents of the biocrystals is also discussed. Studies on the process of decalcification of otoliths in some species indicate the endolymphatic sac as a versatile calcium deposit.