On the role of carbonic anhydrase in the early phase of fish otolith mineralization

Abstract

The first step in the formation of fish otoliths, calcified structures which are responsible for the internalization of gravitational information, is based on the action of so-called Tether- (T-) cells. These T-cells appear during the very early development of the inner ear and persist only a few hours. They are characterized by a kinocilium, which is – in contrast to the kinocilium of the later developing sensory hair cells – not mechanosensory, but binds seeding particles containing glycogen, thereby localizing otolith formation (otolith seeding). Beating cilia distributed throughout the ear agitate seeding particles, thereby inhibiting premature agglutination. In the later development, a protein matrix is formed and mineralization/crystallization takes place. Since the enzyme carbonic anhydrase (CAH) plays a prominent role in otolith mineralization (it provides carbonate for CaCO 3 precipitation), we were prompted to investigate histochemically using larval cichlid fish ( Oreochromis mossambicus), whether CAH might be present as early as T-cells. Indeed, CAH was present in T-cells with prominent amounts of reaction product being located along the kinocilia and around the seeding particles. These results strongly indicate that kinocilia of T-cells act as structural guides for CAH/bicarbonate transportation towards the early otoliths’ calcification sites. Besides its role in calcification, CAH in the very early stage of otolith seeding may moreover aid in the accretion process of the precursor particles.