Columella (auditory system)

Evolution of mammalian auditory ossicles In the auditory system, the columella contributes to hearing in amphibians, reptiles and birds. The columella form thin, bony structures in the interior of the skull and serve the purpose of transmitting sounds from the eardrum. It is an evolutionary homolog of the stapes, one of the auditory ossicles in mammals. In many species, the extracolumella is a cartilaginous structure that grows in association with the columella. During development, the columella is derived from the dorsal end of the hyoid arch. The evolution of the columella is closely related to the evolution of the jaw joint. It is an ancestral homolog of the stapes, and is derived from the hyomandibular bone of fishes. As the columella is derived from the hyomandibula, many of its functional relationships remain the same. The columella resides in the air-filled tympanic cavity of the middle ear. The footplate, or proximal end of the columella, rests in the oval window. Sound is conducted through the oval window to the interior of the otic capsule. This motion ultimately stimulates sensory cells in the inner ear. In the transition of tetrapods from sea to land, the earliest appearance of functional columella appeared in temnospondyls. Crocodilians evolved to lift the head and body off the ground, isolating the head from ground vibrations. Under selective pressure to detect airborne sound vibrations, the columella in crocodilians have become more slender and reduce their mass. The extracolumella, a cartilaginous outgrowth on the distal end of the columella, couples the columella to the tympanum to conduct sound from the exterior air. Birds and modern crocodilians have evolved a trifurcated columella, which forms a Y-shaped support structure on the surface of the tympanic membrane. In birds, this is thought to increase the surface area of the columellar footplate, thus lowering the threshold of hearing and improving the detection of airborne sound waves. In frogs, the extracolumella is simple and club-shaped.