Columella (highlighted) in the skull of the extinct therapsid Dicynodon.

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.[1]

Evolution

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.[2]

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.[2] This motion ultimately stimulates sensory cells in the inner ear.[3]

Depiction of the evolution of the ossicles of the ear. Columella (Co) and extra-columella (E) evolve into the stapes and extra-stapes in embryonic mammals (7).[4]

In the transition of tetrapods from sea to land, the earliest appearance of functional columella appeared in temnospondyls.[5]

Extracolumella

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.[6]

Birds and modern crocodilians have evolved a trifurcated columella, which forms a Y-shaped support structure on the surface of the tympanic membrane.[7] 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.[7][3]

Anatomy in amphibians

Frogs

In frogs, the extracolumella is simple and club-shaped.[3]

Anatomy in reptiles

In reptiles, the columella function to transduce sound through the middle ear as part of the auditory pathway. The columella is relatively straight and moves in a piston-like motion in response to vibration.[3] Due to the rigid bony structure, the columella primarily responds to low-frequency vibrations transmitted through the ground.[2]

Crocodilians

In crocodilians, the columella arises from a proximal and a distal component which develop into the columella and extracolumella, respectively. It is typically trifurcated, with three finger-like projections supporting it against the tympanic membrane.[3] The extracolumella remains cartilaginous while the columella ossifies during development.[8] The connection between the columella and extracolumella remains flexible over the animal's lifetime.[7]

Snakes

Mounted skull of a python with disarticulated upper and lower jaw joints. In snakes, the columella would be attached directly to the quadrate bone (c).

Snakes have lost a tympanic membrane, and hence a distal attachment for the columella. The columella is instead connected to the quadrate bone of the jaw. Thus, snakes are able to detect and localize ground vibrations through the lower jaw, rather than the sides of the head.[3]

Worm lizards

In Amphisbaenia, the extracolumella is particularly lengthened and firmly connects with a layer of skin over dentary bone of the lower jaw. This connection appears to facilitate detection of airborne vibrations in the facial area.[6] The embedding in the skin often occurs at a specially enlarged labial scale. As a result, the amphisbaenian is able to detect substrate vibrations as it burrows through the ground while protecting the internal ear from damage.[9] Amphisbaenians otherwise lack an external ear structure, likely due to selective pressure to protect the middle and inner ears from damage as the animal burrows.[10]

Birds

In birds, the columella is anchored to the conical tympanic membrane at an acute angle, rather than a 90-degree angle relative to the plane of the tympanic membrane. This is thought to provide a lever advantage in conducting airborne sound from the distal to the proximal end of the columella.[6]

Development in chickens

In chick embryos, the primordial columella arises from a mesenchymal condensation. Chondrification of the columella occurs earlier than the extracolumella. During endochondral ossification, the columella ossifies from two origins of periosteum: the shaft and the footplate.[11]

Homology in mammals

Within mammals and other synapsids the columella has evolved into the stapes, a homologous bone within the newly evolved inner ear. As the tympanic cavity evolved to reduce in size, the columella shortened in length. The stirrup-shaped articular processes of the columella inspired a new name for this auditory ossicle, the stapes. The auditory ossicles continue to function in conducting transmitting sound through the auditory pathway; however, they have lost their function in conducting low frequency ground vibrations.

Later-arising reptiles with columella likely evolved stronger limbs and a more crawling posture, which removed the body from the ground and prevented the transmission of ground-conducted sounds. The skin over the ear evolved into the eardrum, which allowed for the detection of high-frequency airborne vibrations. In mammals, the newly specialized ossicles function to transduce and amplify these vibrations along the auditory pathway.[2]

Artificial columella

In humans, artificially made columella may be produced as autografts from cortical bone. These prostheses are used as replacements for the stapes in ear surgery to correct for hearing problems (such as cholesteatoma or re-perforation).[12][13]

References

  1. ↑ Goodrich ES (July 1915). "Memoirs: The Chorda Tympani and Middle Ear in Reptiles, Birds, and Mammals". Journal of Cell Science. 61 (2): 137–160. S2CID 27277400.
  2. 1 2 3 4 Homberger DG, Walker WF (2004). Vertebrate dissection (9th ed.). Belmont, CA: Thomson Brooks/Cole. ISBN 0-03-022522-1. OCLC 53074665.
  3. 1 2 3 4 5 6 Christensen-Dalsgaard J, Manley GA (October 2013). "The malleable middle ear: an underappreciated player in the evolution of hearing in vertebrates.". In Köppl C, Manley GA, Popper AN, Fay RR (eds.). Insights from comparative hearing research. Springer Handbook of Auditory Research. Vol. 49. New York, NY.: Springer. pp. 157–191. doi:10.1007/2506_2013_33. ISBN 978-1-4614-9077-7.
  4. ↑ Olson EC (August 1966). "The middle ear--morphological types in amphibians and reptiles". American Zoologist. 6 (3): 399–419. doi:10.1093/icb/6.3.399. PMID 5949350.
  5. ↑ Manley GA (May 2010). "An evolutionary perspective on middle ears". Hearing Research. 263 (1–2): 3–8. doi:10.1016/j.heares.2009.09.004. PMID 19786082. S2CID 25664943.
  6. 1 2 3 Saunders J (2000). The Middle Ear of Reptiles and Birds. Springer. ISBN 978-1-4612-7036-2.
  7. 1 2 3 Claes R (2018). Understanding functioning and evolution of bird middle ear mechanics: a functional morphological analysis (PDF) (Ph.D. thesis). University of Antwerp.
  8. ↑ Frank GH, Smit AL (1974). "The Early Ontogeny of the Columella Auris of Crocodilus Niloticus and its Bearing on Problems Concerning the Upper End of the Reptilian Hyoid Arch". African Zoology. 9 (1): 59–87. doi:10.1080/00445096.1974.11448520.
  9. ↑ Evans, Susan Evans (2016). "The Lepidosaurian Ear: Variations on a Theme". Evolution of the Vertebrate Ear. Springer Handbook of Auditory Research. Vol. 59. pp. 245–284. doi:10.1007/978-3-319-46661-3_9. ISBN 978-3-319-46659-0.
  10. ↑ Gans, Carl (1972). "The ear and hearing in Amphisbaenia (Reptilia)". Journal of Experimental Zoology. 179 (1): 17–34. doi:10.1002/jez.1401790103.
  11. ↑ Wood JL, Hughes AJ, Mercer KJ, Chapman SC (February 2010). "Analysis of chick (Gallus gallus) middle ear columella formation". BMC Developmental Biology. 10 (1): 16. doi:10.1186/1471-213X-10-16. PMC 2834582. PMID 20158901.
  12. ↑ Kylén P, Albrektsson T, Ekvall L, Hellkvist H, Tjellström A (1987). "Survival of the cortical bone columella in ear surgery". Acta Oto-Laryngologica. 104 (1–2): 158–65. doi:10.3109/00016488709109062. PMID 3310512.
  13. ↑ Rönnblom A, Gladiné K, Niklasson A, von Unge M, Dirckx J, Tano K (December 2019). "A New, Promising Experimental Ossicular Prosthesis: A Human Temporal Bone Study With Laser Doppler Vibrometry". Otology & Neurotology. 41 (4): 537–544. doi:10.1097/MAO.0000000000002556. PMC 7208281. PMID 31821265.
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