In the

auditory system The auditory system is the sensory system for the sense of hearing. It includes both the ear, sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, incre ...

, the columella contributes to hearing in

amphibians Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s and

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s. The columella form thin, bony structures in the interior of the

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

and serve the purpose of transmitting sounds from the

eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pres ...

. It is an evolutionary

homolog In biology, homology is similarity in anatomical structures or genes between organisms of different taxa due to shared ancestry, ''regardless'' of current functional differences. Evolutionary biology explains homologous structures as retained her ...

of the

stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...

, one of the auditory ossicles in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s. 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.

Evolution

The evolution of the columella is closely related to the evolution of the jaw joint. It is an ancestral homolog of the

, 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 The tympanic cavity is a small cavity surrounding the bones of the middle ear. Within it sit the ossicles, three small bones that transmit vibrations used in the detection of sound. Structure On its lateral surface, it abuts the external audit ...

of the middle ear. The footplate, or proximal end of the columella, rests in the

oval window The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a connective tissue membrane-covered opening from the middle ear to the cochlea of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicl ...

. Sound is conducted through the oval window to the interior of the otic capsule. This motion ultimately stimulates sensory cells in the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

EB1911 Reptiles - Evolution of the Ossicular Chain of the Ear

In the transition of

tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

from sea to land, the earliest appearance of functional columella appeared in temnospondyls.

Extracolumella

Crocodilians Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchi ...

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 Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

and modern crocodilians have evolved a trifurcated columella, which forms a Y-shaped support structure on the surface of the

tympanic membrane In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pressur ...

. 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.

Anatomy in amphibians

Frogs

frogs A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to ...

, the extracolumella is simple and club-shaped.

Anatomy in reptiles

In reptiles, the columella function to transduce sound through the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...

as part of the

auditory pathway The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, increas ...

. The columella is relatively straight and moves in a piston-like motion in response to vibration. Due to the rigid bony structure, the columella primarily responds to low-frequency vibrations transmitted through the ground.

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. The extracolumella remains cartilaginous while the columella

ossifies Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by Cell (biology), cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes r ...

during development. The connection between the columella and extracolumella remains flexible over the animal's lifetime.

Snakes

Snakes Snakes are elongated Limbless vertebrate, limbless reptiles of the suborder Serpentes (). Cladistically Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales much like other members of ...

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.

Worm lizards

Amphisbaenia Amphisbaenia (called amphisbaenians or worm lizards) is a group of typically legless lizards, comprising over 200 extant species. Amphisbaenians are characterized by their long bodies, the reduction or loss of the limbs, and rudimentary eyes. A ...

, 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. 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. 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.

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.

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 Endochondral ossification is one of the two essential pathways by which bone tissue is produced during fetal development and bone healing, bone repair of the mammalian skeleton, skeletal system, the other pathway being intramembranous ossificatio ...

, the columella ossifies from two origins of

periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. (At the joints of long bones the bone's outer surface is lined with "articular cartila ...

: the shaft and the footplate.

Homology in mammals

Within mammals and other

synapsids Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...

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.

Artificial columella

In humans, artificially made columella may be produced as autografts from

cortical bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

. These prostheses are used as replacements for the stapes in

ear surgery Otorhinolaryngology ( , abbreviated ORL and also known as otolaryngology, otolaryngology–head and neck surgery (ORL–H&N or OHNS), or ear, nose, and throat (ENT)) is a surgical subspecialty within medicine that deals with the surgical an ...

to correct for hearing problems (such as

cholesteatoma Cholesteatoma is a destructive and expanding growth consisting of keratinizing squamous epithelium in the middle ear and/or mastoid process. Cholesteatomas are not cancerous as the name may suggest, but can cause significant problems because of the ...

or re-perforation).

References

{{Tetrapod osteology, S. Reptile anatomy Bird anatomy Amphibian anatomy Auditory system Ossicles