![Columella in Dicynodon](https://upload.wikimedia.org/wikipedia/commons/1/17/Columella_in_Dicynodon.jpg)
In the
auditory system, the columella contributes to hearing in
amphibians,
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s and
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s. The columella form thin, bony structures in the interior of the
skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
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
The outer ear, external ear, or auris externa is the extern ...
. It is an evolutionary
homolog
In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of prima ...
of the
stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals 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 foo ...
, one of the
auditory ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (inner ear), labyrinth (cochlea). The ...
in
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
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
The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arc ...
.
Evolution
The evolution of the columella is closely related to the evolution of the
jaw joint. It is an ancestral homolog of the
stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals 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 foo ...
, and is derived from the
hyomandibular
The hyomandibula, commonly referred to as hyomandibular one( la, os hyomandibulare, from el, hyoeides, "upsilon-shaped" (υ), and Latin: mandibula, "jawbone") is a set of bones that is found in the hyoid region in most fishes. It usually plays ...
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 auditory ...
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 membrane-covered opening from the middle ear to the cochlea of the inner ear.
Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
. Sound is conducted through the oval window to the interior of the
otic capsule.
This motion ultimately stimulates sensory cells in the
inner ear.
![EB1911 Reptiles - Evolution of the Ossicular Chain of the Ear](https://upload.wikimedia.org/wikipedia/commons/3/34/EB1911_Reptiles_-_Evolution_of_the_Ossicular_Chain_of_the_Ear.jpg)
In the transition of
tetrapods
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...
from sea to land, the earliest appearance of functional columella appeared in
temnospondyls
Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carbo ...
.
Extracolumella
Crocodilians
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...
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 Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
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 sound from the air ...
.
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
In
frogs
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...
, 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, which transfer the vibrations of the eardrum into waves in the ...
as part of the
auditory pathway
Auditory means of or relating to the process of hearing:
* Auditory system, the neurological structures and pathways of sound perception
** Auditory bulla, part of auditory system found in mammals other than primates
** Auditory nerve, also known ...
. 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 cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...
during development. The connection between the columella and extracolumella remains flexible over the animal's lifetime.
Snakes
Snakes
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...
have lost a tympanic membrane, and hence a distal attachment for the columella. The columella is instead connected to the
quadrate bone
The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids.
In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper ...
of the
jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...
. Thus, snakes are able to detect and localize ground vibrations through the lower jaw, rather than the sides of the head.
Worm lizards
In
Amphisbaenia, the extracolumella is particularly lengthened and firmly connects with a layer of skin over
dentary bone
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable 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
The labial scales are the scales of snakes and other scaled reptiles that border the mouth opening. These do not include the median scales on the upper and lower jawsWright AH, Wright AA. 1957. Handbook of Snakes. Comstock Publishing Associates ( ...
. 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
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...
condensation. Chondrification of the columella occurs earlier than the extracolumella. During
endochondral ossification, 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. Endosteum lines the inner surface of the medullary cavity of all long bones.
Structu ...
: the shaft and the footplate.
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.
Artificial columella
In humans, artificially made columella may be produced as
autograft
Autotransplantation is the transplantation of organs, tissues, or even particular proteins from one part of the body to another in the same person ('' auto-'' meaning "self" in Greek).
The autologous tissue (also called autogenous, autogen ...
s 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, and ...
. 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 subspeciality within medicine that deals with the surgical a ...
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 th ...
or re-perforation).
References
{{Tetrapod osteology, S.
Reptile anatomy
Bird anatomy
Amphibian anatomy
Auditory system
Ossicles