opening of ion channels. this causes depolarisation, which is passed as a signal to the brain along the vestibulocochlear nerve. dynamic balance also helps maintain eye tracking when moving, via the vestibulo–ocular reflex. development during embryogenesis the ear develops as three distinct structures: the inner ear, the middle ear and the outer ear. each structure originates from a different germ layer: the ectoderm, endoderm and mesenchyme. inner ear the otic placode visible on this sketch of a developing embryo. after implantation, around the second to third week the developing embryo consists of three layers: endoderm, mesoderm and ectoderm. the first part of the ear to develop is the inner ear, which begins to form from the ectoderm around the 22nd day of the embryo's development. specifically, the inner ear derives from two thickenings called otic placodes on either side of the head. each otic placode recedes below the ectoderm, forms an otic pit and then an otic vesicle. this entire mass will eventually become surrounded by mesenchyme to form the bony labyrinth. around the 33rd day of development, the vesicles begin to differentiate. closer to the back of the embryo, they form what will become the utricle and semicircular canals. closer to the front of the embryo, the vesicles differentiate into a rudimentary saccule, which will eventually become the saccule and cochlea. part of the saccule will eventually give rise and connect to the cochlear duct. this duct appears approximately during the sixth week and connects to the saccule through the ductus reuniens. as the cochlear duct's mesenchyme begins to differentiate, three cavities are formed: the scala vestibuli, the scala tympani and the scala media. both the scala vestibuli and the scala tympani contain an extracellular fluid called perilymph. the scala media contains endolymph. a set of membranes called the vestibular membrane and the basilar membrane develop to separate the cochlear duct from the vestibular duct and the tympanic duct, respectively. parts of the otic vesicle in turn form the vestibulocochlear nerve. these form bipolar neurons which supply sensation to parts of the inner ear (namely the sensory parts of the semicircular canals, macular of the utricle and saccule, and organ of corti). the nerve begins to form around the 28th day. molecular regulation most of the genes responsible for the regulation of inner ear formation and its morphogenesis are members of the homeobox gene family such as pax, msx and otx homeobox genes. the development of inner ear structures such as the cochlea is regulated by dlx5/dlx6, otx1/otx2 and pax2, which in turn are controlled by the master gene shh. shh is secreted by the notochord. middle ear the middle ear and its components develop from the first and second pharyngeal arches. the tympanic cavity and auditory tube develop from the first part of the pharyngeal pouch between the first two arches in an area which will also go on to develop the pharynx. this develops as a structure called the tubotympanic recess. the ossicles (malleus, incus and stapes) normally appear during the first half of fetal development. the first two (malleus and incus) derive from the first pharyngeal arch and the stapes derives from the second. all three ossicles develop from the neural crest. eventually cells from the tissue surrounding the ossicles will experience apoptosis and a new layer of endodermal epithelial will constitute the formation of the tympanic cavity wall. outer ear the ear develops in the lower neck region and moves upwards as the mandible develops. unlike structures of the inner and middle ear, which develop from pharyngeal pouches, the ear canal originates from the dorsal portion of the first pharyngeal cleft. it is fully expanded by the end of the 18th week of development. the eardrum is made up of three layers (ectoderm, endoderm and connective tissue). the pinna originates as a fusion of six hillocks. the first three hillocks are derived from the lower part of the first pharyngeal arch and form the tragus, crus of the helix, and helix, respectively. the final three hillocks are derived from the upper part of the second pharyngeal arch and form the antihelix, antitragus, and earlobe. the outer ears develop in the lower neck. as the mandible forms they move towards their final position level with the eyes. clinical significance hearing loss perforationfluid in the middle ear cavitycomplications of otitis media that can lead to hearing loss, as seen on otoscope.main article: hearing loss hearing loss may be either partial or total. this may be a result of injury or damage, congenital disease, or physiological causes. when hearing loss is a result of injury or damage to the outer ear or middle ear, it is known as conductive hearing loss. when deafness is a result of injury or damage to the inner ear, vestibulochoclear nerve, or brain, it is known as sensorineural hearing loss. causes of conductive hearing loss include an ear canal blocked by ear wax, ossicles that are fixed together or absent, or holes in the eardrum. conductive hearing loss may also result from middle ear inflammation causing fluid build-up in the normally air-filled space, such as by otitis media. tympanoplasty is the general name of the operation to repair the middle ear's eardrum and ossicles. grafts from muscle fascia are ordinarily used to rebuild an intact eardrum. sometimes artificial ear bones are placed to substitute for damaged ones, or a disrupted ossicular chain is rebuilt in order to conduct sound effectively. hearing aids or cochlear implants may be used if the hearing loss is severe or prolonged. hearing aids work by amplifying the sound of the local environment and are best suited to conductive hearing loss. cochlear implants transmit the sound that is heard as if it were a nervous signal, bypassing the cochlea. active middle ear implants send sound vibrations to the ossicles in the middle ear, bypassing any non-functioning parts of the outer and middle ear. congenital abnormalities anomalies and malformations of the pinna are common. these anomalies include chromosome syndromes such as ring 18. children may also present cases of abnormal ear canals and low ear implantation. in rare cases no pinna is formed (atresia), or is extremely small (microtia). small pinnae can develop when the auricular hillocks do not develop properly. the ear canal can fail to develop if it does not channelise properly or if there is an obstruction. reconstructive surgery to treat hearing loss is considered as an option for children older than five, with a cosmetic surgical procedure to reduce the size or change the shape of the ear is called an otoplasty. the initial medical intervention is aimed at assessing the baby's hearing and the condition of the ear canal, as well as the middle and inner ear. depending on the results of tests, reconstruction of the outer ear is done in stages, with planning for any possible repairs of the rest of the ear. approximately one out of one thousand children suffer some type of congenital deafness related to the development of the inner ear. inner ear congenital anomalies are related to sensorineural hearing loss and are generally diagnosed with a computed tomography (ct) scan or a magnetic resonance imaging (mri) scan. hearing loss problems also derive from inner ear anomalies because its development is separate from that of the middle and external ear. middle ear anomalies can occur because of errors during head and neck development. the first pharyngeal pouch syndrome associates middle ear anomalies to the malleus and incus structures as well as to the non-differentiation of the annular stapedial ligament. temporal bone and ear canal anomalies are also related to this structure of the ear and are known to be associated with sensorineural hearing loss and conductive hearing loss. vertigo main article: vertigo vertigo refers to the inappropriate perception of motion. this is due to dysfunction of the vestibular system. one common type of vertigo is benign paroxysmal positional vertigo, when an otolith is displaced from the ventricles to the semicircular canal. the displaced otolith rests on the cupola, causing a sensation of movement when there is none. ménière's disease, labyrinthitis, strokes, and other infective and congenital diseases may also result in the perception of vertigo. injury outer ear injuries to the external ear occur fairly frequently, and can leave minor to major deformity. injuries include: laceration, avulsion injuries, burn and repeated twisting or pulling of an ear, for discipline or torture. chronic damage to the ears can cause cauliflower ear, a common condition in boxers and wrestlers in which the cartilage around the ears becomes lumpy and distorted owing to persistence of a haematoma around the perichondrium, which can impair blood supply and healing. owing to its exposed position, the external ear is susceptible to frostbite as well as skin cancers, including squamous-cell carcinoma and basal-cell carcinomas. middle ear the ear drum may become perforated in the event of a large sound or explosion, when diving or flying (called barotrauma), or by objects inserted into the ear. another common cause of injury is due to an infection such as otitis media. these may cause a discharge from the ear called otorrhea, and are often investigated by otoscopy and audiometry. treatment may include watchful waiting, antibiotics and possibly surgery, if the injury is prolonged or the position of the ossicles is affected. skull fractures that go through the part of the skull containing the ear structures (the temporal bone) can also cause damage to the middle ear. a cholesteatoma is a cyst of squamous skin cells that may develop from birth or secondary to other causes such as chronic ear infections. it may impair hearing or cause dizziness or vertigo, and is usually investigated by otoscopy and may require a ct scan. the treatment for cholesteatoma is surgery. inner ear there are two principal damage mechanisms to the inner ear in industrialised society, and both injure hair cells. the first is exposure to elevated sound levels (noise trauma), and the second is exposure to drugs and other substances (ototoxicity). a large number of people are exposed to sound levels on a daily basis that are likely to lead to significant hearing loss. the national institute for occupational safety and health has recently published research on the estimated numbers of persons with hearing difficulty (11%) and the percentage of those that can be attributed to occupational noise exposure (24%). furthermore, according to the national health and nutrition examination survey (nhanes), approximately twenty-two million (17%) us workers reported exposure to hazardous workplace noise. workers exposed to hazardous noise further exacerbate the potential for developing noise-induced hearing loss when they do not wear hearing protection. tinnitus tinnitus is the hearing of sound when no external sound is present. while often described as a ringing, it may also sound like a clicking, hiss or roaring. rarely, unclear voices or music are heard. the sound may be soft or loud, low pitched or high pitched and appear to be coming from one ear or both. most of the time, it comes on gradually. in some people, the sound causes depression, anxiety, or concentration difficulties. tinnitus is not a disease but a symptom that can result from a number of underlying causes. one of the most common causes is noise-induced hearing loss. other causes include: ear infections, disease of the heart or blood vessels, ménière's disease, brain tumors, emotional stress, exposure to certain medications, a previous head injury, and earwax. it is more common in those with depression and anxiety. society and culture stretching of the earlobe and various cartilage piercings the ears have been ornamented with jewelry for thousands of years, traditionally by piercing of the earlobe. in ancient and modern cultures, ornaments have been placed to stretch and enlarge the earlobes, allowing for larger plugs to be slid into a large fleshy gap in the lobe. tearing of the earlobe from the weight of heavy earrings, or from traumatic pull of an earring (for example, by snagging on a sweater), is fairly common. injury to the ears has been present since roman times as a method of reprimand or punishment – "in roman times, when a dispute arose that could not be settled amicably, the injured party cited the name of the person thought to be responsible before the praetor; if the offender did