We use the sense of balance and hearing to position ourselves in space, sense our surrounding environtment and to communicate. Portion of the ear appear very early in development as specialized region (otic palcode) on the embryo surface that sinks into the mesenchyme to form a vesicle (otic vesicle=otocyt) that form in the inner ear.
This region connects centrally to the nervous system and peripherally through specilized bones to the external ear (auricle). This organisation develop different sources forming the 3 ear parts: inner ear (otic placode, otocyst) middle ear (1st pharingeal pouch and 1st and 2nd arch mesenchyme) and outer ear (1st pharingeal cleft and 6 surface hillocks).
This complex origin, organisation and time course means that abnormal development of any one system can impact upon the development of hearing.
Embriogenic origin overview
External ear:
- Auricle --- Pharingeal arches 1 and 2 (ectoderm and mesoderm)
- External Auditory Meatus --- Pharingeal arch 1 groove or cleft (ectoderm)
- Tympanic Membrane --- Pharingeal Arch 1 membrane (ectoderm, mesoderm, endoderm)
Middle ear:
- Middle ear ossicles
#Stapes --- Pharingeal Arch 2 cartilage neural crest (ectoderm)
- Middle ear muscles
#Stapedius --- Pharingeal arch 2 (mesoderm)
- Middle ear cavity --- Pharingeal Arch 1 pouch (endoderm)
Inner ear:
- Inner ear labyrinth
#Semicircular canals --- Otic vesicle ---otic placode (ectoderm)
#Saccule and utricle --- Otic vesicle --- otic placode (ectoderm)
- Cranial nerve VIII
#Vestibular component --- Otic vesicle and neural crest (ectoderm)
POSTNATAL CHANGES
There are a number of post natal changes associated with growth of the head that affect the newborn to adult auditory tube and its functions. The auditory tubes (Eustachian, otopharingeal or pharingotympanic) space connects the middle ear cavity to nasopharinx portion of pharinx.
Auditory tube function:
- Ventilation --- pressure equalization in the middle ear
- Clearence --- allow fluid drainage from the middle ear , tube is normally closed and opened by muscle
- Birth --- (neonatal to early childhood) the tube is initially short (17-18 mm), narrower and runs almost horizontal, The tube is opened by a single muscle, tensor palati muscle.
- Adult --- the tube is longer (twice as long) , wider and runs at aproximately 45 degrees to the horizontal. Tube is opened by two separate muscle, tensor palati and levator palati.
There are many different abnormalities of hearing development that can result in hearing loss and can be broadly be divided into either conductive or sensorineural loss. These abnormalities can have genetic, environtmental or unknown origins. In addition, abnormalities of the external ear (position and structure) is used as a clinical diagnostic tool for developmental abnormalities in other systems.
- Inner ear --- common cavity, severe cochlear hypoplasia
- Middle ear --- rare and can be part of first arch syndrome, Malleus, Incus, and Stapes Fixation. Cholestoma is epithelium trapped within skull base in development, erosion of bones (temporal bone, middle ear, mastoid)
- Outer ear ---several genetics effects and syndromes, environmental effect. Microtia is abnormally small external ear. Prearicular sinus is occurs in 0,25% births, bilateraln (hereditary) 25-50%, unilateral (mainly the left), duct runs inward can extend into the parotid gland, postnatally site for infection.
- Sensory neural --- Cochlear or central auditory pathway
- Hereditary -- Recessive (severe) and dominant (mild)
- Acquired --- rubella (German measless), maternal infection during 2nd month of pregnancy, vaccination of young girls --- cytomegalovirus ---streptomycin ---antibiotic ---thalidomide
- Disease of outer and middle ear
- Can be produced by otitis media with effusion, that is widespread i young children
- Temporary blockage of outer or middle ear
Position - lower or uneven height, "railroad track" appearance, curve at top part of outer ear is under developed, folded over parallel to curve beneath
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