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Otic Capsule Or Bony Labyrinth

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Otic Capsule Or Bony Labyrinth DEVELOPMENT OF EAR BY DR NOMAN ULLAH WAZIR DEVELOPMENT OF EAR The ears are composed of three anatomic parts: External ear: • Consisting of the auricle , external acoustic meatus, and the external layer of the tympanic membrane. Middle ear: • The internal layer of the tympanic membrane, and three small auditory ossicles, which are connected to the oval windowsof the internal ear. • Internal ear: Consisting of the vestibulocochlear organ, which is concerned with hearing and balance. • The external and middle parts of the ears are concerned with the transference of sound waves to the internal ears, which convert the waves into nerve impulses and registers changes in equilibrium. DEVELOPMENT OF INTERNALEAR The internal ears are the first to develop. • Otic placode: Early in the 4th week, a thickening of surface ectoderm takes place on each side of the myelencephalon,the caudal part of thehindbrain. • Inductive signals from the paraxial mesoderm and notochord stimulate the surface ectoderm to form theplacodes. • Each otic placode soon invaginates and sinks deep to the surface ectoderm into the underlying mesenchyme. • In so doing, it forms an otic pit. • The edges of the pit come together and fuse to forman otic vesicle the primordium of the membranous labyrinth. • The otic vesicle soon loses its connection with the surface ectoderm. • A diverticulum (endolymohatic appendage) grows from the vesicle and elongates to form the endolymphatic duct and sac. the rest of the oticvesicle differentiates into an expanded pars superior (Ventral saccularparts, which give rise to the sacculeand cochlearducts) and an initially tapered pars inferior (Dorsal utricular parts, from which thesmall endolymphaticducts, utricles and semicircular ductsarise). • Three disc like diverticula grow out from the utricular parts of the primordial membranous labyrinths. • The central parts of these diverticula fuse and disappear. • The peripheral unfused partsof the diverticula become the semicircular ducts, which are attached to the utricle. • Localized dilatations, the ampulla, develop at one endof each semicircular duct. • From the saccular part of the otic vesicle, a tubular diverticulum the cochlear duct grows and coils to form the membranous cochlea. • A connection of the cochlea with the saccule, the ductus reuniens,soon forms. The spiral organ (of Corti) differentiates from cells in the wall of the cochlear duct. • Ganglion cells of the vestibulocochlear nerve migrate along the coils of the membranous cochlea and form the spiral ganglion. • Nerve processes extend from this ganglion to the spiral organ, where they terminate on the haircells. • The cells in the spiral ganglion retain their embryonic bipolar condition. • Otic capsule or bony labyrinth Inductive influences from the otic vesicle stimulate the mesenchyme around it to condense and differentiate into a cartilaginousotic capsule Or bony labyrinth • As the membranous labyrinth enlarges, vacuoles appear in the cartilaginous otic capsule and join to form the perilymphaticspace. • The membranous labyrinth is now suspended in perilymph. • The perilymphatic space, related to the cochlearduct, develops two divisions: • Scalatympani • Scalavestibuli. • The cartilaginous otic capsule later ossifies to form the bony labyrinth of the internalear. • The internal ear reaches its adult size and shape by the middle of the fetal period (20–22weeks). DEVELOPMENT OF MIDDLEEAR Tubotympanic recess: Arises from the first pharyngeal pouch. The proximal part of the tubotympanic recess forms the pharyngotympanic tube (auditory tube). • The distal part of the recess expands and becomes the tympaniccavity. • which envelops the auditory ossicles (malleus, incus, and stapes), their tendons and ligaments, and the chorda tympaninerve. • The ossicles are derived from separate origins in the first andsecond arch mesenchyme. • The expansion of the tympanic cavity gives rise to themastoid antrum, located in the petromastoid part of the temporal bone. • The mastoid antrum is almost adult size at birth; however,no mastoid cells are present in neonates. • The middle ear continues to grow through puberty. • The tensor tympani muscle is derived from mesenchyme in the first pharyngeal arch and is innervated by trigeminal nerve. • The stapedius muscle is derived from the second pharyngeal arch and is supplied by facial nerve. DEVELOPMENT OF EXTERNALEAR The external acoustic meatus, develops from the dorsal part of the first pharyngeal groove. The ectodermal cells at the bottom of this funnel-shaped tube proliferate to form a solid epithelial plate, the meatalplug. Late in the fetal period, the central portion of this plug degenerate, forming a cavity that becomes the internal part of the external acoustic meatus. The meatus, relatively short at birth, attains its adult length in approximately the 9th year. • The primordium of the tympanic membrane is the first pharyngeal membrane, which forms the external surface of the tympanic membrane. • The pharyngeal membrane separates the first pharyngeal groove from the firstpharyngeal pouch. • As development proceeds, mesenchyme grows between the two parts of the pharyngeal membrane and differentiates into the collagenic fibers in the tympanic membrane. The auricle develops from six auricular hillocks that arise during the fifth week on the first and second pharyngeal arches. • From ventral to dorsal, the hillocks on the first pharyngeal arch are called the tragus, helix, and cymba concha (or one to three, respectively), and • the hillocks on the second arch are called the antitragus, antihelix, and concha (or four to six, respectively). • During the seventh • week, the auricular hillocks begin to enlarge, differentiate, and fuse to produce the definitive form of the auricle. • As the face develops, the auricle is gradually translocated from its original location low on the side of the neck to a more lateral and cranial site CLINICALCORRELATES CONGENITALDEAFNESS: • The formation of the internal ear is independent of development of the middle and external ears therefore congenital impairment of hearingmay be the result of maldevelopment of the sound-conducting apparatus of the middle and external ears, or of the neurosensory structures of the internal ear. • Congenital deafness may be associated with several other head and neck defects as a part of the first archsyndrome. • Abnormalities of the malleusand incus are often associated with this syndrome. Rubella infection: During development of the internal ear, particularly the 7th and 8th weeks, rubella can cause defects of the spiral organ and deafness. • Congenital fixation : Fixation of the stapes to the bony labyrinth resultsin conductive deafness in an otherwise normal ear. Auricularabnormalities: Severe defects of the externalear are rare, but minor deformities are common. • The auricles are often abnormal in shape and low-set in infants with trisomy18. • In infants affected by maternal ingestion of certain drugs (e.g., trimethadione). AuricularAppendages (skin tags) are common and may result from the development of accessory auricular hillocks. The appendages usually appear anterior to the auricle, more often unilaterally than bilaterally. The appendages, often with narrow pedicles, consist of skin but may contain some cartilage. Anotia: (Absence of the Auricle) is rare but is commonly associated with the first pharyngeal arch syndrome. Anotia results from failureof mesenchymal proliferation. Microtia: (a small or rudimentary auricle) results from suppressed mesenchymal proliferation . This defect often serves as an indicator of associated birth defects, such as an atresia of the external acoustic meatus and middle ear anomalies. The cause can be both genetic and environmental. • Preauricular Sinuses and Fistulas: • Pit-like cutaneous depressions or sinuses are occasionally located in a triangular area anterior to the auricle. • The sinuses are usually narrow tubes or shallow pits that have pinpoint external openings. • Some sinuses contain a vestigial cartilaginous mass. Auricular fistulas (narrow canals): connecting the preauricular skinwith the tympanic cavity or the tonsillar fossa. .
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