Development of Ear

Organs of Special Sense are organs specialized in perceiving sensory information. They include the eyes, ears, the nose and the tongue. The eyes are organs of sight that contain specialized sensory cells that perceive light, shape of objects and colour; they transform light into electrical signals that are interpreted by the brain. The ears are responsible for hearing and balance. They have a cochlear auditory system responsible for hearing and a vestibular system responsible for balance. The contains the olfactory mucosa that comprises sensory olfactory cells specialized in sensing smell and generate impulses that are interpreted by the brain. The tongue has several functions that include sensing taste. It contains specialized structures called taste buds that contain sensory cells and nerve ending specialized in identifying different tastes. The nose and tongue have already been covered in previous chapters (respiratory system and digestive system chapters), thus this chapter will deal only with the development of the eyes and ears. Organs of special senses start to show up during the early stages of embryonic development.

Fig. 19.1.  Organs of special senses show up in early stages of embryonic development.

The ear is one of the organs of special senses, the sense of hearing and in addition it has another important function which is the maintenance of the body balance. It has three major parts which are the external ear, the middle ear, and the inner ear, each with its own functions. The external ear collects sound waves and direct them towards the middle ear which amplifies the sound waves and transmits them towards the inner ear. The inner ear converts the sound waves into electrical signals that are sent to the brain for interpretation. The inner ear has cochlear system responsible for hearing and a vestibular system for maintenance of balance.

The external ear consists of the ear pinna (auricle) and the external auditory meatus (the ear canal), whereas the middle is an air-filled cavity known as the tympanic cavity which contains three inter-articulated tiny bones called the malleus, incus and stapes; the three bones, which are collectively known as the auditory ossicles, are responsible for amplifying and conducting the sound waves from the external ear to the internal (inner) ear. The external auditory meatus and the tympanic cavity are separated by a thin membrane called the tympanic membrane. The vestibular system comprises the semicircular canals, the utricle, and the saccule. The cochlear system comprises the snail-shaped bony cochlea and its contents of the membranous labyrinth.  

Fig. 19.2.  The illustration shows the major components of the outer ear (pinna and ear canal), middle ear (auditory ossicles) and inner ear (semicircular canals and cochlea).

Development of the Inner Ear

The inner ear (internal ear) begins to develop at an early stage of development, during the 4th week of embryonic development. It is ectodermal in origin. The surface ectoderm of the head thicken to form the otic placode on either side of the rhombencephalon. The otic vesicle invaginates and gradually transforms into a vesicle called the otic vesicle (the otocyst), which sinks into the underlying neural crest mesenchyme detaching itself from the surface epithelium. The otic vesicle differentiates into two components, a dorsal portion and a ventral portion. The dorsal portion which develops into the utricle and the semicircular canals functions in maintaining body balance. The ventral portion on the other hand develops into the cochlear duct and saccule, which are key structures for hearing. The cochlear duct elongates and coils assuming the shape of a snail.

Fig. 19.3. The illustration on the right shows the development of the otic placode (above) and its transformation into an otic vesicle in the vicinity of the developing hindbrain (rhombencephalon). The picture on the left shows the development of the ear (otic vesicle) and the eye (optic cup) in a 9-weeks old fetus.

The ectodermal epithelial cells of the cochlea and vestibule give rise to specialized ciliated sensory cells called the hair cells which are crucial for sensing sound waves and detecting head movements. The soft tissues forming ducts of the inner ear collectively referred to as the membranous labyrinth. A protective bony labyrinth develops around the membranous labyrinth. The membranous labyrinth is ectodermal in origin, whereas the bony labyrinth is of neural crest origin. The neural crest-derived mesenchyme surrounding the membranous labyrinth condenses and differentiate into bone via endochondral ossification. Models of hyaline cartilage are first formed are then replaced by bone.

Fig. 18. 14. The illustration above shows the components of the fully developed bony labyrinth, whereas the one below shows the components of the fully developed membranous labyrinth.

Development of the Middle Ear

The middle ear comprises the tympanic cavity and the auditory ossicles. The tympanic cavity is of endodermal origin; it develops from the 1st pharyngeal pouch. This pouch elongate and forms the tubotympanic recess. The distended distal part of the recess develops into the tympanic cavity, whereas the tubular part gives rise Eustachian tube. The tympanic membrane (ear drum) develops from the endoderm of the first pharyngeal cleft and the overlying ectoderm. Thus, the tympanic membrane is ectodermal externally and endodermal internally. The tympanic cavity expands into the mesenchyme that will develop into the auditory ossicle. The mesenchyme is of neural crest origin, so the ossicles are of neural crest origin. The ossicle develops by endochondral ossification; cartilaginous are formed first and then cartilage is replaced by bone.  

Development of the External Ear

The external comprises the auricle and the external auditory meatus. The auricle (pinna) develops in 3rd week of gestation from six swellings known as the auricular hillocks; they all develop from the 1st and 2nd pharyngeal arches. The hillock merge together forming the characteristic shape of the ear pinna. The external auditory meatus develops by deep invagination and elongation of the 1st pharyngeal cleft.

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