The embryological development of the ear is best understood by describing separately the individual development of the three major anatomical features that comprise the ear. Accordingly, although in the fetus, the development of the external ear, middle ear, and internal ear overlap in many areas and stages, the overall development of the ear may be studied by an individual study of these three structure.
The external ear develops from the upper portion of the first external pharyngeal groove and becomes an area of highly modified skin. Comprising ectoderm, the cells in this area, termed the meatal plate, continue to divide until they come in contact with cells destined to form the middle ear structure. The cells at this juncture will ultimately form the tympanic membrane. As development proceeds, the meatal plate thickens and becomes plug-like in form (the meatal plug). Eventually, a cavity forms within the meatal plug and bony tissue forms within the plug ring to form the external ear (external auditory meatus). The features of the auricle of the adult ear develop from six regionalized swellings (hillocks) that original surround the external pharyngeal groove.
Interestingly, a region of the external ear is the only site of ectodermal cell origin that is innervated by a branch of the Vagal nerve. Anatomists assert that this innervation is an evolutionary vestige of the highly developed lateral-line system found in sharks. In sharks, the lateral-line system enables the shark to discern minute pressure changes in the water caused by moving objects at great distances, and in an important and effective sensory organ used in hunting prey.
The auditory tubes (Eustachian tubes) develop from an area of the developing pharynx that is composed of endodermal cells. The cells form a tubotympanic recess and ultimately form the Eustachian tubes and other middle ear structures, including the tympanic cavity. At about the end of the first month of development, the cells surrounding the tubotympanic recess form a primitive middle ear cavity. Some of the cells thicken and elongate into the primitive auditory tubes. Eventually the cells in this area come in contact with the otic capsule. An area of cells chondrifies to become the malleus, incus, and handle of the malleus of the middle ear. A separate area of the otic capsule differentiates to form the stapes.
The developing handle of the malleus comes in contact with the developing chorda tympani nerve that ultimately joins other nerves in the eardrum. These structures become wedged between the ectoderm of the outer ear and endoderm of the middle ear to eventually lie in a thin layer of mesoderm that separates the developing structures.
The malleus and incus become suspended by folds of membrane. These and other folds partially divide the middle ear into a complex series of passages. The otic capsule eventually becomes cartilaginous (condrifies) and then becomes bony (ossifies) as part of the temporal bone. Portions of the convoluted labyrinth of folds become part of the inner ear structure.
The inner ear develops from a thickening of ectoderm near the hindbrain, termed the otic placode. The plate like placode forms an otic pit. When the pit is separated from the surface by further development of surrounding cells, an otic vesicle forms. The otic vesicle is surrounded with a layer of mesoderm to form an otic capsule.
Somewhat like a belt being tightened, the otic capsule becomes contracted about its middle region to form upper and lower chambers. The upper area forms the utriculosaccular area, and the lower chamber the cochlear area. Semicircular canals develop in the utriculosaccular area. Specialized neural cells develop near ampullae that ultimately contribute to sensory feeling associated with balance. Ultimately the utriculosaccular area divides into the utricle and saccule that remain connected by an utriculosaccular duct. The lower cochlear region of the otic capsule grows into the spiral shape found in the adult utriculosaccular area.
Specialized neuroreceptor cells develop within the cochlea to form the spiral organ of Corti. The cells associate with neural ganglia (the spiral ganglion). Eventually regions of the otic vesicle form the membranous labyrinth that becomes filled with lymph fluid. In the adult, the cochlea senses disturbances in the lymphatic fluid caused by sound waves. Hair-like projections respond to change son fluid motion and cause associated nerves to fire electrical signals to the brain. The development of the cochlea is usually complete by the end of the second trimester of development.
Although a newborn's ear looks like an adult ear, anatomists and physiologists continue research into determining exactly when the ear completes development and takes on full adult-like capacity. Small differences may lead to discoveries or insights into developmental abnormalities or delays in the development of balance and walking.
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