External ear development is a lengthy snd complex precess that extends from early embryogenic life until well into the postnatal period. Initial development of the auricle and external auditory canal during the fourth and fifth weeks of gestation is closely associated with anotomycal changes involving the pharingeal arch apparatus of the human embryo. The auricle and external canal are well formed by the time of birth but do not attaintheir full size and adult configuration until about 9 years of age. Sebaceous and modified apocrine glands, which are responsible for cerumen production, begin their development at about 5 months gestation in association with hair follicles in the outer portion of the external canal. Although they appear anatomically mature before birth, these glands do not reach full fuctional capacity until puberty.
As emphasized throughout this special issue, the external ear plays an essential role in auditory function and occupies an important place in the clinical practice in audiology and otology. Its major component, the auricle and external auditory canal, receive sound energy from the environtment and provide some degree of directional and frequency selectivity for the incoming sound and stimulus. They also serve to protect the tympanic membrane from mechanical injury and from abrupt changes in temperature and humidity. Various abnormalities affecting the external ear, particularly those involving congenital defects, are best understood from a developmental perspective.
External ear development is a process that begins in embryonic life, progresses through the fetal period to the time of birth, and continoues postnatally until the age of puberty, when the glands of the external canal become fully functional. (As ussually defined, the embryonic phase of human development extends from 2 weeks gestational age up to the seventh or eight week, while the fetal period is the interval from about 8 weeks gestation to term).
The embryonioc pharingeal arch apparatus provide the structural foundation for formation of the external ear. The pharingeal arches are conspicious external feature of the human embryo and are significantly involved in various aspects of head and neck development.
By the end of the forth week of gestation, four well-defined pairs of pharingeal arches are externally visible in the neck region of the human embryo. The first two of these, the mandibular and hyoid arches, are important contributors to external ear development. During the fifth gestational week, nodular swellings of tissue known as the hillocks of his appear on the first and second pharingeal arches. Six such hillocks , three on either side of the first pharingeal cleft, can be distinguished. Most investigators believe that the auricle is formed by growth, differentiation and fusion of these six tissue condensation. During the initial stages of its development, the auricleis located in the general area of the neck, behind the lower jaw, but by the 20th week of gestation it has moved upward to attain its adult location and overal configuration. In a 4-5 years old child, the auricle is about 80 percent adult size. It reaches full adult size by approximately 9 years of age.