Development of the human external ear

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.

Brain Development in Children

Early childhood is the most and rapid period of development in a human life. The years from conceptions through birth to eight years of age critical to the complete and healty cognitive, emotional and physical growth of children.

A Child's brain develops in response to both genes and the environtment. It is the interaction between the genes and the environtment that really shape the developing brain, a dance between biology and experience.While the gene provide the initial map for development, it is the experiences and relationships babies and children have every day that literally shape their brains. Families have an extremely important ongoing influence on children's development. The community and service environtments in which children and families interact also play a key role in supporting optimal development.

The rapid development of children's brains begin in the prenatal stage and continous after birth. Although cell formation is virtually complete before birth, a new born baby has about a 100 billion brain cells, brain maturation, and important neural pathway and connections are progressively developed after birth in early childhood. Therefore, early childhood is a period in development where environtment  actually has important impact on determaining how the brain and the central nervous system grows and develops, Environtment affects not only the number of the brain cells and the number of connections among them but also the way these connections are "wired". The process of eliminating excess neurons and synapses from the dense, immature brain, which continues well into adolescence, is most dramatic in the early yeras of life, and it is guided to large extent by the child's sensory experience of the outside world. Scientific evidence suggest that if the brain doesn't receive the appropriate stimulation during this criticalwindow, it is very difficult for the brain to rewire itself at a later time.

Genes provide the initial map for brain development, beginning with the basic connections in the brain from birth. Significance wiring occurs during the first years of a child's life and this effectively programs child development. At three, a child has around 1000 trillion brain connections or synapses, which in later development are selectively pruned. When adolescence is reached, brain synapses will number around 500 trillion, and this number remains relatively stable into adulthood, The prunning of brain synapses indicates the tremendous influence experience and environtment play in shaping a young brain. It is the experiences and relationships that infants and young children have that continuosly develop their brains and build the neural circuits that will be the foundation for later development. New research in an area called epigenetics, even suggest that a person's genes can potentially develop in response to some environtmental factors.
Inadequate nutrition before birth and in the first years of life can seriously interfere with brain development and lead to such a neurological and behavioral disorders as learning disabilities and mental retardation. There is considerable evidence showing that infants exposed to good nutritions, and adequate psychosocial stimulation had measurably better brain function at twelve years of age than those raised in less stimulating environtment,

Stress is a feature of the normal development of positive and adaptive coping. Everyday stress responses of a moderate and brief nature can result in mild increases of hormone levels (cortisol) and short-lived increases in heart rate. These kinds of tolerable stress responses help in the development of adaptive coping when buffered by stable and supportive relationships and are an important part of healthy development.

Excessive or long -lasting stress is known as toxic stress and can have negative impact on brain development. Example of toxic stress include: physical or sexual abuse, neglect or lack of affection, parental mental illness, family violence, poverty and lack of adequate housing. Ongoing stress factors that are not buffered bay caring and positive relationships disrupt brain architecture leading to a lower treshold of activation of the stress management system, which in turn can lead to life long problems in learning , behaviour, and both physical and mental health,

Although manageable levels of stress are normal and growth promoting, toxic stress in the early years can damage brain development. It is in situations where ongoing stress is likely, that intervening as early as possible is critical to achieving the best possible outcomes for the child, Caring and positive relationships are essential to ensure stress levels promote resilience for babies and children.

The architecture of the brain (the neural circuits) is built in a hierarchical "bottom-up" sequence. This means the foundation is paramount, as higher level circuits are built on lower level ones. Each newly acquired skill aides in the sequential development of the next, Attaining the more complex and higher order skills becomes  much more difficult when the foundation is shaky. As the foundations are built upon, brain circuits stabilise making them much harder to change and this highlights the importance of getting them right the first time, Positive early experiences result in optimal brain development, which in turn provides the foundation for the other skills and abilities children need  for succes at school and for life.

These are critical periods, or "prime times" for various aspects of brain development. The brain is programmed for events and experiences to happen at particular times for the best wiring and brain development. For example, language developments depends on adequate hearing  and if hearing loss is not diagnosed at an early age and the brain can not receive the sounds that lead to language development, the language part of the brain begin to "close up". The quality of child's earliest environtments and the availability of appropriate expereiences at the right stages of development are crucial to brain development and the foundation for learning in later life,

Brain Development Period

ANTE NATAL:

• All five senses begin to function before birth
• Prenatal sensory experiences actually help shape the brain and nervous system
• Prenatal experiences prime the attachment behaviours of the infant

0-3 YEARS:

• A rapid periode of brain development which can be fostered by relationships with caregivers and supported by optimal community environtments for families and children
• Brain development is vulnerable to toxic stress (depending on length and number of stressors for the child)

BY SCHOOL AGE:

• Children build on the solid foundation of the first five years
• It is more difficult for children to take advantage of the learning environtment of schools if they have not had an optimal home environtment, there is restricted access to quality early childhood sevices and they have experienced a poor quality community environtment

ADOLESCENCE

• Brain development prioritises the connections used most often, Resulting in"pruning" of brain networks or circuits
• As children entered this period, more intensive rsources are required if children have missed the opportunities for optimal caregiving and environtments in the preceeding years

Rethinking The Brain

Old thinking :

• How the brain develops depens on the genes that you were born with
• The experiences that you have before age three have a limited impact on later development
• A secure relationship with primary caregiver creates a favorable context for early development and learning
• Brain development is linier, the brain's capacity to learn and change grows steadily as an infant progesses towards adulthood
• A toddler's brain is much less active than the brain of college student

New thingking :

• How the brain develops hinges on a complex interplay between the genes that you are born wth and the experiences you have
• Early experiences have decisive impact on the architecture of the brain, and on the nature and extent of adult capacities
• Early interactions don't just create a context, they directly affect the way that the brain is wired
• Brain development is non linear, there are prime times for acquiring different kinds of knowledge and skills
• By the time children reach age three, their brains are twice as active as those of adults. Activity levels drop during adolescence

The brain reaches half its mature weght by about six months and 90 percent of its final weight by age eight. This rapid development is reflected in children's capabilities and what they do. Althoughevery children unique, it is widely accepted that development follows a basic pace and pattern of development in all children.

What Children Do at This Age

Birth to 3 months:

• Begin to smile, tract people and objects with their eyes
• Prefer faces and the bright colours
• Turn toward sound
• Discover feet and hands

 

4 to 6 months:

• Smile
• Develop preferences generally to parents and older siblings
• Repeat actions with interesting results
• Listen intently
• Respond when spoken to
• Laugh and gurgle
• Imitate sounds
• Explore hands and feet
• Put objects in mouth
• Sit when propped
• Roll over
• Grasp objects without using thumb

 

7 to 12 months :

• Remember simple events
• Identify themselves and body parts, and familiar voices
• Understand their own name and other common words
• Say first meaningful words
• Explore objects and hidden objects
• Put objects in containers
• Sit alone
• Put themselves up to stand and walk

 

1 to 2 years :

• Imitate adult actions
• Speak and understand words and ideas
• Experiment with objects
• Walk steadily, climb stairs and run
• Recognize ownership of objects
• Develop friendships
• Solve problems
• Show pride in accomplishments
• Begin pretend play

 

2 to 3,5 years :

• Enjoy learning new skills
• Learn language rapidly
• Gain increase control of hands and fingers
• Act more independently

 

3,5 to 5 years :

• Develop a longer attention span
• Talk a lot, ask many questions
• Test physical skills and courage with caution
• Reveal  feeling in dramatic play
• Like to play with friends, don't like to lose, share and take turns sometimes

 

5 to 8 years :

• Gain curiosity about people and how the world works
• Show more interest in numbers, letters, reading and writing
• Gain more convidence and use words to express feelings and cope
• Play cooperatively
• Develop interest in final products

Prenatal Central Nervous System Development

Central Nervous System development plays the central role in the primary argument. That argument is that the prenatal nervous system is particularly vulnerable to environmenttal perturbations because it is rapidly developing during that time period. Futher, many of the learning and behavioral problems that occur in childhood and adolescence have their origins in these prenatal perturbation on central nervous system development.

To help conceptualize fetal central nervous system development, metaphorically link the development of the central nervous system to the construction of a house. In the same way that a blueprint guides house construction, an individual's genome serves as ablueprint for the brain. Some of the DNA in the genome creates proteins that buid structures, while others are"timing genes" that manage the sequencing of the building process, Neurons and glial cells function as the foundational materials of bricks, wood and cement. Axons, dendrites and synaptic connections among neurons serve as the wiring for electricity and the telephone.

The construction of this elaborate communication structure we call the brain is complex, but there are general principles that guide that process. First, while genes provide the blueprint, central nervous system development is a complex process that results from the interplay of genetically governed biological processess and a number of experential,environtmental factors. Second, despite this complex genetic and environtmental interaction, the formation of brain regions occurs according to precisety sequenced schedule with more phylogenetically primitive region (e.g, lymbic system, forebrain) developing before more complex structure (e.g, cerebral cortex). Third, within these regions, brain development is most vulnerable to insult during periods of most rapid growth and development. Thus, the timing of an insult may be more important than the dose or nature of the insult in fluencing the pattern of malformation. The fourth guiding principle is that of all organs in the body, the brain is most vulnerable to teratogenic disruptionbecause of the extended amount of time it requires for development. Fifth, birth does not mark a particular milestone in the development of the brain. The brain continues to develop throughout the lifespan, although the most significant development occurs early in the development during the fetal period and the first years of life.

HEARING AND BALANCE DEVELOPMENT

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

#Malleus and incus --- Pharingeal Arch 1 cartilage neural crest (ectoderm)
#Stapes --- Pharingeal Arch 2 cartilage neural crest (ectoderm)

• Middle ear muscles

#Tensor Tympani ---PharingealArch 1 (mesoderm)
#Stapedius  --- Pharingeal arch 2 (mesoderm)

• Middle ear cavity --- Pharingeal Arch 1 pouch (endoderm)

Inner ear:

• Inner ear labyrinth

#Cochlea --- Otic vesicle ---  Otic placode (ectoderm)
#Semicircular canals --- Otic vesicle ---otic placode (ectoderm)
#Saccule and utricle  --- Otic vesicle --- otic placode (ectoderm)

• Cranial nerve VIII

#Auditory component --- Otic vesicle and neural crest (ectoderm)
#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

Auditory postnatal :

• 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.

Abnormalities

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.

1. Inner ear --- common cavity, severe cochlear hypoplasia
2. 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)
3. 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.

Congenital deafness

1. 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

2. Conductive --- Disease of outer and middle ear

• 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

Fetal  alcohol syndrome
Position - lower or uneven height, "railroad track" appearance, curve at top  part of outer ear is under developed, folded over parallel to curve beneath