Ear

Outer ear
The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came.

The sound waves enter the auditory canal, a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12 kHz. At the far end of the ear canal is the eardrum (or tympanic membrane), which marks the beginning of the middle ear.

Middle ear
Sound waves traveling through the ear canal will hit the tympanic membrane, or eardrum. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the malleus (hammer), incus (anvil) and stapes (stirrup). These ossicles act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the oval (or elliptical) window. The malleus articulates with the tympanic membrane via the manubrium, where the stapes articulates with the oval window via its footplate. Higher pressure is necessary because the inner ear beyond the oval window contains liquid rather than air. The sound is not amplified uniformly across the ossicular chain. The stapedius muscle reflex of the middle ear muscles helps protect the inner ear from damage. The middle ear still contains the sound information in wave form; it is converted to nerve impulses in the cochlea.

Inner ear
The inner ear consists of the cochlea and several non-auditory structures. The cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure across the basilar membrane separating two of the sections. Strikingly, one section, called the cochlear duct or scala media, contains an extracellular fluid similar in composition to endolymph, which is usually found inside of cells. The organ of Corti is located at this duct, and transforms mechanical waves to electric signals in neurons. The other two sections are known as the scala tympani and the scala vestibuli, these are located within the bony labyrinth which is filled with fluid called perilymph. The chemical difference between the two fluids (endolymph & perilymph) is important for the function of the inner ear.

Regeneration of inner ear hair cells
The drug LY-411575 has been proven to help with regeneration of hair cells in the inner ear by blocking notch signalling.

http://www.dailymail.co.uk/health/article-2259688/Drug-reverse-permanent-deafness-regenerating-hair-cells-inner-ear.html

http://www.scientificamerican.com/article.cfm?id=hear-raising-compound-reg

Organ of Corti
The organ of Corti forms a ribbon of sensory epithelium which runs lengthwise down the entire cochlea. The hair cells of the organ of Corti transform the fluid waves into nerve signals. The journey of a billion nerves begins with this first step; from here further processing leads to a panoply of auditory reactions and sensations.



Hair cell
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at the top, for which they are named. These cilia are the mechanosensors for hearing. Lightly resting atop the longest cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia and allowing electric current into the hair cell.

Hair cells, like the photoreceptors of the eye, show a graded response, instead of the spikes typical of other neurons. These graded potentials are not bound by the “all or none” properties of an action potential.

At this point, one may ask how such a wiggle of a hair bundle triggers a difference in membrane potential. The current model is that cilia are attached to one another by “tip links”, structures which link the tips of one cilium to another. Stretching and compressing the tip links may open an ion channel and produce the receptor potential in the hair cell. Recently it has been shown that cdh23 and pchh15 are the adhesion molecules associated with these tip links. It is thought that a calcium driven motor causes a shortening of these links to regenerate tensions. This regeneration of tension allows for apprehension of prolonged auditory stimulation.

Neurons
Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter glutamate communicates signals from the hair cells to the dendrites of the primary auditory neurons.

There are far fewer inner hair cells in the cochlea than afferent nerve fibers. The neural dendrites belong to neurons of the auditory nerve, which in turn joins the vestibular nerve to form the vestibulocochlear nerve, or cranial nerve number VIII.

Efferent projections from the brain to the cochlea also play a role in the perception of sound. Efferent synapses occur on outer hair cells and on afferent (towards the brain) dendrites under inner hair cells.

Tissue Engineering of the ear


The Vacanti mouse is an example of basic tissue engineering.

The earmouse, as it became known as, was created by Dr. Charles Vacanti, an anesthesiologist at the University of Massachusetts and Dr. Linda Griffith-Cima, an assistant professor of chemical engineering at M.I.T. in 1995. Created to demonstrate a method of fabricating cartilage structures for transplantation into human patients, a resorbable polyester fabric was infiltrated with bovine cartilage cells and implanted under the skin of a hairless mouse. The mouse itself was a commonly used strain of immunocompromised mouse, preventing a transplant rejection.

Cow cartilage cells were grown on a mouse to form the shape of a human ear using a mold yet this created public uproar as people thought it was genetic engineering. Cartilage is synthesized by a specialized cell called a chondrocyte.

Tissue engineering is the use of a combination of cells, engineering and materials methods, and suitable biochemical and physio-chemical factors to improve or replace biological functions.

Cartilage is extensive extracellular matrix material secreted by chondrocytes, and composed mainly of collagen, but also including large, spongy carbohydrate chains (glycose amino glycans) and minor proteins.

In tissue engineering cells are seeded into artificial tissue structures supporting three dimensional tissue formations called scaffolds. These scaffolds must deliver and retain cells and biochemical factors and enable diffusion of vital cell nutrients and expressed products. They must also be biodegradable.

See also:

http://www.earreconstruction.com/tissue.html