Introduction

The ear is a complex sensory organ responsible for hearing and balance. It is divided into three parts and involves an intricate interplay between structures to transmit sound waves into neural impulses read by the brain. This article will provide a brief overview of the parts of the ear, their corresponding histologies, and some critical clinical conditions that can cause hearing loss.

Structure

The ear divides into three distinct parts: the outer ear, middle ear, and inner ear.

The external ear includes the auricle (pinna) and the external ear canal; both have a lining keratinized stratified squamous epithelium.[1] The outer ear functions to direct sound energy and vibrations into the ear canal to the tympanic membrane. The external ear canal subdivides into a lateral portion composed of elastic cartilage (a continuation of the cartilaginous auricle) and a medial part composed of bone—small hairs and specialized apocrine glands that produce cerumen line the inside of the canal. Cerumen is necessary for cleaning and lubrication and has anti-microbial properties. However, too much cerumen or impaction can occlude the auditory canal and worsen hearing, ultimately resulting in conductive hearing loss. The sensory innervation of the external ear is from various nerves, including the greater auricular nerve, lesser occipital nerve, auriculotemporal nerve (branch of the mandibular nerve), and branches of the facial (CN VII) and vagus nerves (CN X). Of clinical importance is the cough reflex that occurs due to stimulation of the auricular branch of the vagus nerve.[2]

The tympanic membrane is a connective tissue structure that separates the external ear from the tympanic cavity. It is contiguous with skin on the outside (keratinized stratified squamous epithelium) and has a mucous membrane (non-keratinized stratified squamous epithelium) inside. The membrane is innervated by cranial nerves V and X laterally and IX medially.[3] 

The middle ear begins from the tympanic membrane and extends to the oval window of the inner ear, with a lining of non-keratinized stratified squamous epithelium.[2] This tympanic cavity is filled with air and is connected to the nasopharynx with the Eustachian tube, meaning the middle ear pressure equalizes with the pharynx. The primary role of the middle ear is to transmit vibration from the tympanic membrane to the inner ear through three small bones, the ossicles. In order from the tympanic membrane to the inner ear are the malleus (attached to the tympanic membrane), the incus, and the stapes (attached to the oval window). There are two clinically essential muscles in the middle ear, the stapedius and the tensor tympani. The stapedius muscle attaches to the stapes and is the smallest muscle in the human body and receives its innervation via the facial nerve (CN VII). The tensor tympani muscle attaches to the malleus, innervated by a branch of the trigeminal nerve's mandibular division. These muscles are necessary for the acoustic reflex, which functions to dampen loud sounds to prevent damage to the organ of Corti in the inner ear. It is important to note that branches of the facial nerve are in the middle ear space, and nerve damage during surgical procedures can lead to ipsilateral facial paralysis.[4]

The inner ear is in the bony labyrinth of the temporal bone and has two distinct parts that serve two separate functions. Within the inner ear exists a cavity filled with semicircular canals which function to sense equilibrium. The cavity in which they lie is called the vestibule and is where the vestibular part of the VIII cranial nerve forms. The cochlea is a spiral cavity containing the organ of Corti that is responsible for converting fluid waves into nerve signals using hair cells that are interpreted by the central nervous system.

Function

Overall the function of the ear is hearing and maintaining balance. Sound waves are transmitted to the tympanic membrane by the external ear, then undergo modulation by the ossicles of the middle ear. They are then carried into the inner ear by the oval window that results in mechanical fluid movement within the inner ear that converts the mechanical energy into electrical energy. The electrical input travels to the brain within the vestibulocochlear nerve (CN VII), and the information registers as sound in the temporal region of the brain. The ear also maintains balance using the semicircular canals within the inner ear system. The three channels that are at right angles with one-another contain fluid that shifts according to head movement causing depolarization of delicate hair cells. These depolarizations essentially convert mechanical energy into electrical energy and are transmitted to the central nervous system via the vestibular branch of the vestibulocochlear nerve.[5]

Microscopically, sensory hair cells serve as mechanoreceptors that are stimulated by inner ear fluid movement. The human ear can hear sound waves with frequencies between 20 hertz and 20 kilohertz.

Tissue Preparation

The process of preparing an ear tissue sample for histologic observation can be different depending on which portion of the ear is under assessment. External and middle ear preparations are similar to the development of any other tissue. Initially, the fabric is fixed with a chemical solution to preserve the sample, followed by processing and embedding in a mold. Finally, the mold is sliced into sections, and the tissue sample is prepared onto a microscopic slide. The prepared slide is then stained based on the preparer's needs, with the most commonly used stain being the hematoxylin and eosin stain.

There have also been advances in techniques to prepare the three-dimensional histological reconstruction of the inner ear and other complex tissue structures. One study embedded the compositions in an epoxy resin, followed by serial grinding and digitization.[6]

Histochemistry and Cytochemistry

The most commonly utilized histological stains for ear research are the hematoxylin and eosin (H&E) stain. This stain contains both acidic and basic dyes, which allows for a general visualization of anatomical structures and different cell types. Regarding the ear, the H&E stain provides for the study of the middle and inner ear structures in pathological models. The dyes can give relevant information about reactive inflammation and structural changes.

The trichrome stain contains three different dyes and is useful for distinguishing structures in samples. This stain can differentiate muscle from connective tissues like collagen and helps analyze reactive changes in response to inflammation.

Immunohistochemistry lets the researcher identify the specific location of proteins in samples. This technique has been used to detect markers for apoptosis, neural tissue, and changes in protein expression in pathologies such as otosclerosis.[7]

Microscopy, Light

When observing a specimen of the ear under light microscopy, several elements are identifiable. A cross-sectional preparation of the external ear will show keratinized stratified squamous epithelium with underlying cartilaginous structure, and cerumen-producing glands may be visible. The middle ear will show non-keratinized stratified squamous epithelium.

The most complex structure seen with a microscope will be the inner ear. The bony labyrinth, membranous labyrinth, kinocilium, stereocilia, semicircular canals, cochlea, and the organ of Corti may undergo a histological examination. A simple cross-section of the cochlea will show two fluid-filled channels, the spiral ganglion, the organ of Corti, and bone. Highly specialized inner ear cells include hair cells, pillar cells, Boettcher cells, Claudius cells, spiral ganglion neurons, and Deiters cells (phalangeal cells).[8]

The external ear has a pinna, external auditory canal, cerumen, and tympanic cavity. The pinna is keratinized, stratified squamous epithelium skin on it. It is made of subcutaneous fibroconnective tissue, elastic fibrocartilage, and fat.[9]

The external auditory canal also has keratinized stratified squamous epithelium that covers the external tympanic membrane as well. Cerumen is wax produced by the sebaceous glands.

The middle ear has a Eustachian tube, which has a ciliated columnar epithelium, and it changes to pseudostratified, ciliated columnar with goblet cells closer to the pharynx. The tympanic membrane is lined by a flat, single layer of cuboidal epithelium.[10]

The inner ear has a columnar epithelium overlying vascular stroma in the vestibular labyrinth.

Microscopy, Electron

Electron microscopy allows for the visualization of objects at the levels of 10^-10m that are not possible with other techniques. Scanning electron microscopy has been used to study the topographical details of structures. This technique has allowed researchers to get panoramic views of the cochlea and enable studies in healthy and diseased inner ear organs. Cholesteatoma debris can be seen using this technique, and toxic damage, permitting the observation of pathological changes.[11]

Pathophysiology

Infections are the most common pathologies seen in patients. It can either occur as otitis externa, otitis media, tympanitis, or labyrinthitis. Otitis externa is mainly due to the disruption of protective barriers leading to infection of the external ear. The two main pathogens that cause it are Pseudomonas aeruginosa and staphylococcus aureus. In contrast, otitis media is a middle ear infection. It is mostly seen post upper respiratory tract viral infection. The post-infection edema could obstruct the eustachian tube leading to the accumulation of the secretions in the middle ear. This favors the growth of microbial agents. It can also have a bacterial origin by Streptococcus pneumoniae, Moraxella catarrhalis, and non-typeable Haemophilus influenza.[12]

Labyrinthitis is caused due to viruses such as herpes simplex virus, adenovirus, etc. It causes an inflammation of the vestibular nerve presenting as peripheral vertigo in the patients.

Cholesteatoma is a benign collection of keratinized squamous epithelium in the middle ear. It is caused due to prolonged negative pressures that lead to the retraction of the tympanic membrane or a perforation in the tympanic membrane, which causes the keratinized squamous debris to accumulate.[13]

Meniere's disease is another common disease caused by fluid build-up in the endolymphatic system.[14]

Benign paroxysmal positional vertigo presents as a spinning sensation secondary to calcium debris (canalithiasis) within the semicircular canals disrupting the endolymph movement in the canals.[15]

Clinical Significance

Hearing loss affects up to 1.1 billion people to some degree worldwide, and about half is preventable through public health measures.[16] Preventable methods include immunization, proper prenatal care, avoidance of loud noises, and shunning certain medications. There are also specific syndromes that can affect the ear structure and hinder its function. Some of the causes of hearing loss are listed:

Presbycusis: A progressive loss of ability to hear high-frequency sounds with age.

Noise-induced hearing loss: Loud sounds can damage the stereocilia of the hair cells within the inner ear. Up to 12.5% of children from ages 6 to 19 have permanent hearing loss due to exposure to loud noises.

Genetics: Most hearing loss disorders are inherited conditions passed on in a recessive manner. Some syndromes that include hearing loss are Alport syndrome, neurofibromatosis type 2, and Usher syndrome.

Medications: Some ototoxic medications include loop diuretics, quinine, and macrolide antibiotics.

Otosclerosis: An autosomal dominant condition where the ossicles in the middle ear become fixed and cannot vibrate accordingly. It presents as painless progressive hearing loss.[17]