Histology of Ear and Eye

Histology of the Ears and Eyes Dr. Leandro Vila The Ear -

Divided into the outer ear, middle ear and inner ear

Outer ear Pinna – collects soundwaves External auditory canal / meatus Tympanic membrane – lateral portion, aka eardrum Middle ear Air-filled cavity Otitis media – presence of fluid / pus Usually sealed by the medial portion of the tympanic membrane Ossicles – malleus, incus and stapes (smallest bones in the body) Skeletal muscles – stapedius and tensor tympani o Stapedius – CN VII o Tensor tympani – CN V Middle ear is connected to the nasal cavity by means of nasopharynx o In pediatric patients, the Eustachian tube is oriented horizontally to the nose such that when the child lies on his/her side, the mucus may enter the middle ear o When the fluid accumulates, it can push the tympanic membrane and cause it to rupture, leading to otitis media (“luga”) o Destruction of the tympanic membrane will cause conductive hearing loss because soundwaves will no longer be vibrated  difficulty in deciphering soundwaves  hearing impairment o In the adult, the Eustachian tube will be directed posteriorly and downward, reducing the likelihood of developing otitis media o Eustachian tube is usually closed except when yawning and chewing o Yawning and chewing equalizes pressure of the middle ear with the atmospheric pressure especially when ascending mountains or in places with increased altitude and decreased barometric pressure o Yawning and chewing opens the nasopharynx to equalize the barometric pressure or else the pressure will push against the middle ear (sense of fullness in the ear)  may eventually cause the tympanic membrane to rupture Inner ear -

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Sometimes referred to as the labyrinth Made up of snail-like structures (cochlea) Connected to 3 canals (semi-circular canals) by the vestibule A bony structure made up of the cochlea, vestibule and semicircular canals (lateral, posterior and superior canals) o Lateral because it is oriented towards the external ear Inside the cochlea is the cochlear duct Inside the vestibule: utricle and saccule Inside the semicircular canals: semicircular ducts Inner ear is divided into bony and membranous labyrinth o Bony labyrinth  Cochlea, vestibule and semicircular canals  Has a fluid inside called the perilymph, a paracellular fluid – fluids which are transiently available on one particular space but goes back to the blood vessel (another example: CSF)  Perilymph comes from plasma ECF such that their contents are similar  If plasma ECF sodium content is high, perilymph sodium content is also high

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Membranous labyrinth  Cochlear duct, semicircular ducts, utricle and saccule  Fluid inside is known as the endolymph, also a paracellular fluid  But although it also comes from the plasma, the endolymph is modified such that the contents of the endolymph is similar to that of the ICF

Cochlea and Stapes Stapes has a communication with the cochlea (inner ear) Commonly, the stapes will cover an opening of the labyrinth (oval window) The oval window will come in communication with the base of the stapes Below which, there is a round window The cochlea turns 2 and ½ or 2 and ¾ times inward, forming a canal Inside the canal, a duct divides the cochlea into 3 compartments: o Scala vestibuli – superior space, withoval window, in communication with the stapes such that when soundwaves vibrate the tympanic membrane, it will also vibrate the ossicles, incus, stapes and eventually transmit the vibration to the perilymph fluid  Scala vestibuli has a communication with the apex of the cochlea together with the scala tympani and this junction is called the helicotrema  Such that the scala vestibuli and the scala tympani both contain the same fluid (perilymph) o Scala tympani – inferior space, with round window  Perilymph has less Na and Cl than the perilymph in scala vestibuli o Scala media – space in between the scala vestibuli and scala tympani Reissner’s membrane / vestibular membrane – boundary between the scala vestibuli and scala media o Made up of 2 layers of simple squamous epithelium (one each from scala vestibuli and scala media) Basilar membrane – separates scala media and scala tympani o Resting on the basilar membrane are the receptors for hearing (collectively termed as Organ of Corti) These structures are arranged in a way then when soundwaves vibrate, it will be transmitted to the scala tympani and the pressure will exit through the round window If the perilymph is vibrated in the scala tympani, it will also vibrate Reissner’s membrane and the basilar membrane Vibrating the basilar membrane will also vibrate the organ of Corti, the sound receptors resting on the basilar membrane Organ of Corti Receptors for hearing The ear is not only for hearing, it is also for balance o During tinnitus, the head has a tendency to tilt towards the origin of the vibration and you have a tendency to fall on the same side where you are having tinnitus o CN VIII is termed as vestibulo-cochlear rather than auditory/cochlear nerve  Remember that the semicircular canals, along with the utricles and saccules are needed also for balance  Vestibular and cochlear nerve unites to form the vestibulo-cochlear nerve (CN VIII)  Cochlear nerve from the cochlea  Vestibular nerve from the vestibule

Receptors for Balance Crista ampullare – in the semicircular canals, functions in balance for moving the head side-to-side and balance during angular acceleration (when making a rapid or sudden turn) o Cupula - a gelatinuous membrane in the crista ampullare Macula – in the saccule and utricle, functions in balance for static and linear acceleration (when riding an elevator) o Macula et saccule – in the saccule o Macula et utricule – in the utricle  Otolitic membrane - a gelatinous membrane found in the macula  Otoconia / otolitic stones – calcium carbonate particles on top of the otolitic membrane Tectorial membrane – gelatinous membrane on top of the hair cells of the organ of Corti o Spiral limbus – structure that secretes the tectorial membrane o Interdental cells – cells that secrete the tectorial membrane o Function of hair cells on tectorial membrane:  3 outer hair cells : 1 inner hair cell  1 inner hair cell contributes up to 90-95% of the stimuli in the spiral ganglion for hearing  Moves laterally or medially depending on the stimuli on the tectorial membrane Supporting cells of the hair cells (receptors): o Pillar cells o Phalangeal cells Endolymph Fluid in the scala media Stria vascularis - a stratified squamous non-keratinized structure that secretes the endolymph o Has a unique electrogenic pump for K so it can pump plenty of K in endolymph of the scala media (important for depolarization) o This is why the endolymph is considered to have the same composition as the ICF (K-rich as opposed to the perilymph which is Na-rich) Hair Cells With dendrite, cell body and axon The axons will all converge to form the spiral ganglion (predominantly inner hair cells) Hair cells on the base are shorter and stouter o Requires high frequency in order to vibrate Hair cells on the apex are thinner and longer o Vibrates even at low vibration and frequency The Eye -

3 layers of the eye: o Tunica fibrosa - fibrous coat (outermost)  Cornea - anterior 5/6 (dense regular CT)  Sclera (dense irregular CT) o Tunica vasculosa - vascular and muscular layer (middle)  Choroid (brownish, vascularized)  Separated from the sclera by the episcleral membrane – Bruch’s membrane  Ciliary body (expanded part anterior to choroid)  Iris (narrow, anterior to ciliary body) o Tunica nervosa - nervous layer (innermost)  Retina – does not reach the lens  Ora serrata – anterior termination of the retina

Parts of the Eye The cornea is consist of 5 layers: o Corneal epithelium (outermost layer)  Stratified squamous non-keratinized epi o Bowman’s membrane  Clear, fibrous layer responsible for the stability and strength of the cornea o Substancia propria  Bulk of the cornea o Descemet’s membrane o Corneal endothelium (innermost layer)  Simple squamous epithelium that maintains the transparency of the cornea Anterior chamber - space behind the cornea Pupil - space between the iris and cornea Posterior chamber – space behind the iris Vitreous space – space behind the lens o Makes up the vitreous body because it is made up of vitreous humour, it maintains the shape of the eyeball Corneoscleral / iridocorneal junction – junction between cornea and sclera o Corneoscleral angle – where the canal of Schlemm is found Canal of Schlemm – drains the aqueous humour Aqueous humour – fluid between the anterior and posterior chambers Ciliary processes – projections that secrete the aqueous humour Uvea / uveal tract – pigmented middle layer (Wikipedia) Suspensory ligament – suspends the lens to the ciliary body Ciliary body – made up of ciliary processes and ciliary muscle which contracts towards the lens to relax it (makes lens thicker) and relaxes towards the lens to constrict it (makes lens thinner) Iris – has pigmented epithelium which contains melanin o Melanin – gives color to the eye, amount of melanin dictates the color of the iris (abundant melanin = black, moderate = brown, few = gray/blue/violet) o Light-colored irises may connote connective tissue diseases (blue eyes is associated with osteogenesis imperfecta) Dilator pupillae – radial muscle near the pigmented epithelium Constrictor pupillae – circular muscle at the center of the pupil Retina – inner layer of the optic cup gives rise to the retina proper, outer layer of the optic cup gives rise to pigment epithelium. The retina has 10 layers: o Pigment epithelium o Photoreceptor layer – layer of rods and cones, dendrites of the photoreceptors o Outer limiting membrane – formed by Muller’s cells (columnar cells) o Outer nuclear layer – formed by nerve cell bodies of the photoreceptor o Outer plexiform layer - synapse between the axon of the photoreceptor and the dendrite of the bipolar cell, horizontal cells are found here (modifies activity of photoreceptors in stimulation to the bipolar cells) o Inner nuclear layer – formed by nerve cell bodies of the bipolar cells, amacrine cells are found here o Inner plexiform layer – synapse between the axon of the bipolar cell and the dendrite of the ganglion cell o Ganglion cell layer – formed by nerve cell bodies of the ganglion cell o Nerve fiber layer – formed by axon of the ganglion cell o Inner limiting membrane – also formed by Muller’s cells

Prepared by: Paolo Warren (Med-IA)