Histology of the Eye & Ear

Key questions

1. Q1Name the three tunics of the eye and the parts of each.

   Outer fibrous (tunica fibrosa): sclera + cornea. Middle vascular (uvea): choroid, ciliary body, iris. Inner nervous: retina (neural + anterior non-neural).
2. Q2How do the sclera and cornea differ?

   Sclera: posterior 5/6, white, dense connective tissue with flat type I collagen bundles, very few vessels. Cornea: anterior 1/6, transparent, 0.5–1 mm thick, avascular.
3. Q3List the five layers of the cornea from outside in.

   Epithelium (stratified squamous, non-keratinized) → Bowman's membrane → stroma (substantia propria, ~90% of thickness, collagen lamellae at right angles) → Descemet's membrane → endothelium (simple squamous).
4. Q4Where is aqueous humor made, and where does it drain?

   Made by the ciliary processes into the posterior chamber; flows through the pupil to the anterior chamber; drains through the trabecular meshwork into the canal of Schlemm at the corneoscleral junction (limbus). Blocked drainage → glaucoma.
5. Q5What is the epithelium of the ciliary processes, and what does it continue as?

   A double layer of low columnar epithelium: one pigmented, one non-pigmented. It continues forward to line the posterior iris and backward to the ora serrata.
6. Q6What determines eye color?

   The amount of melanin in the pigmented epithelium of the iris: less = blue, intermediate = green, more = brown, none (albino) = pink from reflected blood.
7. Q7Which muscle constricts and which dilates the pupil, and what controls each?

   Sphincter pupillae: parasympathetic (CN III via ciliary ganglion). Dilator pupillae: sympathetic (superior cervical ganglion).
8. Q8Which retinal cells form the "straight" pathway, and which modulate it?

   Straight: photoreceptors → bipolar cells → ganglion cells. Modulating: horizontal and amacrine cells. Support: Müller glia and RPE.
9. Q9Rods vs cones: pigment, function and number?

   Rods (100–130 million): rhodopsin; light intensity only; more sensitive, so they work in dim light (night and peripheral vision). Cones (3–6.5 million): iodopsin variants; need bright light; color and visual acuity.
10. Q10What is the difference between the anatomic and visual axes?

    Anatomic axis: center of the pupil to the optic nerve. Visual axis: center of the pupil to the center of the fovea; it divides the eye into nasal and temporal halves.
11. Q11What is special about Descemet's membrane and the corneal endothelium?

    Descemet's membrane is the thickest basement membrane in the body and is made by the endothelium. The endothelium keeps aqueous humor out of the stroma, which keeps the cornea transparent.
12. Q12Why can corneas be transplanted so successfully?

    The cornea is avascular, so immune cells rarely reach the graft. High yield.
13. Q13What are the two layers of the choroid, and which diseases involve the uvea?

    Choriocapillaris (capillaries next to Bruch's membrane and the retina) and choroidal stroma. Choroidal melanocytes can give rise to ocular (uveal) melanoma; uveitis is inflammation of the iris, ciliary body and/or choroid.
14. Q14Which layer of the ciliary epithelium is pigmented?

    The outer layer (toward the stroma, continuous with the RPE) is pigmented; the inner layer (facing the posterior chamber, continuous with the neural retina) is non-pigmented.
15. Q15How do the iris muscles look in cross-section?

    Sphincter: rings of smooth muscle near the pupil margin. Dilator: a thin longitudinal (radial) band along the back of the iris.
16. Q16What forms the blood–retina barrier on the choroid side?

    Bruch's membrane plus the retinal pigment epithelium.
17. Q17Where does a retinal detachment split, and why do photoreceptors suffer?

    Between the photoreceptor layer (outer nuclear layer/segments) and the pigmented epithelium. Photoreceptors lose RPE support: the RPE phagocytoses and recycles their shed discs (~every 10 days), and blocking this causes retinal degeneration.
18. Q18Why does the fovea give the sharpest vision?

    Highest density of cones, no blood vessels, and the inner retinal neurons are pushed aside to the foveal rim, so light reaches the cones directly.
19. Q19What is special about the fovea centralis and the optic disc?

    Fovea: thin pit in the macula lutea made only of cones → sharpest vision. Optic disc: where the optic nerve and central retinal artery exit/enter; no photoreceptors → blind spot.

Key questions

1. Q20Name the four parts of the lens from outside in.

   Lens capsule (proteoglycans + type IV collagen) → lens epithelium (simple cuboidal to columnar, anterior) → differentiating lens fibers (making crystallins) → mature lens fibers (no nuclei).
2. Q21What suspends the lens, and what are the fibers made of?

   Zonule fibers (oxytalin fibers) running from the ciliary processes to the lens capsule.
3. Q22Name the three chambers of the eye and what fills each.

   Anterior chamber (cornea ↔ iris) and posterior chamber (iris ↔ lens): aqueous humor. Vitreous chamber (behind the lens): vitreous body.
4. Q23What is a cataract, and how is it treated?

   Clouding (opacity) of the lens, partly from oxidative change to lens proteins — the most common cause of vision loss (especially after 40) and the leading cause of blindness worldwide. Treated by removing the lens and placing an intraocular lens implant.
5. Q24What is the hyaloid canal?

   A channel through the vitreous body left behind when the fetal hyaloid artery degenerates.
6. Q25Nearsighted vs farsighted?

   Nearsighted (myopia): image forms in front of the retina — near objects clear, distant blurred. Farsighted (hyperopia): the opposite — need glasses to read.
7. Q26What layers make up the eyelid, and which glands does it have?

   Skin, orbicularis oculi (skeletal), levator palpebrae (upper lid), tarsal plate (fibroelastic), conjunctiva. Glands: Meibomian (sebaceous, in tarsus), Zeis (sebaceous, on lash follicles), Moll (sweat, into lash follicles).
8. Q27What is the conjunctiva?

   A thin mucous membrane covering the anterior sclera and lining the inner eyelids; stratified columnar to stratified squamous epithelium with goblet cells (mucin).
9. Q28Describe the lacrimal apparatus.

   Lacrimal gland (upper lateral orbit; serous tubuloalveolar acini) → tears across the cornea → lacrimal canaliculi (lower medial) → lacrimal sac → nasolacrimal duct → nasal cavity.
10. Q29Which nerves supply the extraocular muscles?

    CN VI → lateral rectus. CN IV → superior oblique. CN III → all others (superior, inferior, medial rectus, inferior oblique) + levator palpebrae. (LR6 SO4)3.

Key questions

1. Q30What are the three parts of the outer ear?

   Pinna (auricle), external auditory canal (meatus), tympanic membrane.
2. Q31What is the pinna made of?

   Elastic cartilage and connective tissue, tightly covered by skin.
3. Q32What lines the external auditory canal, and which glands are in it?

   Stratified squamous epithelium continuous with skin, with hair follicles, sebaceous glands and ceruminous glands (modified apocrine tubular glands → cerumen/earwax).
4. Q33Describe the three layers of the tympanic membrane.

   Outer: thin epidermis (ectoderm). Middle: fibrous connective tissue (collagen, fibroblasts). Inner: simple cuboidal/squamous epithelium continuous with the middle ear lining (endoderm).
5. Q34How do ceruminous glands differ from sebaceous glands?

   Ceruminous glands are modified tubular (apocrine) sweat glands — larger, deeper and paler on a slide — that make cerumen (protects against bacteria and water). Sebaceous glands empty into hair follicles and make oily sebum (protects against fungi and bacteria).
6. Q35Describe the parts of the tympanic membrane and the cone of light.

   A small, lax pars flaccida at the top and a large, taut pars tensa below (most perforations). The handle of the malleus attaches centrally and ends at the umbo, which produces the cone of light: 5 o'clock in the right ear, 7 o'clock in the left.
7. Q36Which nerve crosses the middle ear, and what happens if it is compressed?

   The chorda tympani (CN VII) passes between the malleus and incus, carrying taste from the anterior 2/3 of the tongue. CN VII runs through the middle ear, so compression there can cause facial palsy and taste loss.
8. Q37What do the stapedius and tensor tympani protect against?

   Stapedius (smallest skeletal muscle, CN VII): loud sounds, including your own voice. Tensor tympani (CN V3): the sound of chewing.
9. Q38What is in the middle ear, and how does it connect to the pharynx and inner ear?

   Air-filled space with the ossicles (malleus, incus, stapes), tensor tympani and stapedius, and the chorda tympani. The auditory (eustachian) tube connects it to the pharynx (opens on swallowing to equalize pressure); the oval and round windows connect it to the inner ear.
10. Q39What do the ossicles and middle ear muscles do?

    Ossicles form a lever system that amplifies sound from the eardrum to the oval window. Tensor tympani (on malleus) and stapedius (on stapes) dampen vibration.
11. Q40Which reptile jaw bones became the malleus and incus?

    Articular → malleus; quadrate → incus (stapes is shared). "Mammals have an ear full of reptile jawbones."
12. Q41Bony vs membranous labyrinth?

    Bony labyrinth: cavities in the temporal bone (vestibule, semicircular canals, cochlea) filled with perilymph. Membranous labyrinth: ducts and sacs inside it (utricle, saccule, semicircular ducts, cochlear duct) filled with endolymph.

Key questions

1. Q42Name the three sensory regions of the inner ear and what each senses.

   Organ of Corti in the cochlear duct (hearing); maculae of the utricle and saccule (linear acceleration, gravity); cristae ampullares of the semicircular canals (rotation).
2. Q43Name the three scalae of the cochlea and their fluids.

   Scala vestibuli (perilymph), scala media / cochlear duct (endolymph, made by the stria vascularis; holds the organ of Corti), scala tympani (perilymph). Vestibuli and tympani meet at the helicotrema.
3. Q44What are the components of the organ of Corti?

   Hair cells with stereocilia sitting on the basilar membrane, supporting cells, and the overlying tectorial membrane; signals go to the spiral ganglion (CN VIII).
4. Q45How does sound become a nerve signal?

   Stapes pushes the oval window → pressure wave up the scala vestibuli, across the scala media / helicotrema, down the scala tympani → round window bulges out. The basilar membrane moves against the tectorial membrane → stereocilia bend → hair cells fire.
5. Q46Describe the macula.

   Found in the utricle and saccule: support cells + two types of hair cells (type I goblet-shaped with an afferent nerve calyx; type II columnar with afferent + efferent endings), each with stereocilia and one kinocilium, under a gelatinous layer topped by calcium carbonate otoconia (otoliths).
6. Q47How does the crista ampullaris differ from the macula?

   It sits in the ampulla of each semicircular canal; its hair cells project into a tall gelatinous cupula with NO otoliths. The cupula spans the ampulla, and endolymph lag during head turns bends it → rotation (angular acceleration).
7. Q48How many rows of hair cells are in the organ of Corti, and what does each type do?

   Four rows: one row of inner hair cells (send sound information to the spiral ganglion) and three rows of outer hair cells (amplify).
8. Q49Where along the cochlea are high and low pitches detected?

   High frequencies at the base; low frequencies at the apex.
9. Q50What happens when stereocilia bend toward vs away from the kinocilium?

   Toward: tip links pull K⁺ channels open → K⁺ from endolymph flows in → depolarization → Ca²⁺ entry → neurotransmitter release onto CN VIII. Away: tip links slacken → channels close → hyperpolarization.
10. Q51What does the stria vascularis do?

    Its specialized epithelium secretes endolymph (high K⁺) and supports the survival and nutrition of the organ of Corti.
11. Q52How many semicircular canals are there, and why?

    Three, at right angles to each other, so each detects rotation in a different plane.
12. Q53What links stereocilia together?

    Side (tip) links connect stereocilia in the maculae and cristae; bending toward the kinocilium opens channels → depolarization.
13. Q54Which drugs are ototoxic?

    Aminoglycoside antibiotics (e.g., neomycin) and chemotherapy agents such as cisplatin — they damage the cochlea, auditory nerve and sometimes the vestibular system.