Histology of Eye and Ear

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Tarsal Plates

• Dense CT making up "skeleton" of eyelid.

Meibomian Glands

• Sebaceous glands within tarsal plates.

Sty

• Inflamed sebaceous glands.

Three Tunics of Eye

Outer Tunica Fibrosa:

• CT forming cornea anteriorly and sclera posteriorly.

Middle uveal or vascular coat:

• Consisting from anterior to posterior of iris, ciliary body and choroid layer.

Retinal Coat:

• Consisting of Pigmented retina extending from posterior surface of iris, ciliary body and whole back of the eyeball in front of the choroid.

• Neural retina covers back of eyeball up to the ciliary body at a juncture called the ora serrata.

Zonules of Zinn

• Support lens.

• Filled with nutritive aqueous humor produced by epithelium of ciliary processes.

Vitreous Body

• Homogenous gelatinous material whose source is unknown.

• Gives shape to the eyeball and helps hold retina in position.

Sclera

• Posterior 5/6 of tunica fibrosa composed of type I collagen with some elastic fibers.

• Avascular with a high water content.

• Is continuous with the dura mater at the optic nerve.

• Extraocular muscles attach to the sclera.

Cornea

• Anterior 1/6 and is clear.

• Provides 2/3 focusing power of eye. Nutrients diffuse from aqueous humor.

Epithelium

• 5-6 layers of SSE non-keratinizing.

• Densely innervated for pain respond by blinking and lacrimation.

• Mitosis replaces cells every 7-10 days.

Bowman's Layer

• Collagen firmly attached to the basal lamina of the epithelium.

• Protects against bacterial invasion.

Lamina/Substantia Propria

• Stromal layer of type I collagen and ground substance.

Descemet's Membrane

• Thick basal lamina of the endothelium which transports water out to maintain clarity.

Suprachoroid Layer

• Adjacent to sclera, loose CT with elastic fibers.

Vessel Layer

• CT stroma with many choroidal arteries and veins. Numerous melanocytes to absorb scattered light.

Choriocapillaris

• Single layer of fenestrated capillaries which nourish surrounding tissues including the retina.

Bruch's Membrane

• Glassy membrane is a network of collagen and elastin between the basal lamina of choriocapillaris and retinal pigment epithelium.

Iris

• Posterior surface is pigmented and anterior is a discontinuous layer of stromal cells.

• Separates anterior and posterior chambers.

Sphincter Pupillae

• Circular band of smooth muscle constricts pupil and under parasympathetic control.

Dilator Pupillae

• Radial smooth muscle dilates pupil and is under sympathetic control (bug eyed in emergency).

Eye color is the result of varying amounts of melanin against the dark pigmented choroid.

Ciliary Body

• Thickening of the choroid that forms a ring around the eye on the inner aspect of the sclera.

• Ciliary body is composed of loose CT and circular and radial smooth muscle and numerous fenestrated capillaries.

• Inner edge has ciliary processes which are attached to the lens by a series of fibers called zonules of Zinn.

Vessels of ciliary body are the source of aqueous humor to the posterior chamber that passes the pupil to the anterior chamber and then to the trabecular meshwork of Fontana and thence to the annular canal of Schlemm located near the attachment of the ciliary body with the sclera and then to venous drainage. Blockage of aqueous drainage can lead to increase of intraocular pressure (glaucoma) and blood flow and result in ischemia and eventually blindness.

Lining of Fontana and Schlemm:

Fontana and Schlemm are lined by simple squamous epithelium (endothelium).

Irdio-Corneal Drainage Angle:

Angle between margin of cornea and iris is the irdio-corneal drainage angle.

Lens

• Derived from surface ectoderm. Lens capsule is composed of collagen IV and proteoglycans.

• Lens is avascular with no CT, instead it is composed entirely of modified epithelial cells, the lens fibers that lose their nucleus.

• Anterior surface is cuboidal epithelium with a lateral germinal zone whose cells divide slowly during life.

• Lens provides 1/3 of focusing power and accommodation by changing its shape.

• Eye at rest, zonules of Zinn stretch lens flatter to adjust for distance vision.

• Presbyopia - age related hardening of lens (reading glasses necessary).

Retina

• Derived from the brain.

• Has 10 layers including the retinal pigment epithelium (retinal pigment epithelium).

• Light must pass through all the layers before encountering the photoreceptors that point away from the incoming light.

Six Cell Types in the Eye:

Photoreceptors:

• Detect light.

Laterally Bipolar cells

• Receive impulses from photoreceptors.

Amacrine cells

• Distribute impulses laterally to ganglion cells.

Ganglion cells

• Axons exit via the optic nerve.

Muller cells

• Are glia-like and extend throughout all the layers.

Retinal Pigment Epithelium

• Cuboidal epithelium with no free surface.

• The basal lamina and tight junctions form the blood-retina barrier.

• Free surface encloses the tips of rods and cones.

• Retinal pigment epithelium nourishes the outer parts of rods and cones and phagocytoses cast off outer parts that are shed.

• Connection between retinal pigment epithelium and retina is not firm and this is where detached retina frequently occurs.

• Leads to death of photoreceptors.

Photoreceptors have inner segment and outer segment of rods and cones with optic disks that are continuously shed. Rods contain rhodopsin ( a vitamin A derivative photopigment) and are sensitive to dim light and do not function in daylight. Cones contain 3 types of photopsin and is sensitive to color vision in daylight.

Cones are prominent in the fovea centralis where there are no blood vessels and the other retinal layers are thinner. Optic disk or papilla is a region where photoreceptors are absent, therefore the blind spot.

The Ear

External ear with auricle (pinnae), external auditory meatus (EAM) and tympanic membrane. EAM lined by apocrine cells called ceruminous glands that produce cerumen (ear wax). Middle ear - tympanic cavity with ear ossicles and Eustachian tube. Inner ear with bony labyrinth and membranous labyrinth.

Middle ear is an air filled box. Tympanic membrane is the lateral wall and bony labyrinth is the medial wall. Connects to mastoid air cells ( a resonating chamber) and Eustachian tube.

Parts of Ear:

1. Tympanic membrane.

2. Malleus.

3. Incus.

4. Stapes.

5. Oval window.

Movements of tympanic membrane are transmitted by the ossicles to the fluid filled chambers of the inner ear at the oval window and is dissipated at the round window.

Inner ear:

• 3 semicircular canals.

Bony labyrinth:

• filled with perilymph.

• Vestibule with oval and round windows as well as spiral cochlear canal.

Membranous Labyrinth:

• Suspended in perilymph of bony labyrinth.

Membranous Labyrinth Mediates:

• Audition (cochlear duct) and vestibular sense (utricle, saccule and semicircular canals.)

Source of Perilymph:

1. Ultrafiltrate of plasma.

2. Derivative of CSF via perilymphatic duct connecting to subarachnoid space.

Inside cochlear canal of bony labyrinth is endolymph filled cochlear duct (cochlea) and is divided into: scala vestibuli (perilymph) scala media (endolymph from stria vascularis) with organ of Corti scala tympani (perilymph).

Vestibular membrane = Reissner's membrane.

Spiral Lamina:

Bony modiolus has a spiral projection is called the spiral lamina.

More Ear Facts

• Inner and outer phalangeal cells support inner and outer hair cells.

• Tectorial membrane has stereocilia of sensory hair cells embedded in it.

Semicircular Canals

• Semicircular canals with expanded ampulla near the junction with the utricle; raised as the crista ampullaris.

• Crista has sustentacular and sensory hair cells with stereocilia.

• Stereocilia are embedded in a gelatinous cupola.

• Semicircular canals mediate head position and rotation.

Utricle and Saccule

Utricle and saccule have sustentacular cells and sensory hair cells. Hair cells are embedded in a glycoprotein otolithic membrane with calcium carbonate otoliths. Provides information on linear acceleration and position of head relative to gravity.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

Medical Images

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