EYE

Function: sense of sight

Location: it is suspended in the bony orbital socket by six extraocular muscles that control its movements

Origin:

• Surface ectoderm - lens; epithelium of the cornea, conjunctiva, and lacrimal gland
• Neural ectoderm - vitreous body (partly mesenchyme); epithelium of the retina, iris, and ciliary body; muscle of puppilary sphincter and dilator; optic nerve
• Mesoderm - sclera; stroma of cornea, conjunctiva, ciliary body, iris, and choroid; extraocular muscles; covering of the optic nerve; connective tissue and blood vessels, bony orbit, and vitreous

Anatomic features

• Layers - concentric coats:
  • Corneascleral coat - the outer or fibrous layer that includes the sclera and the cornea
  • Uvea, the middle layer or vascular coat that include the choroid, and the stroma of the ciliary body and iris
  • Retina, the inner layer that includes an outer pigment epithelium, the inner neural retina, and the epithelium of ciliary body and iris
• Chambers:
  • Anterior chamber, occupying the space between the cornea and the iris
  • Posterior chamber, occupying the space between the posterior surface of the iris and the anterior surface of the lens
  • Vitreous space, occupying the space between the posterior surface of the lens and the neural retina
• Refractile (dioptric) media:
  • Cornea, the anterior window of the eye
  • Aqueous humor, the watery fluid located in the anterior and posterior chambers
  • Lens
  • Vitreous body (or humor)

Location: anterior one-sixth of the eye.

Function: the anterior window-like transparent region that has a convexity.

Composition:

• Corneal epithelium - nonkeratinized stratified squamous epithelium with numerous free nerve endings
• Bowman's membrane - homogeneous lamina with collagen fibrils
• Stroma (substantia propria) - about 60 thin lamellae of collagen fibers in orthogonal array and ground substance
• Descement's membrane - thick basal lamina
• Corneal endothelium - provides metabolic exchange between cornea and aqueous humor

SCLERA

Function: the opaque layer that provides attachment for intrinsic muscles of eye

Composition: dense connective tissue

• Episclera - the external layer, is loose connective tissue adjacent to periorbit fat
• Sclera proper (Tenon's capsule), dense network of thick collagen fibers
• Lamina fusca -the inner aspect of the sclera, contains thinner collagen fibers as well as pigment cells and elastic fibers

IRIS

Function: contractile diaphragm anterior to lens with central aperture - pupil. It changes in size in response to light intensity

Composition: layers from anterior to posterior

• Fibroblasts and melanocytes in discontinuous layer
• Arterior stromal sheet (lamella) - a thin avascular layer
• Loose connective tissue with many small blood vessels
• Posterior membrane - discontinuous layer of smooth muscle. Two muscles - the sphincter pupillae and the dilator pupillae are composed by myoepithelial (pigmented) cells
• A double layer of pigmented epithelial cells

CILIARY BODY

Location: between iris and choroid

Composition: the thickening of the choroid with 17-34 grooves

• Ciliary epithelium - two layers of simple columnar epithelium cover the internal surface

• Ciliary muscle

Function: is responsible for lens accommodation.

• Vascular region that extends into the ciliary processes.

• 75 radial ridges or ciliary processes - the extensions from which zonule fibers emerge and extend to the lens

CHOROID

Function: vascular layer that provides nutrients to the retina, pigment scattered and reflected light to minimize glare within the eye

Location: between sclera and retina

Composition:

• Suprachoroid lamina - loose connective tissue with numerous melanocytes, free smooth muscle cells, blood vessels, and lymphatic capillaries

• Choriocapillary layer - loose connective tissue with melanocytes, the fenestrated capillaries, and veins
• Bruch's membrane (lamina vitrea) - thin amorphous hyaline membrane consists of collagen and elastic fibers as well as basal lamina of retinal epithelial cells

RETINA

• Optic papilla (disk) - the site were the optic nerve joints the retina, a blind spot
• Fovea centralis with macula lutea around it - area of greatest visual acuity
• Pigment epithelium

• Absorption of light passing through the retina to prevent reflection
• Isolation of the retinal cells from blood-borne substances
• Participation in restoration of photosensitivity to visual pigments that were dissociated in response to light
• Phagocytosis and disposal of membranous discs from the rods and cones of the photoreceptor cells

• Cells of neural retina:

• Photoreceptors - about 120 million retinal rods with photopigment rhodopsin and about 7 million cones with photopigment iodopsin

Function: they convert light striking the retina into electrical impulses that are transmitted to the brain. Rods provide the "a black and white picture". Corns provide visual image composed of color

Composition:

• Outer segment - the site of photosensitivity
• Connective stalk - a cilium that joins the outer and inner segments
• Inner segment - the metabolic machinery to support the activity of photoreceptors

• Conducting neurons - bipolar and ganglion cells.
• Association and other neurons - horizontal, centrifugal, and amocrine
• Supporting cells - Muller's cells and neuroglial cells

• Layers of neural retina

• Layer of rods and cones - contains the outer and inner segments of photoreceptor cells
• External (outer) limiting membrane - the apical boundary of Muller's cells
• Outer nuclear layer - contains the cell bodies (nuclei) of retinal rods and cones
• Outer plexiform layer - contains the processes of retinal rods and cones and processes of the horizontal, amacrine, and bipolar cells that connect to them
• Inner nuclear layer - contains the cell bodies (nuclei) of horizontal, amacrine, bipolar and Muller's cells
• Inner plexiform layer - contains the processes of horizontal, amacrine, and bipolar cells and processes of ganglion cells that connect to each other
• Ganglion cell layer - contains the cell bodies (nuclei) of ganglion cells
• Layer of optic nerve fibers - contains processes of ganglion cells that lead from the retina to the brain
• Internal (inner) limiting membrane - composed of the basal lamina of Muller's cells

CRYSTALLINE LENS

Location: transparent, avascular, biconvex structure that is suspended between the edges of the ciliary body by zonule (suspensory) ligament

Function: accommodation to bend light rays so that they focus on the retina

Composition:

• Lens capsule - a thick basal lamina of anterior lens cells
• Subcapsular epithelium - a simple cuboidal epithelium presents only on the anterior surface
• Lens fibers - highly elongated, thin, flattened structures without nuclei and other organelles. They are filled with proteins - crystallins

VITREOUS BODY (HUMOR)

Location: the posterior segment of the eye

Function: refractile media

Composition: transparent jelly-like substance containing about 99% water, collagen, and glycosaminoglycans

ACCESSORY STRUCTURES

• Eyelids

• Thin and elastic skin
• Tarsal plate - dense fibrous and elastic tissue
• Meibomian glands and glands of Zeis- sebaceous glands
• Glands of Moll - sweat glands
• Eyelashes - shot, stiff, curved hairs on anterior edge of lid margin

• Conjunctiva - thin, transparent mucous membrane

• Lacrimal (tarsal) gland

• Extraocular (extrinsic) muscles -striated skeletal muscles