outer  (fibrous or supporting) layer: sclera and cornea

middle (vascular or uveal) layer: choroid, part of the ciliary body and part of the iris

inner (retinal) layer: retina, part of the ciliary body, and part of the iris ("non-visual retina")

Name and describe the 10 layers of the retina.

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1. inner limiting membrane - Inner portion of Muller’s cells (modified astrocytes) and basal lamina

2. Nerve fiber layer - unmyelinated axons of ganglion cells

3. Ganglion cell layer - large multipolar conducting neurons

4. inner plexiform layer - Bipolar cell axons, ganglion cell dendrites, amacrine cell processes

5. inner nuclear layer - Bipolar, horizontal, amacrine and Muller cell nuclei

6. outer plexiform layer - Rod and cone cell processes, bipolar & horizontal cell processes

7. outer nuclear layer - rod and cone nuclei; where signal transduction occurs

8. outer limiting membrane - junctional complexes of Muller's and photoreceptor cells

9. layer of rods and cones - Inner and outer segments of photoreceptor cells

10. retinal pigment epithelium (RPE) - Simple cuboidal epithelium, melanin producing; phagocytose rod and cone plasma infoldings that contain the photosensitive compounds when shed; tight junctions form blood-retina barrier; process vitamin A and pump ions

CC: Retinal detachment occurs when the rod and cone layer separate from the retinal pigment epithelial layer, depriving the rod and cone layer of nourishment

Describe rods and cones, their function and where they are found. Describe convergence.

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rods and cones are photosensitive cells -found in the posterior part of the retina (periphery = rods only); highly polarized neurons: one pole -> single photosensitive dendrite and at the other -> synapses with cells of the bipolar layer.

Found between the outer limiting membrane and the RPE

inner segment: lots of mitochondria, rER, and glycogen; Modified cilium with basal body

basal body: at constriction of outer and inner segments

outer segment: Site of photosensitivity
--Disks formed from plasma membrane  infolding
--Free floating disks in rods, attached disks in cones
--Disks shed at apex, phagocytosed by RPE

Rods
• Rods > Cones
• Sensitive to low light
• Rhodopsin: an opsin + retinal

Cones

• less cones than rods
• No cones in peripheral retina
• Sensitive to light (red, green, blue)
• Iodopsin: Retinal + one of three cone opsins

Convergence: assoc. of nerve fibers w/ rods and cones;

--at anterior margin of the retina, a number of rods synapse with a single bipolar neuron; a number of neurons then synpase with a single ganglion cell.

retinal blood supply: 2 sources

1. central retinal a/v via optic nerve and spread out over its inner surface

- avoid the macula lutea

- penetrate only the inner retina

- Capilaries in the inner nuclear layer

- venules drain the inner plexiform layer

- arterioles run in the nere fiber layer

- VESSEL-FREE: outer plexiform and outer nuclear

2. capilaries in the choroid (by diffusion) - nourish rods and cones

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Describe the components of the choroid. Explain why the choriocapillary layer is particularly important.

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choroid - highly vascular tunic in the post. 2/3 of the eye, with loose CT righ in collagen and elastic fibers, fibroblasts, melanocytes, macrophages, lymphocytes, mast cells, and plasma cells.

4 layers

1. suprachoroid (epichoroid): elastic fibers and melanocytes, outer part that binds to the sclera

2. vessel layer: large vessels

3. capillary layer (choriocapillary): flattened vascular channels, lined with fenestrated endothelium. important for nutrition and normal maintenance for the outer retinal areas (rods, cones, pigment epithelium)

4. Bruch's membrane: includes the basal lamina of the pigment epithelium, collagen, and elastic fibers;

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Describe the general relationships of the ciliary body and iris. Describe the morphology of these structures.

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Ciliary body:  anterior expansion of the choroid at the level of the lens, thickened ring of tissue lying just inside the anterior portion of the sclera; stroma has loose CT, rich in microvasculature, elastic fibers, and melanocytes, all surrounding smooth muscle

a. middle coat of eye (vascular layer): contributes ciliary muscle (smooth), which is firmly anchored anteriorly to the sclera, posteriorly free to pull the choroid

b. double layer of cuboidal epithelium covers the ciliary body. epithelium is continuous with the inner layer of the eye (retinal layer)

c. cilaiary process epithelium makes aqueous humor and the zonules fibers, which have plasma membrane infolding, Na-K ATPases to produce the aqueous humor. Tight junctions between the ciliary epithelial cells form the blood-aqueous barrier.

Iris: the most anterior extension of the middle layer, partially covering the lens, leaving a round opening (pupil)

a. middle coat - consists of stroma, spiraled blood vessels, and melanoctyes

i. number of melanocytes determine the eye color. Blue eyes have few melanocytes, while brown eyes have numerous melanocytes.

b. retinal components - dilator and sphincter muscles are smooth muscle, derived from pigment epithelium.

i. double layer of epithelium on post. surface of the iris is continuous with the double layer of the ciliary body epithelium

ii. dilator pupillae: thin sheet of smooth muscle, radially oriented on posterior of the iris, derived from pigment epithelium. Sympathetic innervation; circular band at pupil margin

iii. sphincter: parasympathetic innervation

c. anterior surface - lacks epithelium!!!, but has stromal cells facing the anterior chamber

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Close vision: muscle tenses, lens relaxes -> lens assumes globular shape; sphincter of iris contracts

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far vision: muscle relaxes, lens tenses;

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Describe the structure and function of the sclera and cornea.

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Sclera - opaque white posterior 5/6 of the external layer; collagen fibers arranged in irregular sheets;

a. lamina cribosa: formed at the exit of the optic nerve

b. sclera is continuous with the meninges covering the optic nerve

c. conjunctiva covers the anterior aspect, which is continuous with the corneal epithelium at the limbus (corneoscleral junction)

i. lines the outer surface of the globe and the inner portion of the eyelid

Cornea - anterior 1/6 of the eye;  transparent (relatively dehydrated), and avascular, free nerve endings; resp. for MAJOR BENDING OF LIGHT

a. corenal epithelium: non-keratinized stratified squamous; regenerates, fine "vermiform" ridges, which help retain layer of tears

b. Bowman's membrane (anterior limiting membrane): basement membrane of corneal epithelium with some condensation of underlying stroma; does not regenerate, but forms a scar

c. stroma or substantia propria: layers of collagen fibrils of uniform size, uniform layers, at rt. angles => transparency; hydration maintained by the ground substance contributes to tranparency and uniform spacing of the collagen fibers;

i. corenal fibroblasts (keraocytes) produce and are within stroma
d. Descemet's membrane (posterior limiting membrane): thick basement membrane of inner (Descemet's) epithelium; tough layer, penetrating objects stop here; does regenerate

e. Corneal (descemet's) endothelium (not vascular endothelium): simple squamous; provides for metabolic exchange between aqueous humor and cornea; pumps that keep epithelium dehydrated;

i. damage to here can result in corneal edmea with decreased vision.

Describe the structure of the lens.

Lens - a trannsparent biconvex structure immediately behind the iris, used to focus light on the retina; avascular, highly elastic (decreases with age)

1. Capsule - completely surrounds lens, very elastic, anchors zonule fibers; thick basal lamina; collagen IV and proteoglycans

2. Anterior epithelium: single layer of cuboidal cells; form new lens fibers at equator; lens slowly inc. in size throughout life;

3. lens fibers - highly elongated, thin flattened structures; lose their nuclei and organelles; gap junctions present; fill cytoplasm with crystalins => strong

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Describe the production, circulation, fate and composition of aqueous humor.

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Aqueous humor

Production: epithelium of the ciliary body

Circulation: fills anterior and posterior chambers -> trabecular meshwork -> corsses endothelium -> canal of Schlemm -> veins

Composition: amino acids, glucose, ascorbic acid, water, Na+, Cl-

a. water escapes from the fenestrated capillaries in the stroma of the ciliary body following the active transport of Na+ and Cl-

b. from the intracellular spaces and the ciliary channel--a narrow space between the apical domains of the non-pigmented and pigmented ciliary epithelial cells---aqeuous humor drains to the post. chamber

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Describe the structure of the optic nerve.

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Optic nerve - made up of the axons of ganglion cells; it is covered with meninges; contains glial cells; becomes myelinated after leaving the eye at the lamina cribrosa;

septa, which distinguish it from peripheral nerve, cause small incomplete angular fascicles to be formed;

round-to-oval glial cell nuclei: no Schwann cells;

a. peripheral nerve has elongated, spindle-shaped nuclei of Schwan cells and fibrocytes

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Describe the structure of the eyelid and the accessory glands of the eye and state their functions.

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I. Eyelid - pliable structures containing skin, muscle, and conjunctiva that protect the eyes. The skin is present only on the external surface; loose and elastic, lacking fat, and has very small hair follicles and fine hair, except at the distal edge of the eyelid (large hair follicles present)

a. Associated with the follicles of eyelashes are sebaceous glands and modified apocrine sweat glands

b. two epithelial surfaces: the outer surface is thin skin, and the inner is conjunctiva

skeletal muscle (orbicularis oculi) and tarsal plate (dense fibroelastic CT for support) and tarsal glands present (oils)

c. eyelashes may be seen

II. Conjunctiva - mucous membrane covering inner portion of eyelid & outer portion of the globe of the eye; mostly columnar, scattered goblet cells; contribute to tear formation

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III. Glands of the eye

Lacrimal glands: serous glands in the upper lateral side of the orbit produce most of the volume of tears

Meibomian glands (Tarsal glands): Sebaceous glands forming a plate/column of glands emptying into the edge of the eyelid; the oily secretions reduce evaporation of tears; help lubricate the ocular surface

Other glands: sebaceous gland of eyelashes, apocrine sweat glands of eyelashes, accessory lacrimal glands; all of these also contribute to tear production

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Fovea centralis - the depressed center of the macula lutea, which consists of cones (highest visual acuity)

macula lutea - surrounds the fovea centralis, a region where all layers of the retina are present and the two plexiform layers are rich in various carotenoids => yellow color

Here, a single cone synapses with a single bipolar neuron which synapses with a single ganglion cell--1:1 innervation (lowest convergence); convergence increases more peripheral to th emacula lutea.

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