34.0 THE EYE
a) Define and use properly the following words: Eye, Tunica fibrosa, Sclera, Conjunctiva,
Cornea, Bowman's membrane, Stroma, Collagen Type I, Descemet's membrane,
Corneal endothelium, Tunica vasculosa, Choroid, Melanocytes, Melanin granules,
Tapetum Lucidum, Bruch’s membrane, Ciliary Body, ora serrata, Inner nonpigmented
epithelium, Outer pigmented area, Ciliary processes, Ciliary muscle, Iris, sphincter and
dilator muscles, iridocorneal angle, cornealscleral trabecular meshwork, Canal of
Schlemm, Tunica neurosa, Retina layers, Retinal pigment epithelium, Rods and cones,
membranous discs, Bipolar cell , Amacrine cell, Horizontal cell, Radial glial (Müller cell),
ganglion cells, Eyelid, Palpebral glands, Nictitating membrane (Third eyelid).
b) Describe and associate basic structure/function for the following: all structures listed
above.
c) Identify by microscopy: Eye, Tunica fibrosa, Sclera, Conjunctiva, Cornea, Bowman's
membrane, Stroma, Descemet's membrane, Corneal endothelium, Tunica vasculosa,
Choroid, Melanocytes, Melanin granules, Bruch’s membrane, Ciliary Body, ora serrata,
Inner nonpigmented epithelium, Outer pigmented area, Ciliary processes, Ciliary muscle,
Iris, sphincter and dilator muscles, iridocorneal angle, cornealscleral trabecular
meshwork, Tunica neurosa, Retina, Retinal pigment epithelium, Photoreceptor cells,
area containing Bipolar cell and Amacrine cell and Horizontal cell and Radial glial (Müller
cell), ganglion cells, Eyelid, Palpebral glands.
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THE EYE
I.
GENERAL INFORMATION
1. Photosensory organ
a. Light passes through cornea, lens, several refractory structures
b. Focused by lens on the retina for transduction of light
c. Light causes chemical changes in the photosensitive cells of the retina, which
trigger nerve impulses that travel to the brain
2. Three tunics
a. Fibrous
i. Tough outer coat
ii. Composed of sclera and cornea
b. Vascular
i. Pigmented and blood vessels
ii. Composed of choroids, ciliary body and iris
c. Neuroepithelial
i. The retina, lining the innermost coat
ii. Photosensitive and small non-photosensitive areas
II.
TUNICA FIBROSA
1. Sclera
a. Thick, collagen and elastic fibers
b. Optic nerve leaves through area cribrosa perforation
2. Conjunctiva
a. transparent mucous membrane that covers the sclera
b. stratified squamous epithelium
c. lines the inner surfaces of the eyelids
d. Comprised of many small blood vessels in connective tissue, with tiny secretory
glands
e. produce tear film that lubricates and protects
f. these blood vessels dilate making the white part of the eye look red
3. Cornea
a. Epithelium
i. nonkeratinized stratified squamous epithelium
ii. rich in sensitive nerve endings
iii. turnover time of seven days
b. Bowman's membrane (subepithelial)
i. thick, specialized basement membrane
ii. ends at the limbus (sclerocorneal junction)
c. Stroma
i. 90% of corneal thickness
ii. collagen fibers (Type I most common) are arranged in thin parallel
lamellae at different angles
iii. precise right angle alignment required for transparency
iv. long, slender fibroblasts present among the collagen fiber bundles
d. Descemet's membrane (posterior limiting lamina)
i. thick basement membrane
ii. Types IV-VIII collagen mostly
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e. Corneal endothelium
i. simple squamous endothelium
ii. secretion of Descemet’s membrane
iii. Na+ transport; water follows passively; maintains stroma hydration
III.
TUNICA VASCULOSA
1. Choroid
a. Between retina and sclera
b. Outer layer
i. Collagen and elastin
ii. Numerous melanocytes
c. Numerous fenestrated capillaries
i. Oxygen and nutrients to outer retina
ii. Next to pigmented epithelium
d. Tapetum Lucidum
i. Connective tissue, mostly collagen
ii. Layers of flat cells
i. Contain stacks of rods, melanosomes, maybe zinc
iii. Function is RELECTION of light back to retina
iv. Increases night vision
v. Gives the eye shine (green in cat for example)
vi. Species differences
i. Humans do not have tapetum
ii. Herbivores (cows, sheep, horses) have thick fibrous layer
e. Bruch’s membrane
i. Basal complex
ii. Barrier with collagen, species vary in thickness
2. Ciliary Body
a. Anterior continuation of choroid, begins with ora serrata
b. Connects with iris
c. Consists of epithelium, vascular layer and ciliary muscle
d. Inner nonpigmented epithelium
i. Basement membrane toward chamber (upside down)
e. Outer pigmented area
i. Continuous with retinal pigmented epithelium
ii. Supported by basement membrane of Bruch’s
f. Ciliary processes
i. Projects into the posterior chamber
ii. Production of aqueous humor
iii. Attach lens by fibers
iv. Outer free edge outlines pupil
g. Ciliary muscle
i. Smooth muscle
ii. Attached to elastic tendons of the choroid
3. Iris
a. Separates anterior and posterior chambers
b. Consists of:
i. stroma of pigmented
ii. vascularized connective tissue
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iii. sphincter and dilator muscles
i. determine shape of pupil; slit (cat) or circle (dog)
iv. posterior epithelium pigmented
c. pigmentation determines eye color
d. iridocorneal angle
i. drainage of aqueous humor
ii. venous drainage important
iii. cornealscleral trabecular meshwork
4. Species differences
a. Canal of Schlemm only found in human
b. Domestic cats have vertical slit pupil. Why?
i. greater and more accurate control of how much light enters their eyes
ii. did the domestic cat and lion evolve from the same ancestor?
i. If so, why does the lion hunt during the day and have circular pupil
like humans?
IV.
TUNICA NEUROSA
1. Retina
a. 10 distinct layers (from outside, adjacent to the choroid, to inside)
i. retinal pigment epithelium
ii. layer of rods and cones
iii. outer limiting membrane
iv. outer nuclear layer
v. outer plexiform layer
vi. inner nuclear layer
vii. inner plexiform layer
viii. ganglion cell layer
ix. optic nerve fiber layer and
x. inner limiting membrane
2. Histological markers (start from the choroid area)
a. Retinal pigment epithelium
i. Melanin granules absorb light
ii. Tight junctional complex limit
iii. Rods and cones indent the apical surface
iv. Function
1. Prevent light reflection and bounce
2. Pass nutrients
3. Degrade (phagocytose) and recycle shed outer segments of
photoreceptors from the rod cells
4. esterifies vitamin A derivates in the SER
b. Rods and cones
i. Outer area (toward lens)
1. Axon, synapse; Nucleus; cytoplasm with mitochondria, Golgi
ii. Inner area (near pigmented epithelium
1. Extension of cytoplasm via a cilium
2. Contains photoreceptors (expansion of the cilium)
iii. Photoreceptor segments- Rods
1. Stacks of membranous discs, free floating discs enclosed within
the plasmalemma
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iv.
v.
vi.
vii.
2. Discs are continuously shed at the tip and phagocytized by the
pigmented epithelium at lights on.
Photoreceptor segments- Cones
1. Shorter and the photoreceptor is fat and upside down pyramid
2. Stacks of membranous discs that remain part of the plasmalemma
and thus one side is exposed to the outside.
3. Disc segments are phagocytized by the pigmented epithelium at
the time lights go off.
Purpose of placing next to pigmented epithelium
1. to keep from overheating and getting bounced light
2. stacks of membranes
Nuclei ever present layer, stains hematoxylin
Next layer is axon terminals
1. Stains lighter
2. Filled with mitochondria, tubulin, synapse vesicles
3. Also dendrites from nerves
4. This makes sense because must transmit signal to nerve
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c. Inner nuclear layer (modifying nerve cells)
i. Bipolar cell
1. Specific for rods or cones
ii. Amacrine cell
1. Nuclei on the inner side
2. Function
a. Inhibitory + Excitatory
b. Acuity of vision
c. Brightness and contrast
iii. Horizontal cell
1. Nuclei on the outer side (distinct)
2. Function
a. Inhibitory directly on rods and cones
iv. Radial glial (Müller cell)
1. Bisect all layers
2. Provide mechanical support and nutrition
3. Basal lamina forms the inner limiting membrane
d. Ganglion layer
i. Large ganglion cells
ii. Nerve fibers layer (axons)
iii. Intermixed with capillaries
IV. All converge to exit via optic disk to form optic nerve
V.
OTHER COMPONENTS OF VISION
1. Dual blood supply - capillaries of the choroid layer supply the outer one third, and
branches of the retinal artery course over the interior surface and supply the inner two
thirds
2. Muller cells are neuroglial cells that extend between the vitreous body and the inner
segments of the rods and cones, where Muller cells end by forming zonulae adherentes
with the photoreceptor cells represented by the external limiting membrane.
3. Basal laminae of the Muller cells compose the inner limiting membrane.
VI.
EYELID
1. Outer and inner structures
a. Keratinized stratified squamous epithelium
i. Outer skin
b. Stratified squamous to simple columnar epithelium with goblet cells
i. Inner skin
c. Dense connective tissue
i. Tarsus
2. Palpebral glands
a. Tarsal glands
i. Sebaceous
ii. On the margin of the eyelid
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b. Sweat glands
i. Sebaceous
ii. Connect with the eyelashes
VII.
NICTITATING MEMBRANE (Third eyelid)
1. Fold of conjunctiva
2. Lymphatic nodules and glands in the cartilage area
3. Species differences
a. Dog and ruminant
i. Hyaline cartilage
ii. Deep Harderian gland in bovine
b. Horse and pig
i. Elastic cartilage
http://videoforcats.com/catvision.htm
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