Eye Histology

Question 1

what are the two compartments of the eye?

Answer 1

• divided by lens, suspensory ligaments and ciliary body
  1. anterior compartment: filled with aqueous humor (has anterior and posterior chambers seperated by iris)
  2. posterior compartment: filled with vitreous humor

Question 2

what three tissues can eye be divided into?

Answer 2

1. Corneo-scleral layer: (outer fibrous tunic)
   Makes up the sclera in the posterior (5/6) wall of the eye and cornea in anterior of eye
2. Uveal layer (middle vascular tunic)
   - Makes up the choroid in the posterior (5/6) wall of the eye
3. Retinal layer (neural tunic/ inner tunic)

Question 3

corneal - scleral layer

Answer 3

outermost tunica layer

• cornea is transparent
• sclera is opaque and lined by middle/vascular layer that is pigmented and absorbs light
• limbus = zone of transition with cornea and sclera
• this layer is primarily for protection of innerstructures of eye. maintains shape and consistency of the eye.

Question 4

Uveal layer: what does it contain?

Answer 4

• choroid = vascular layer in posterior 2/3 of eye, this layer thickens to be ciliary body in anterior of eye
• ciliary processes, extend inward from ciliary body
• vascular layer continues as the iris anteriorly
• vascular layer is pigmented, reducing reflection of light
• blood vessels travel through this layer.
• the anterior portion contains muscle of ciliary body and dilator constrictor of iris

Question 5

Retinal layer/ neural tunic: what is it composed of?

Answer 5

two layers:

1. outer pigmented layer
2. inner retinal layer
   - the posterior 2/3 of retina is sensitive to light
   - retina contains photoreceptor neurons (rods and cones), conducting neurons (bipolar and ganglion cells), association neurons (horizontal and amacrine cells)
   - axons from retinal ganglion cells pass across the surface of the retina and converge on the papilla, or optic disk, and leave the eye through many openings of the sclera to form the optic n.

Question 6

conjunctiva

Answer 6

• a stratified to columnar epithelium with mucous secreting goblet cells that are supported by a thin lamina propria
• it lines the anterior surface of the eyeball up to the cornea, and the inner surface of the eyelid.

Question 7

tarsal glands

Answer 7

glands located deep to the conjunctiva and under the eyelid

- secrete lipid-containing product that retards evaporation

Question 8

lacrimal glands

Answer 8

produce tears to protect the cornea.

Question 9

what is the sclera?

Answer 9

• opaque, white, posterior 5/6 of the eye
• in first layer
• nearly avascular
• continuous with cornea
• composed of collagen and elastin

Question 10

5 layers of the cornea

Answer 10

• transparent, avascular, but highly innervated
  1. outer epithelium (nonkaratinzed squamous)
  2. Bowman’s membrane (thin basal lamina- noncellular layer)
  3. Substantia Propria (“stroma”, dense collagenous tissue witih sparse keratinocytes)
  4. Descemet’s membrane: thick basal lamina
  5. corneal endothelium (resp. for active transport of fluid out of SP and allowing diffusion of metabolites from aqueous humor)

Question 11

which corneal cell layer has ability to heal?

Answer 11

corneal endothelium

Question 12

where do nn. exist in the cornea?

Answer 12

myelinated nn. can be found in the stroma. after crossing bowman’s layer, nn. become unmyelinated and extend toward the surface.
- this is what makes the cornea so sensitive

Question 13

myopia

Answer 13

= nearsightedness

• light rays are focused in FRONT of the retina
• this occurs b/c cornea is too curved, or lens is too powerful for length of globe

Question 14

hyperopia

Answer 14

“farsightedness”
light rays are focused behind the retina
- lens and cornea are too weak, or length of the globe is too short.

Question 15

astigmatism

Answer 15

due to incorrect shape of the cornea

Question 16

LASIK

Answer 16

“laser assisted in-situ keratomileusis”

• for both myopia and hyperopia
• procedure:
• a corneal flap is raised using a microkeratome
• A UV or “cool laser” is used to ablate a precise amount of the exposed corneal stroma
• the flap is irrigated and replaced
• NO sutures are applied (the flat remain in place due to the dehydration pump action of the corneal endothelium)
• corneal epithelial cells migrate from the periphery to the wound

Question 17

choroid…. what are its three layers? what is its main fn?

Answer 17

• found in uveal layer of eye - posterior pigmented portion of the vascular layer
• contains: loose CT, fibroblasts, melanocytes
  1. Bruch’s membrane: seperates choroid from retina
  2. choriocapillaris: middle layer, rich in capillaries, connects with larger capillaries in stroma layer
  3. choroidal stroma: outer layer of choroid, larger vessels, many fibroblasts and melanocytes (helps capture light not taken in by photoreceptors)

Question 18

Drusen

Answer 18

• the accumulation of amyloid material in the inner side of bruch’s membrane, right up against the retina
• large drusens push the photoreceptors away from their blood supply and can cause degerneration.
• this is the earliest sign of age-related macular degeneration

Question 19

Ciliary body: where is it? what does it contain?

Answer 19

= forward continuation of the uveal (vascular layer)

• wedge shaped: located b/w the iris and the vitreous body
• important for production and filtration of aqeous humour!

contains:
- loose CT,
- pigmented epithelilum,
- nonpigmented epithelium (filtrate plasma to create aqeous humor),
- ciliary muscle smooth m.
- zonule fibers (radiate from ciliary processes to lens, forms supsensory ligament of the lens)

Question 20

ciliary muscle

Answer 20

helps with acommodation of lens

• located within the ciliary body
• composed of three bundles that open the canal of schlemm
• two stretch the ciliary body
• when ciliary mm. contracts it elongates and moves towards the lens, resulting in less tension placed on zonular fibers and the lens can shorten
• necessary for close vision

Question 21

suspensory ligaments

Answer 21

attached to choroid body

• alters the shape of the lens
• as ciliary mm contract, ciliary body stretches in length, thereby releasing tension on the suspensory ligaments, lens gets thicker and more convex
• necessary for close vision

Question 22

distant vision?

Answer 22

ciliary mm. are relaxed. tension is placed on the suspensory ligament. there is an elongation and flattening of the lens.

Question 23

structure of ciliary channel? whats in aqueous humor?

Answer 23

• aqeuous humor is composed of AA’s, glucose, ascorbic acid, H2O and NaCl
• ciliary channel is located b/w nonpigmented ciliary epithelial cells and pigmented ciliary epithelial cells.
• their apical domains face each other

Question 24

how is aqueous humor produced?

Answer 24

• produced by the ciliary epithelium lining the ciliary processes. Water escapes from the fenestrated capillaries in the stroma of the ciliary body following the active transport of Na and Cl
• fluid from intercelluar spaces and the ciliary channel - a narrow space b/w the apical domains of nonpigmented and pigmented ciliary epithelial cells - reaches the posterior chamber as aqueous humor
• both cell types are linked by desmosomes and gap jns.

Question 25

pigmented vs. nonpigmented ciliary epithelial cells?

Answer 25

- pigmented cells are on the inside, closer to stroma: actively transport substances through them into ciliary channel - nonpigmented are in contact with aqeous humor: actively tranport fluid into the posterior chamber

Question 26

how does aqueous humor circulate?

Answer 26

- Enters the posterior chamber, passes through the papillary aperture between the iris and lens and enters the anterior chamber. - Drains through the trabecular meshwork into the Canal of Schlemm and directly into venous circulation. (canal of schlemm --> aqeuous vv. --> episcleral vv.)

Question 27

ciliary processes:

Answer 27

located on medial surface of ciliary body - composed of epithelial folds with a core rich in fenestrated capillaries - contains pigmented and nonpigmented ciliary epithelial cells - production of aqeous humor

Question 28

why is drainage of aqeous humor important?

Answer 28

controls the intraoccular eye pressure

Question 29

corneal-irideal angle

Answer 29

- area b/w descemets's membrane of the cornea and the anterior surface of the iris - canal of schlemm forms a complete circle here

Question 30

Glaucoma

Answer 30

- an obstruction in drainage of aqeuous humor leads to increased intraocc. eye pressure that damages the retina and causes blindness if not treated. - causes ganglion cell death - produces pain and nausea as typical symptoms

Question 31

open-angle glaucoma

Answer 31

the most common form - occurs when the corneal-irideal angle is open - presents with increased IOP - can be due to increased production or decreased outflow - canal of schlemm may be blocked

Question 32

closed-angle glaucoma

Answer 32

- corneal-irideal angle is obstructed - results when the aqeous humor is unable to reach the trabecular meshwork b/c an inflamm. process of the uvea blocks fluid access to the meshwork. can't reach canal of schlemm - primary: anatomical cause - secondary: inflammation/hemmorrhage or tumor growth

Question 33

trabeculoplasty

Answer 33

surgery that aims to restore aqeous flow via using laser to burn small holes in trabecular meshwork around the limbus

Question 34

Iris

Answer 34

colored, anterior extension of ciliary body - controls pupillary aperture - seperates anterior and posterior chambers - two concentric rings: pupillary and ciliary zones - melanocytes give it its color

Question 35

two concentric rings of iris?

Answer 35

- pupillary zones: nearest the pupil, thickest area | - ciliary zone, nearest the ciliary body and has widest area

Question 36

what makes up the layers of the retina?

Answer 36

1. anterior surface: fibroblasts and pigmented melanocytes embedded in EC matrix - eye pigmentation is determined here via # if melanocytes 2. stroma: well vascularized loose CT with vessels arranged radially 3. contractile elements: myoepithelial cells making up dilator pupillae mm (symp) and smooth m. making up sphincter pupillae m. (PS) 4. posterior surface: pigmented epithelium continuous w/ ciliary body

Question 37

two contractile elements of the retina?

Answer 37

1. dilator pupillae muscle: made by myoeptihelial cells, symp innervation 2. sphincter pupillae: PS: made up of smooth muscle cells

Question 38

Lens, how is it supported?

Answer 38

transparent, biconvex, avascular | - supported by zonule fibers making up suspensory ligaments

Question 39

three components of the lens

Answer 39

1. lens capsule: thick, transparent basal lamina, holds shape, suspensory ligaments attach here. 2. Lens epithelium: single layer of cuboidal cells only present on anterior and lateral surface 3. Lens fibers: hexagonal and terminally differentiated cells, lose nuclei and organelles (more posterior). filled with crystallins (proteins that increase refractory index of lens)

Question 40

cataracts

Answer 40

leading cause of blindness in the world - caused by lens not shedding nonviable cells. lens fibers do not have intracellular mechanism to deal with accumulation of substances. - risk factors: age, smoking, sun, alcohol, DM, malnutrition, inactivity - causes a hazy image to be seen - no pharmaceuticals available, however surgical replacement with synthetic lens can help.