Cornea

what is a cornea

Cornea is the transparent structure overlaying the iris

which structure is continuous with the cornea

sclera

how much light is transmitted by the cornea

95%

what are some properties of the cornea

protective , avascular, highly sensitive

which tunic is the cornea part of

outer fibrous tunic

why does vision become blury underwater

the refractive index of water is higher than the refractive index of air

the anterior cornea is

• Aspherical

• Ellipsoid

• Spheroid

• Prolat

how wide is the adult cornea

12 mm horizontal 11mm vertically

how wide is the optical zone of the cornea

4 mm

what is the average thickness of the central cornea

0.53 mm

what is the average thickness of the peripheral cornea

0.71mm

what is the average radius of curvature of the cornea

8mm

what are the layers of the cornea

1. epithelium

2. bowman’s layer

3. stroma

4. descemet’s membrane

5. endothelium

what structure lies above the epithelium

tear film

Bowmans layer is made up of

connective tissue

how much of the cornea is made up of the stroma

95%

what is the function of the epithelium layer of the cornea

barrier function and protection

what are the types of cells in the epithelium

1. squamous cells ( anterior)

2. Wing cells ( middle )

3. basal cells / columnar (posterior)

how many cells thick is the epithelium

5-7 layers thickest in the periphery

what are the two specializations of the epithelium

1. microvilli

2. glycocalyx

what is the structure of the squamous epithelial cells

• Very smooth surface

• Flattened nucleus

• Fewer organelles than deeper cells

• Plasma membrane contains a glycocalyx

• Contains microvilli

what is the structure of wing epithelial cells

• Middle layer

• 2-3 layers thick

• Wing-like lateral processes

• Were basal cells

what are properties of basal epithelial cells d

• A single layer of columnar cells

• Basal surface attaches to basement membrane (secreted by the basal cells)

• Mitotic layer of cells (regeneration occurs here)

how long is the corneal epithelium renewal cycle

7-10 days

what is the second layer of cornea

Bowmans layer

Bowmans layer is knowns as the

transition layer between the epithelium and the stroma

what are some structural features of the Bowmans layer

• Dense, tough fibrous sheet

• Acellular

• Collagen fibers

• Fibers run in random directions

Bowmans layer does not contain any

cells

what is the function of bowmans layer

unknown function likely to support the structure

Bowmans layer does not

regenerate

when is the bowmans layer produced

prenatally

which cells replace the bowmans layer if absent or removed

basal epithelial cells

how much of the cornea is made up of the stroma thickness

90%

which corneal layer maintains corneal transparency

stroma

what is the stroma made up of

• Connective tissue

• Cells

• Keratocytes (corneal fibroblasts)

• Extracellular matrix

• Collagen fibres

• Ground substance

what are fibrils

many linked collagen fibres

what is the diameter of fibril

25-35nm

what are bunches of fibrils called

lamellae

what is the arrangement of lamellae in the stroma

• Distributed throughout stroma

• Lie parallel to corneal surface

• Arranged with regular spacing between them

• Each lamellae extends across the entire cornea

• Interweaving occurs between lamellae (latticework)

• All fibrils separated from others by an equal distance (equidistant space between fibrils)

what is the role of a keratocyte

Synthesise collagen and extracellular matrix components

what other cells are found in the stroma

• White blood cells

• Macrophages

• Lymphocyte

• Leukocyte

what is the purpose of ground substance in the stroma

Fills the area between fibrils

what is ground substance made up of

1. proteoglycans

2. Glycosaminoglycan

what are the types of proteoglycans

• Decorin

• Lumican (Controls the diameter of collagen fibrils within a very narrow range)

• Keratocan

• Mimican

what do proteoglycans do

maintain tensile strength of cornea

what do Glycosaminoglycans do

contribute to high hydration levels

what are the properties of Glycosaminoglycans

• Hydrophilic

• Negatively charges

how many layers of laminae make up the descemets membrane

2

with age the descemets membrane

thickens

the descemets layer regenerates ______

continuously

what is the innermost layer of the cornea

endothelium

The endothelium is made up of

A single layer of squamous cells with microvilli on apical surface

what is the shape of the endothelium

polyhedral mostly hexagonal

what do injured endothelial retinal cells do

cells die, existing cells migrate and spread out to cover the injured area

what are the mechanisms in endothelium cells to remove excess water

High levels of mitochondria (supplies energy)

• Presence of metabolic pumps to remove excess ions and maintain levels within the cornea

• Each cell estimated to have 1.5 X 106 Na+/K+ ATPase pumps

• Cells can adapt the number of pumps based on changes in endothelial membrane permeability

what are the basic structures of nerve cells

1. axon - conducting

2. axon terminals

3. dendrites

4. myelin sheath - produces schwan cells

5. cell body

what is a sensory neuron

detects internal and external stimuli

what is a motor neuron

controls muscles and glands

what is an interneuron

transmits signals between sensory and motor

how many pairs of cranial nerves emerge from the brain stem

12

what is the main nerve controlling the eyes and the face

trigeminal nerve

what is the pathway of nerves that run to the cornea

trigeminal → ophthalmic (v1) → nasocilary → long and short ciliary nerves → supply to cornea

how many nerve endings are per square millimeter in the cornea

7000

how many nerve endings are there in the cornea

910,000

where are the corneal ciliary nerve fibers running

corneal stroma and lose their myelin sheath

stimulation of corneal nerve fibres triggers

1. blinking

2. lacrimation ( tear formation)