Cornea Anatomy Flashcards
Corneal Functions*
- Transparent: smooth optical surface, avascular
- Major refractive surface: 2/3 refractive power of eye
- mechanical strength - collagen: protection, maintain ocular contour
- barrier - ocular biodefense system
- Protection: highly innervated, barriers, and mechanical strength
corneal-scleral junction is called ______
aka limbus
richly vascularized transition zone
cornea radius of curvature
anterior surface: 7.8 mm (steep center, flatter peripheral)
post sufrace: 6.5 mm (steeper than anterior)
optical zone: 4.mm
cornea refractive power
~43 D
Cornea diameter?*
anterior surface - oval: 10.6 mm vertical and 11.7 mm horizontal
posterior surface - round: 11.7 mm around
megalocornea and microcornea
megalocornea: cornea diameter > 13 mm
microcornea: cornea diameter < 10 mm
Thickness of cornea*
center average: 544 +/- 34 um
periphery: 670 um
hydration determines thickness of cornea
thickness fluctuates on diurnal basis -> AM thicker
Layers of the cornea*
- Epithelium
- Bowman’s membrane
- Stroma
- Decemet’s membrane
- Endothelium
(mnemonic: erect penises blasting organisms stopped the doctors exam needlessly)
Epithelium*
non-keratinized stratified squamous (4-6 layers thick)
superficial cells exfoliate (7-10 day lifespan)
barrier function and light transmission
Epithelium - squamous layer*
terminally differentiated cells: 2-3 layers thick
no mitosis
less active metabolically: fewer organelles
most superficial (slough off 7-10 days)
epithelium squamous layer surface modifications*
microplicae and microvilli
glycocalyx: interacts with mucin layer of tear film
- maintain hydrophilic properties of epithelium, enhance tear film stability, barrier against pathogens
barrier: tight jxn, desmosomes, gap jxn
- anterior corneal barrier, lost when cell exfoliate
fluorescein dye
evaluate barrier jxn,
focal epithelial defects allow penetration of dye: spk (superficial punctate keratitis)
epithelium - wing cells
extensive interdigitations: desmosomes, gap jxns,
highly innervated
2-3 cell layers
cytokeratins: abundant intermediate filaments
- influences shape of wing cells
epithelium - basal cells*
single layer
desmosomes
mitotically active: originate from stem cells, transient amplifying layer
metabolically active: glycogen granules, metabolic pumps, secretes basal lamina
dendritic cells and lymphocytes
adherence of epithelial cells (hemidesmosome send anchoring fibers to lamina densa of BM) – essential to wound healing
hemidesmosomes, anchoring fibrils (7): penetrate to stroma, anchoring plaques
epithelium - dendritic langerhans cells
antigen-presenting cells:
immune response cells in cornea
numerous in periphery of cornea - “professional”
central cornea - “immature”
epithelial regeneration*
constantly state of turn-over with exfoliating apical cells being replaced by underlying cells in a weekly basis
basal cells are only epithelial cells capable of mitosis
during normal apical cell exfoliation basal cell proliferates and replace lost cells in 7-14 days
loss of basal cells and defective regeneration will lead to corneal scar formation
how is BM connected to bowman’s layer*
2 part process: cell attached to BM and BM to bowman’s to secure epithelium to stroma
anchoring fibrils and anchoring plaques running from lamina densa to bowman’s layer
Bowman’s layer*
anterior limiting lamina
no cells, no orgnization to fibrils, and thin
8-14 um thick
randomly arranged, small diameter collagen: 1,3,5,6
-not ordered into bundles - dense fibrous sheet
acellular - modified condensation of the anterior stroma
secured to stroma -> hard to penetrate and prevents shearing
-provides shears strength (stromal resistance to sublayer sliding)
barrier
striated collagen fibrils from anterior and sometimes deeper stroma become contiguous with bowman’s layer
-anchor bowman’s layer to stroma: strong attachment
bowman’s layer can not be stripped away from cornea
stroma
made of collagen (lamallae), fibroblasts (keratocytes), corneal connective tissue (substantia propia)
stroma lamallae*
sheets of collagen fibers
~200-300 lamellae
unique arrangement of collagen fibrils in adjacent lamellae (more disorganized anteriorly and peripherally)
extend limbus to limbus
tensile strenght
stable shape
transparency importance
collagen: type 1 and type 5 collagen heterodimers
collagen fibers are uniform and regular in diameter
distance between fibers is consistent (fibrils surrounded by EC matrix)
stroma keratocytes*
fibroblasts that sandwich between lamallae
- connected to each other by gap jxn and tight jxn
- form a fxnal synctium: (communicating network) tight junction, gap junction
majority in intralaminar space
sig. degree of mobility when activated: can migrate to wound margins and synthesize collagen GAGs, and MMPs
synthesize intracellular crystallins
some leucocytes also found in stroma
circumcorneal annulus
approaching limbus (1.5 - 2.0 mm wide)
believed to maintain curvature of cornea
collagen fibrils within a lamellae turn and run circumferentially around the cornea
Stroma - non collagenous proteins *
hydrophilic mucopolysaccharide in ground substance
- proteoglycans: protein with GAG side chain
- highly negatively charged
- surround collagen fibrils to create uniform spaces between fibrils
corneal crystallin in keratocytes
stroma - transparency of cornea*
regularity of collagen fibers thickness
regularity of distance between collagen fibers
lamellar arrangement in stroma (any disruption (edema, scarring) loss of transparency)
Stroma anterior 1/3 *
collagen fibrils are thicker than in bowman’s layer, but thinner than posterior stroma
lamellae cross at oblique angles (more disorganized)
lamellae are thin
highest density of keratocytes
contribute to shear resistance
less of a tendency to accumulate water than posterior stroma
stroma posterior 2/3*
collagen fibers thicker
precise arrangement of lamellae (orthogonal)
provides tensile strength
greater tendency to accumulate water more anterior stroma
descemet’s membrane*
basal lamina of endothelium
10-15 um thick
anterior layer (banded): lattice of precisely arranged sheets - does not change in thickness
posterior layer (non-banded): less ordered arrangement, type 4 collagen, laminin, fibronectin, secreted thorughout life, provides strong barrier to infection and perforating injury
connected to endothelium by hemi-desmosomes, but not strongly attached to stroma
endothelium*
monolayer of hexagonal cells (400k - 500k)
metabolically active: ion transport systems, synthesize Descemet’s
no mitosis; does not divide in vivo
thickness: 10um birth; 4uq adult
consumes a lot of energy (2nd to photoreceptors in the eye)
pinocytosis (vesicles)
microtubules
mitochondria
lateral interdigitation
desmosomes, gap jxn
focal tight jxn “leaky” -> allow duffusion of glucose and nutrients from aqueous
overlapping marginal folds
club-like microvilli
cell junctions
zonula occludens: claudins, occludens, actin, protein ZO-1
-transmembrane protein complex determines tightness of jxns; the more the tighter
zonula adherens: cadherin E
macula adherens: attachment plaques, proteins, keratin fibers
gap jxn: cell to cell communication, connexion proteins
endothelium - cell density*
cell density changes with age
birth 5k - 6k cells/mm2
child 3k - 4k cells/mm2
adult 1.4k - 2.5k
critical cell density for normal endothelial fxn: 400 - 700 cell/mm2
density is greatest in periphery then paracentral, and finally central
endothelium - cell density dysfxn*
polymegthism - different sizes
pleomorphism - different shapes
guttata - endothelium thinning over thickened descemet membrane in the center
cornea innervation*
sensory cholinergic innervation
long posterior ciliary nerves responsible for sensory innervation
limbal (pericorneal) plexus - radial arrangement
70-80 large nerve trunks; loss of myelin 1-2 mm within cornea, rich afferent innervation, 300-400x epidermis
cornea innervation - plexi of nerve fibers *
- limbal plexus
- stromal plexus
- sub-epithelial (beneath bowman’s layer) - loss of schwann cell covering -> naked nerve ending -> highly sensitive to stimuli
- sub basal plexus: between basal cell and basement membrane
cornea innervation details*
overlap in innervation
-sensitive to small stimuli, approximate sense of spatial location of stimulus, greatest sensitivity –central cornea (has more free nerve endings)
little innervation in posterior stroma
no innervation in descemet’s membrane and endothelium
respond to mechanical, thermal, and chemical stimuli
- some fibers are stimuli specific but majority are polymodal
- primarily perceived as pain
- neurotrophic fxn
- inflammatory response: sustained pain
sympathetic stimulation may inhibit epithelial wound healing
cornea innervation - neurotrophic fxn*
sensory afferent innervation maintains normal epithelial structure and fxn particularly the epithelium
cornea sensory nerves release:
neuropeptides (substance P), growth factors (insulin like growth factor and epidermal growth factor), induce survival development and fxn of neurons
loss of corneal sensation: destruction of normal corneal integrity – epithelial breakdown
neutrophic keratitis: dec. corneal sensation, persistent epithelial defects, delayed epithelial wound healing, ulcers infections melting perforation
dec. corneal sensation: acoustic neuroma, viral infections - herpes, diabetes mellitus
cornea innervation - reflex blinking
stim. of corneal nerves -> reflex arc
afferent pathway CN 5
efferent pathway CN 7
cornea innervation - reflex tearing
stimultion of corneal nerves -> reflex arc
parasympathetic of lacrimal gland
cornea blood supply*
arteries:
ophthalmic -> muscular -> anterior ciliary -> episcleral -> conjunctival -> superficial marginal plexus breaks to:
peripheral corneal arcades (1mm to cornea)
and
recurrent conjunctival vessels
point: does not do much/not much nutrients/ essentially avascular
