The outer eye and cornea in health and disease
Items | Functions |
Orbital rim | Cushioning effect of periocular tissues Local barriers |
Eyebrows | Catch small particles |
Eyelashes (Cilia) | Catch small particles Sensor to stimulate reflex eyelid closure |
Blinking | Augments lacrimal pump |
Tear film | Dilutes toxins and allergens Contains proteins (control normal flora) |
Mucin | Stabilizes tear film Demarcates living cells of ocular surface from environment |
Development of the anterior segment
Gestation age | Events | |
Week | ||
4 | Eye begins to develop Evagination form neuroectoderm | |
5 | Invagination of optic vesicle | double-layered optic cup of neuroectoderm |
Surface ectoderm forms lens placode | Corneal & conjunctival epithelium Eyelid epidermis | |
1st wave of mesenchymal cells from neural crest of surface ectoderm extend under epithelium from limbus | Begin forming corneal endothelium | |
6 | Subsequent wave of mesenchymal cells from neural crest | Begin forming corneal stroma & sclera |
Month | ||
2 | Eyelid fuse Conjunctiva begin to develop within eyelid folds Ocular surface epithelium differentiates | |
3 | Present all corneal components except Bowman's layer | |
4 | Scleral spur forming Appear of Bowman's layer | |
6 | Eyelid begin to open | |
At birth | Infant's globe size: 80% of its adult size | |
< 2y/o | Postnatal sclera & cornea are distensible and become more rigid |
Anatomy
Eyelid
- Unique to vertebrates
Human: highest ratios of interpalpebral fissure width to body mass - Skin
blend into surrounding periorbital skin
0.5mm thick at eyelid margin to 1mm at orbital rim - Hairs
- Vellus hair
- Eyelashes (cilia)
2X numerous upper than lower
replace every 3-5 months
regrow in 2wks when cut; in 2 months if pulled out
- Vellus hair
- Epidermis
similar to other facial skin
abruptly change to nonkeratinized stratified squamous epithelium at mucocutaneous junction of eyelid margin (along row of meibomian gland orifices)- Glands
with holocrine sebaceous gland & eccrine sweat gland
near eyelid margin: apocrine sweat gland (glands of Moll) & sebaceous glands (glands of Zeis)
- Glands
- Sensory nerves
1st division of CN V
through supraorbital nerve & its lacrimal, supratrochlear, and infratrochlear branch - Movement
- Orbicularis oculi muscle
innervated by zygomatic branch of CN VIIMovement
Muscle portion
Close eyelids
Volition
Orbital & preseptal
Subconscious blink
Pretarsal
- Blink
every 3~4 seconds when not concentrating on visual task
eyelid moving toward each other & medially (pumping action within lacrimal drainage system) - Maintain eyelid closure during sleep
active tonus of orbicularis & sustained inhibition of elevator muscles
- Orbicularis oculi muscle
- Blood supply
facial, superficial, temporal, and infraorbital branches of facial a. (external carotid a.)
dorsal, nasal, frontal, supraorbital, and lacrimal branches of ophthalmic a. (internal carotid a.) - Venous drainage
anterior facial & superior temporal v. into jugular v.
ophthalmic v. into cavernous sinus - Lymphatics drain
primarily to preauricular lymph node
medial aspect of eyelid & conjunctiva to submandibular & submental nodes
in deep upper & lower eyelids
provide structural foundation for muscle & orbital septum attaching
Portion | Thick (mm) | Height (mm) |
Upper | 0.75 | 10 |
Lower | 4 |
embed with meibomian glands (with multiple lobules that open into common secretory duct)
- Orbital septum
separate orbicularis muscle from orbital fat
extend from tarsi to periosteum of orbital rim
embed with meibomian
Conjunctiva
- Conjunctival sac
bulbar conjunctiva (fornix on 3 sides & medial semilunar fold) & palpebral conjunctivaOrigin
Functions
Smooth-muscle fibers
levator muscle
Maintain superior fornix
Fibrous slips
Horizontal rectus tendons
Into temporal conjunctiva & plica
Form cul-de-sacs during horizontal gaze
Functions Caruncle Fleshy tissue mass containing hairs & sebaceous glandPalisades of VogtConjunctiva blend at limbus
Series radiating ridges
Contain corneal stem cellsConjunctiva tarsalTightly adherent to underlying tarsusbulbarLoosely adherent to Tenon's capsuleSubstantia propriaLoose connective tissueCell morphology of conjunctiva epitheliumstratified cuboidal (tarsus) ↔ columnar (fornices) ↔ squamous (globe)multiple surface folds
- Goblet cells
up to 10% of basal cell of conjunctival epithelium
most numerous in tarsal & inferonasal bulbar conjunctiva
- Goblet cells
- Conjunctiva-associated lymphoid tissue (CALT)
lymphocytes + leukocytes
especially in fornices
lymphocytes interact with mucosal epithelial cells through reciprocal regulatory signals mediated by growth factors, cytokines, & neuropeptides - Circulation
capillaries fenestrated and leak fluorescein (like choriocapillaris)
Conjunctiva | Blood supply |
Palpebral | Share blood supply with eyelids |
Bulbar | Anterior ciliary a. (ophthalmic a.) |
- Sensory innervation
CN V ophthalmic division: lacrimal, supraorbital, supratrochlear, & infraorbital branches
Cornea
- Transparent, avascular tissue
aspheric, major astigmatism of optical system, 0.25% elasticity & stretches at normal IOP
11-12 mm horizontally; 10-11 mm vertically
refractive index: 1.376 (1.3375 for calibrating keratometer for combied optical power of anterior & posterior curvature of cornea)- Corneal cap
central anterior corneal surface
represent cornea curvature radius with spherocylindrical convex mirror
average radius of curvature: 7.8 mm(6.7-9.4mm)
→ 74% or 43.25D of total 58.6D power of human eye
- Corneal cap
- Nutrition
glucose diffusing from aqueous humor
oxygen diffusing through tear film
peripheral: oxygen from limbal circulation - Sensory
1 of body's highest nerve ending densities
sensitivity 100x of conjunctiva
from long ciliary n. form subepithelial plexus
neurotransmitters: acetylcholine, catecholamines, substance P, calcitonin gene-related peptide, neuropeptide Y, intestinal peptide, galanin, & methionine-enkephalin
Epithelium
- Stratified squamous epithelial cells
5% (0.05mm) of total corneal thickness
form optically smooth surface with tear film
Epithelial cells | Functions |
Superficial | tight junctions prevent penetration of tear fluid into stroma |
Basal | Secrete continuous, 50-nm-thick basement membrane |
- Differentiation
process: 7-14 days
continuous proliferation of perilimbal basal epithelial cells
→ maturated with microvilli coated outermost surface
(dark by SEM and bright by specular microscopy)
→ desquamate into tears
Stroma
- Keratocyte
vary in density & size throughout stroma
form spiraling 3D network - Extracellular matrix
collagens & proteoglycans
type I & V fibrillar collagens interwine with type VI collagen filaments- Proteoglycans
concentrations & ratio vary from anterior to posterior
posterior wetter than anterior (3.85mg H2O/mg dry weight VS 3.04)Associated with
Decorin
Dermatan sulfate
Lumican
Keratin sulfate
- Lamellae
- Proteoglycans
Stroma | Characteristics |
Anterior | Short, narrow sheets with extensive interweaving between layers |
Posterior | Long, wide, thick lamellae extending from limbus to limbus with minimal interlamellar connections |
- Transparency
- Collagen fibrils
embedded in ECM with lattice arrangement
transparent because size of lattice elements smaller wavelength of visible light
act as diffraction grating: reduce light scattering by destructive interference- Scattering
greater anteriorlyRI
Epithelium
1.401
Stroma
1.380
Posteriorly
1.373
- Water content
stroma: 78%
controlled by intact epithelial & endothelial barriers - Endothelial pump
linked to ion-transport system controlled by temperature-dependent enzyme (ex. Na+,K+-ATPase) - Swelling pressure (SP)
negatively charged stromal glycosaminoglycans
imbibitions pressure of stroma: IOP-SP
- Scattering
Endothelium
- Cells
Closely interdigitated, arranged in mosaic pattern of mostly hexagonal shapes
highest density in periphery
not proliferate in vivo, but can divide- Enlargement & spread of neighboring cell to cover defective cell loss area
- Enlargement & spread of neighboring cell to cover defective cell loss area
- Descemet's membrane
basement membrane of corneal endothelium
↑thickness from 3μm at birth to 10-12μm in adults
(endothelium gradually lay down posterior amorphous nonbanded zone)
Biomechanics of the cornea
- Histology
Histology
Biomechanics
Anterior lamellae
Collagen fibrils: stretch from limbus to limbus package in lamellae
Parallel fashion; embedded in ECM of glycoaminoglycans
Slide easily over each other (low shear resistance)
Stroma
Inelastic, anisotropic
Distribute tensile stress unequally throughout thickness, depend on hydration
- Biomechanics
stress: related to IOP (not liner manner)
Cornea state | Stress distribution |
Dehydrated | Principally to posterior layers or uniformly over entire structure |
Healthy or edematous | Anterior lamellae take up strain |
- Experimental moder
inhibit intralamellar stress by cross-linking collange prior to PTK
prevent cornea thickening peripherally to central ablation
Incisional effect
- Local flattening of cornea
depend on direction, depth, location, & number
closer incisions approach visual axis the greater their effect
Incision | Flattening | Effect greater by |
Radial | Meridian of incision & 90⁰ away | Longer incision, (<11-mm diameter); then reverse effect |
Tangential | Meridian of incision Steppening in meridian 90⁰ away (≤magnitude of decrease in primary meridian) [coupling] | Larger angle |
- Incisional surgery
optimum effect: 85-90% deep incision (allow intact posterior lamellae & maximum anterior bowing of other lamellae)
numbers of incisions: by empirically- important variables for radial & astigmatic surgery
patient age, optical zone size, number of incisions, & length of incisions
not significant predictors: IOP & preoperative corneal curvature - effect
depend great on stroma hydration; less on IOP
hyperopic refractive shift: occur when exposed to high or low altitude PO2 (swelling in wound's vicinity)
daily refraction fluctuation: hyperopic in morning; related to stromal hydration
- important variables for radial & astigmatic surgery
Sclera
- Compositions
primarily: type I collagen & proteoglycans (decorin, biglycan, & aggrecan)
elastin & glycoproteins (fibronectin)
fibroblasts lie along collagen bundles - Normally densely white tissue
more translucent: thinning or water content change (<40% or >80%)- Senile scleral plaque
calcium phosphate deposition just anterior to insertions of medial & lateral rectus muscle
dehydrated; blue color (underlying uvea)
- Senile scleral plaque
- Nerve innervations
anterior sclera: long posterior ciliary n.
intrasclera loop (Axenfeld loop) from branch of long posterior ciliary n. sometimes forms visible nodule over ciliary body