Chapter 1 Structure and Function of the External Eye and Cornea

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
    
• 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)
    
• Sensory nerves
  1^st division of CN V
  through supraorbital nerve & its lacrimal, supratrochlear, and infratrochlear branch
  
• Movement
  
  • Orbicularis oculi muscle
    innervated by zygomatic branch of CN VII
    

    	Movement	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
    
• Circulation
  
  • 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
    
• Tarsal plates
  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 conjunctiva
  

  	Origin	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
    
• 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
    
• 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 (type IV collagen, laminin, & other proteins)

• 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
    

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 anteriorly
      

      	RI
      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
      

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
    
• 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 (arcuate or linear)	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
    

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)
    
• 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