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Notes: Chapter 1 - Cornea and Sclera: Anatomy and Physiology

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1. Introduction

• Cornea + Sclera = outer shell of the eyeball (the ocular tunic); cornea = anterior 1/3.
• Cornea = transparent window, avascular, exposed to environment.
• Sclera = opaque dark box for retinal image formation, covered by conjunctiva/Tenon's capsule.
• Limbus = highly vascularized transition zone; reservoir of pluripotent stem cells.
• Dense collagen is the structural backbone of both, but their microscopic architecture differs - explaining their different optical properties.

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2. Corneal Dimensions and Optics

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3. The 5 Layers of the Cornea ("Every Brave Student Deserves Excellence")

1. Epithelium
2. Bowman's layer
3. Stroma
4. Descemet's membrane
5. Endothelium

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4. Tear Film ("LAM" front to back)

• Thickness ~7 µm; Volume ~6.5 µL; >98% water.

• Dry eye: classified as lipid, aqueous, or mucin deficiency.

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5. Corneal Epithelium

• Superficial cells: flat, 40-60 µm wide, microvilli + microplicae, tight junctions (barrier), desmosomes.
• Wing cells: winglike processes, gap junctions, desmosomes.
• Basal cells: columnar monolayer, only mitotically active layer, hemidesmosomes (attach to BM), gap junctions, glycogen-rich.

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6. Bowman's Layer

• Acellular; ~12 µm thick; present in humans, absent in rodents.
• Random arrangement of collagen types I and III (fiber diameter 20-30 nm).
• Made by stromal keratocytes.
• Does NOT regenerate after injury - but epithelium forms normally even without it (proven by excimer laser ablation).
• Physiological role remains unclear.

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7. Corneal Stroma

• >90% of corneal thickness; ~300 collagen lamellae.
• Keratocytes = only 2-3% of stromal volume (form interconnected network).
• Collagen fiber periodicity: 67 nm; diameter 22.5-35 nm.
• Regular spacing of fibrils = optical transparency (destructive interference of scattered light).
• Scleral stroma = similar composition but irregular fiber arrangement = opacity.
• Key proteoglycans: keratan sulfate (lumican, keratocan) and dermatan sulfate (decorin). These control fibril spacing.
  • Lumican-null mice: larger eyes; keratocan-null mice: smaller eyes.

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8. Descemet's Membrane

• Endothelial basement membrane; thickens throughout life.
• Highly elastic; reflects stromal shape changes.
• Rupture (e.g., birth trauma, forceps injury) → aqueous enters stroma → stromal edema.
• Does NOT regenerate after rupture.
• Fuchs' dystrophy: atypical striated collagen deposition in Descemet's membrane.

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9. Corneal Endothelium

• Single hexagonal cell layer; leaky barrier to aqueous humor.
• Normal hexagonality: 70-80% of cells.
• Cell density decreases with age; no significant proliferation in vivo.
• After injury: cells enlarge (polymegethism) and spread without dividing.
• Pleomorphism = loss of hexagonality (early indicator of damage).
• CV of mean cell area = most sensitive index of endothelial dysfunction.
• Na+ osmotic gradient drives water into stroma (aqueous 143 mEq/L vs stroma 134 mEq/L) - endothelial pump counteracts this.
• Loss of function → corneal edema → bullous keratopathy.

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10. Epithelial Maintenance & Wound Healing

• Source of all corneal epithelial cells; located in basal layer of limbal epithelium.
• Best marker: ABCG2
• LSC deficiency in: aniridia, Stevens-Johnson syndrome, severe alkali burns → conjunctivalization of cornea.

1. Cell migration (sliding) to cover defect - fibronectin/integrin system, provisional matrix.
2. Cell proliferation to restore numbers - growth factor driven.

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11. Key Growth Factors

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12. Corneal Innervation

• Most densely innervated tissue in the body; ophthalmic branch (V1) of CN V.
• Substance P present in trigeminal ganglion cells supplying the cornea.
• Loss of corneal sensation → neurotrophic keratopathy (persistent epithelial defects).
• Causes of reduced sensation: HSV/VZV, diabetes mellitus, topical anesthetic abuse.

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13. Stromal Wound Healing & Complications

• Keratocytes transdifferentiate into myofibroblasts after injury → produce collagen for repair.
• Collagen degradation during infection (3 pathways):
  1. Direct bacterial collagenase (e.g., pseudomonal elastase).
  2. MMP release by keratocytes/neutrophils.
  3. Pseudomonal elastase activates pro-MMPs.
• Corneal neovascularization: driven by VEGF + TGF-β; occurs with trauma, infection, alkali burns, LSC deficiency.

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14. Embryology

• First wave: forms endothelium; second wave: forms stroma.
• Fully mature cornea not present even at 6 months gestation.
• Defective neural crest migration → Peters' anomaly, Axenfeld-Rieger syndrome.

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Quick-Reference Clinical Correlations

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Chapter 2 Notes: The Conjunctiva - Anatomy and Physiology

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1. Introduction - What the Conjunctiva Does

1. Protects soft tissues of the eyelid and orbit
2. Allows extensive eye movement
3. Main site for aqueous and mucous tear components (lipid comes from eyelid sebaceous/meibomian glands)
4. Provides immune surveillance (MALT) and antimicrobial agents

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2. Embryology

• Arises from surface ectoderm + neural crest tissues near the optic vesicle.
• Eyelid formation: 8 weeks (32-37 mm stage) from surface ectoderm folds that fuse.
• Conjunctival epithelium differentiates from cutaneous/corneal epithelium by week 10, definitely by week 12.
• Fornix epithelial budding forms: lacrimal gland (superotemporal) + accessory glands of Wolfring and Krause (fornices).

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3. Anatomy & Key Structures

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4. Epithelium

• Non-keratinized stratified epithelium; 6-9 layers (less orderly than cornea's 5-6).
• Contains goblet cells, Langerhans cells, dendritic melanocytes.

• Possibly uniformly distributed in bulbar + forniceal conjunctiva (most recent evidence)
• OR at the mucocutaneous junction / limbus
• Corneal stem cells are at the limbal basal layer (definite)

• Transmembrane mucins: MUC1, MUC4, MUC16
• Anchor goblet cell MUC5AC to surface; bind immunoglobulins; protect/hydrate/lubricate.

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5. Goblet Cells

• Unicellular mucin glands; ~5-10% of basal cells; likely apocrine secretion.
• Produce MUC5AC (gel-forming mucin; cysteine-rich; primary large soluble mucin in tear film).

• Parasympathetic: acetylcholine + VIP (from sphenopalatine ganglion)
• Receptors: muscarinic M1/M2/M3 + adrenergic receptors
• Sympathetic nerves also surround goblet cells
• Sensory nerves do NOT directly surround goblet cells

• Goblet cell loss = earliest sign of squamous metaplasia
• Vitamin A is essential for conjunctival differentiation

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6. Substantia Propria (Stroma)

• Lymphocytes, plasma cells; part of CALT/MALT.
• Develops postnatally - neonates cannot form follicles (this layer not yet developed).

• Vessels, lymphatics, nerves.

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7. Vascular Supply

• Arterial supply: anterior ciliary arteries (from ophthalmic artery).
• Surface capillaries are fenestrated (allow rapid leakage in inflammation → chemosis).
• Chemosis = plasma protein leakage faster than epithelial cell clearance.
• Palisades of Vogt vessels = more competent, leak less.
• Venous drainage: bulbar veins → episcleral venous plexus → intrascleral plexus.

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8. Lymphatic Drainage

• Medial drainage → submandibular nodes
• Lateral drainage → preauricular nodes

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9. Innervation

• Sensory: CN V1 (ophthalmic) + V2 (maxillary)
• Key neuropeptides:

• In humans: accessory lacrimal glands (Wolfring, Zeiss), Moll's glands, and goblet cells are innervated by VIP-containing fibers.

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10. Normal Conjunctival Flora

• Protected by: lid sweeping + tear lysozyme/lactoferrin + antibody migration.
• Same organisms usually in both eyes; eyelid flora mirrors conjunctival flora (but not vice versa - only ~50% of lid organisms found on conjunctival swab).
• Most common organism: Staphylococcus epidermidis (coagulase-negative staph).

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11. Conjunctiva vs. Cornea (High-Yield Comparison Table)

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12. Wound Healing Concept

• Conjunctival epithelium on a denuded cornea = unstable, trauma-prone, prone to defects.
• Transdifferentiation does NOT occur in humans (current consensus).
• Restoration is by limbal stem cells or ectopic corneal epithelial cells already in the conjunctiva.

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