Ocular Biochemistry

Tear Film & Corneal Chemistry - Surface Shield

• Tear Film (3 layers): Protects & nourishes cornea. 📌 Lovely Apple Muffin (Lipid, Aqueous, Mucin).
  • Lipid Layer (Outer): Meibomian glands; ↓ evaporation, smooth optics. Waxy esters, cholesterol.
  • Aqueous Layer (Middle): Lacrimal glands; O₂ supply, antimicrobials (lysozyme, lactoferrin, IgA), pH ~7.4.
  • Mucin Layer (Inner): Goblet cells; ↑ wettability (MUC5AC), traps debris.
• Corneal Chemistry: Maintains transparency & hydration.
  • Stroma: 78% water; Type I collagen (ordered); Proteoglycans (keratan/chondroitin sulfate) for hydration.
  • Epithelium/Endothelium: Na+/K+ ATPase pumps (endothelium for deturgescence). High glutathione (antioxidant).
  • Glucose metabolism: Aerobic preferred.

⭐ The corneal endothelium's Na+/K+ ATPase pump is crucial for maintaining corneal deturgescence and transparency.

Aqueous & Lens Dynamics - Fluid & Focus

• Aqueous Humor (AH):
  • Production: Ciliary body epithelium. Active secretion (Na+/K+ ATPase, carbonic anhydrase; ~80%) & ultrafiltration.
  • Composition: Plasma-like; ↓ protein, ↑ ascorbate & lactate.
  • Flow: Posterior chamber → pupil → anterior chamber.
  • Drainage:
    • Trabecular (80-95%): Trabecular meshwork → Schlemm's canal. Pressure-dependent.
    • Uveoscleral (5-20%): Ciliary muscle → suprachoroidal space. Pressure-independent.
  • Normal IOP: 10-21 mmHg.
• Lens:
  • Avascular; high protein (crystallins).
  • Metabolism: Anaerobic glycolysis. Sorbitol pathway (aldose reductase) active in hyperglycemia → osmotic damage (diabetic cataract).
  • Transparency: Maintained by crystallin order, glutathione (GSH), Na+/K+ ATPase.
  • Accommodation: Ciliary muscle contraction → zonules relax → ↑ lens curvature.

⭐ The lens has the body's highest protein concentration (~33%), mainly crystallins, crucial for its refractive power and transparency.

Vitreous & Retinal Pixels - Gel & Vision Quest

• Vitreous Humor:
  • Composition: ~99% water, network of Collagen (Type II, IX, XI), Hyaluronic Acid (HA), proteoglycans. Provides viscoelasticity.
  • Functions: Globe integrity, shock absorption, refractive medium.
  • Ageing: Syneresis (liquefaction), PVD (Posterior Vitreous Detachment) → floaters.
• Retinal Phototransduction:
  • Photoreceptors:
    • Rods: Scotopic (low light), Rhodopsin, achromatic. ~120 million.
    • Cones: Photopic (bright light), Photopsins (S, M, L types for color), chromatic. ~6 million.
  • Visual Cycle: Light isomerizes 11-cis-retinal to all-trans-retinal.
    • Regeneration in RPE via RPE65 enzyme.
  • Dark Current: $cGMP$ opens $Na^+$ channels → depolarization.
  • Light Signal Cascade:

⭐ The "Dark Current" (Na+ influx) is unique; light stops this current, leading to hyperpolarization, not depolarization, of photoreceptors.

Ocular Metabolism & Defense - Energy & Guards

• Energy Source: Primarily glucose.
  • Lens & Cornea: Anaerobic glycolysis; HMP shunt for NADPH (antioxidant).
  • Retina: High aerobic metabolism (TCA cycle).
• Metabolic Pathways:
  • Glycolysis: $Glucose \rightarrow ATP + Lactate$.
  • HMP Shunt: Produces NADPH (for glutathione reduction) & ribose-5-P.
  • Sorbitol Pathway: $Glucose \xrightarrow{Aldose Reductase} Sorbitol$; implicated in diabetic cataracts.
• Antioxidant Systems:
  • Enzymatic: SOD, catalase, glutathione peroxidase.
  • Non-enzymatic: Vitamin C (ascorbate), Vitamin E, glutathione (GSH).

  ⭐ Aqueous humor contains ~20x higher Vitamin C (ascorbate) than plasma, offering potent UV/oxidative protection.

High‑Yield Points - ⚡ Biggest Takeaways

• Vitamin A is vital for the visual cycle and rhodopsin regeneration.
• Lens metabolism is mainly anaerobic glycolysis; sorbitol accumulation causes diabetic cataracts.
• Corneal transparency relies on endothelial Na+/K+ ATPase and controlled hydration.
• Aqueous humor formation involves carbonic anhydrase, a key glaucoma drug target.
• The tear film has three layers: lipid, aqueous, and mucin, crucial for ocular surface integrity.
• Key ocular antioxidants include Vitamin C in aqueous humor and glutathione in the lens.
• Dominant collagens: Type I (cornea, sclera), Type II (vitreous), Type IV (lens capsule, Descemet's).