10. Aqueous Humor

What delineates the anterior chamber?

Cornea to iris

What delineates the posterior chamber?

Back of iris to vitreal surface

Schlemm’s Canal

Also called sinus venosus sclerae
Located anterior to scleral spur
Provides an exit for AH and nourishment for adjacent tissues
Lined by endothelial cells with incomplete basement membrane

Schlemm’s canal external wall

Next to limbal stroma

Schlemm’s canal internal wall

Adjacent to the juxtacanalicular tissue (JCT)

Juxtacanalicular Tissue (JCT)

region between Schlemm’s canal and the trabecular meshwork
Composed of loose connective tissue matrix; Collagen types I, IV, V, and VI; elastin, laminin, fibronectin, chondroitin sulfate, hyaluronic acid …

Trabecular Meshwork Location

Apex lies at Schwalbe’s line, base lies at scleral spur.
Inner side lines the anterior chamber
Outer side lies against the JCT and close to Schlemm’s canal

Trabecular Meshwork appearance

Has a triangular shape
Has flattened perforated sheets that branch
Lattice work of branching and interconnected meshwork

Anatomical Divisions of the trabecular meshwork

• Corneoscleral meshwork

  • external

  • attaches to scleral spur

  • starts at Schwallbe’s Line at edge of cornea

  • spaces are smaller compared to uveal meshwork

• Uveal meshwork

  • internal (closer to anterior chamber)

  • attaches to the ciliary stroma and muscle

  • Holes are 2x larger than corneoscleral meshwork

Composition of trabecular meshwork

• Inner cores of extracellular matrix (collagen, aggregates of elastic tissue, ground substance)

• Thin basement membrane

• Covered by TM endotheilum (continuous with corena endothelium)

  • gap junctions and some tight junctions

What occupies the spaces in the meshwork?

Proteoglycans

Scleral Spur

• Made of dense collagen fibers

• an annular ridge

• an attachment site

What happens to AH flow when the ciliary muscle contracts?

Decrease in AH flow

Pathways for AH production:

• One delivers plasma-derived proteins directly to the posterior chamber

• One delivers plasma-derived proteins directly to the anterior chamber

Aqueous production occurs via:

• Diffusion = movement of molecules along a gradient from higher to lower concentration
  Facilitates movement of small, uncharged molecules

• Ultrafiltration = movement of fluid along a hydrostatic pressure gradient from higher to lower pressure

• Active transport = movement of ions and molecules across cell membranes against opposing concentration or electrical gradients. Water moves via osmosis.

What is AH secreted by?

The epithelium of the ciliary process. The molecules pass from the vasculature through the pigmented and nonpigmented epithelium

Role of Iris epithelium in AH flow

Forces the AH to go around the lens and exit through the pupil

Classic Theory Aqueous flow

Secreted into the posterior chamber and goes through the pupil into the anterior chamber. Convention currents moves ``AH to exit via aqueous outflow structures.

Secondary Path Aqueous Flow

Nutrients enter the AH by secretion from the ciliary body stroma vasculature → iris stroma → anterior chamber
(red arrows)

AH exit from anterior chamber

1. Uveoscleral Outflow- unconventional outflow (20%)

2. Trabecular Outflow - conventional outflow (80%); “passive” passage through the meshwork

Uveoscleral Outflow Pathway

Absorbed across the uveal meshwork → absorbed into the face of the ciliary body and iris root → absorbed into the ciliary muscle veins

Trabecular Outflow Pathway

Uveal meshwork → corneoscleral meshwork → JCT → Giant Vacuole → Schlemm’s canal

What forms in Schlemm’s canal to move AH across the inner wall?

Giant vacuoles

External collector channels move AH from __1.__ to __2.__

1. Schlemm’s canal

2. Deep Scleral plexus

Internal collector channels are:

evaginations of Schlemm’s canal and are long and branching

Deep Scleral plexus

Encircles the limbus
Fed into by External collector channels and empties into interascleral plexus

Intrascleral plexus move AH from __1.__ to __2.__

1. deep scleral plexus

2. episcleral plexus

Episcleral plexus move AH from __1.__ to __2.__

1. intrascleral plexus

2. episcleral veins AND conjunctival plexus of veins

What parts of the Aqueous outflow carries blood?

• intrascleral plexus

• episcleral plexus

• episcleral veins

Aqueous veins (of Ascher) move AH from __1.__ to __2.__

1. Schlemm’scanal

2. Episcleral plexus

Functions of AH

• Nutrition

• waste removal

• maintenance of IOP

• optical

• protection

Blood:aqueous barrier

• enables the AH to have a lower protein concentration relative to the blood

• exists mainly because of the cellular junctions between the ciliary epithelial cells and the non-fenestrated blood vessels in of the iris

Glaucoma

Optic neuropathy that is typically characterized by optic nerve damage

Classifications of glaucoma

• open vs closed angle glaucoma

• normotensive glaucoma

• primary vs secondary glaucoma