Schwalbe's Line

• Just anterior to the

apical portion of the

trabecular meshwork

is a smooth area

called as zone S.

• It varies in width from

50 to150 μm .

This anterior border of

this zone marks

transition from

1. trabecular to corneal

endothelium

2. termination of t he

Descemet’s membrane

3. Insertion of trabecular

meshwork into corneal

stroma.

• The posterior border is demarcated by a discontinuous elevation, called Schwalbe's line, formed by the oblique insertion of uveal trabeculae into limbal stroma.

• Secretory cells, called Schwalbe’s line cells, produce a phospholipid material that facilitates aqueous humor flow through the canalicular system.

Scleral Spur

• It is a fibrous ring projecting

from inner aspect of

anterior sclera which runs

parallel to the limbus

• Attached

anteriorly : trabecular meshwork

posteriorly : sclera and longitudinal fibers of ciliary muscle.

• varicose axons characteristic of mechanoreceptor nerve measure stress in the scleral spur due to ciliary muscle contraction or changes in IOP.

Trabecular Meshwork

Trabecular meshwork is a sieve like structure bridging scleral sulcus and converts it into Schlemm’s canal

It is divided into three portions:

(a) uveal meshwork,

(b) corneoscleral

meshwork, and

(c) juxtacanalicular tissue

Uveal Meshwork

• This inner most portion is adjacent to the aqueous humor in the anterior chamber

• It is arranged in cord or rope like trabeculae that extend from the iris root to the Schwalbe's line .

• The arrangement of the trabecular bands creates irregular openings that vary in size from 25 to 75 μm.

Corneoscleral Meshwork

• This portion extends

from the scleral spur

to the anterior wall

of the scleral sulcus

• It consists of 8-14

sheets of trabeculae

that are

interconnected via

cytoplasmic

processes.

They are perforated by

elliptical openings which

become progressively

smaller as the trabecular

sheets approach

Schlemm's canal .

• These perforations are not aligned and have

a diameter ranging from 5 to 50 μm

• The anterior tendons of the longitudinal ciliary muscle fibers insert on the scleralspur and posterior portion of the corneoscleral meshwork.

• This anatomic arrangement suggests an important mechanical role for the cholinergic innervation of ciliary muscle on trabecular meshwork function

Ultrastructure of Meshwork

• Both the uveal and corneoscleral trabecular bands or sheets are composed of four concentric layers

1. An inner connective tissue core is composed of collagen fibers, with 64nm periodicity. The central core contains collagen types I and III and elastin.

2. Elastic fibers are arranged in a spiraling pattern with periodicity of 100nm.

3. Cortical zone also called as glassy membrane

4. An outer endothelial layer provides a continuous

covering over the trabeculae

Juxtacanalicular Tissue

The outermost portion of the meshwork (adjacent to Schlemm's

canal)

This structure has three layers consisting of a layer of connective

tissue lined on either side by endothelium

The inner trabecular endothelial layer is continuous with the

endothelium of the corneoscleral meshwork

The central connective tissue layer has variable thickness and is

unfenestrated with several layers of parallel, spindle shaped cells

loosely arranged in a connective tissue ground substance.

The outermost portion of the trabecular meshwork is the inner wall

endothelium of Schlemm's canal.

This endothelial layer has significant morphologic characteristics,

which distinguish it from the rest of the endothelium in both the

trabecular meshwork and in Schlemm's canal.

Schlemm's Canal

This 360-degree endothelial-lined channel

It is a single channel but occasionally

branches into a plexus-like system .

The endothelium of the outer

wall is a single cell layer that

is continuous with the inner

wall endothelium but has a

smoother surface with larger,

less numerous cells and no

pores .

The outer wall also differs in

having numerous, large outlet

channels

Lip-like thickenings are present around the openings of

the outlet channels and septa are noted to extend from

these openings to the inner wall of Schlemm's canal,

which help keep the canal open.

Collector channels

• Schlemm's canal is connected to episcleral and conjunctival veins by a complex system of intrascleral channels.

• Two systems of intrascleral channels have been identified:

(a) An indirect system of numerous(15-20), finer channels, which form an intrascleral plexus

before eventually draining into the episcleral venous system and

(b) A direct system of large

caliber vessels, which run a

short intrascleral course and

drain directly into the episcleral

venous system, they are about

6-8 in number and also called

as aqueous.

• However, others refer to the

proximal, or intrascleral,

portion of these vessels as

outflow channels because the

structural pattern of the outer

wall of Schlemm's

canal extends into the first third

of these channels

Episcleral and Conjunctival

Veins

The aqueous vessels join the episcleral and conjunctival

venous systems by several routes.

Most aqueous vessels are directed posteriorly, with most

of these draining into episcleral veins, whereas a few

cross the subconjunctival tissue and drain into

conjunctival veins.

The episcleral veins drain into the cavernous sinus via

the anterior ciliary and superior ophthalmic veins,

while the conjunctival veins drain into superior

ophthalmic or facial veins via the palpebral and angular

veins .

Trabecular Outflow

It is the main outlet (85-95%) for aqueous humor.

Various mechanisms described for aqueous transport

are:

• Vacuolation theory

• Sonderman’s channels

• Contractile microfilaments

• Endothelial pores

Vacuolation theory

Vesicles and large vacuoles are seen in

endothelium.

These vacuoles open and close

intermittently to transport aqueous

This is a pressure dependent passive

transport as no. and size of pores increase

with increase in IOP.

Uveoscleral Outflow

It accounts for 5-15% of total aqueous drainage.

It increases with increase in IOP until IOP is equal to episcleral venous pressure, thereafter it is independent of IOP.

Aqueous humor passes through the root of the iris and interstitial spaces of the ciliary muscle to reach the suprachoroidal space.

From there it passes to episcleral tissue via scleral pores

surrounding ciliary blood vessels and nerves, vessels of optic

nerve membranes, or directly through the collagen substance

of the sclera.

A lower hydrostatic pressure is present in the suprachoroidal

space than in the anterior chamber and this pressure

difference is the driving force for uveoscleral outflow.

The main resistance to uveoscleral outflow is the tone of

ciliary muscle