Aqueous humor outflow

1. Aqueous humor outflow system Jagdish dukre

2. 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 .

3. This anterior border of this zone marks transition from trabecular to corneal endothelium termination of t he Descemet’s membrane Insertion of trabecular meshwork into corneal stroma.

4. 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.

5. 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.

6. 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

7. 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.

8. 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.

9. 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

10. The anterior tendons of the longitudinal ciliary muscle fibers insert on the scleral spur 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

11. Ultrastructure of Meshwork Both the uveal and corneoscleral trabecular bands or sheets are composed of four concentric layers An inner connective tissue core is composed of collagen fibers, with 64nm periodicity. The central core contains collagen types I and III and elastin. Elastic fibers are arranged in a spiraling pattern with periodicity of 100nm.

12. 3. Cortical zone also called as glassy membrane 4. An outer endothelial layer provides a continuous covering over the trabeculae

13. 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

14. 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.

15. Schlemm's Canal This 360-degree endothelial-lined channel It is a single channel but occasionally branches into a plexus-like system .

16. 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

17. 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.

18. 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

19. (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

20. 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.

21. 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 .

22. 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

23. Vacuolation theory

24. 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.

25. 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.

26. 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

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