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The Aqueous Outflow System as a Mechanical Pump

Murray A. Johnstone

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This animation illustrates a mechanism for pulsatile aqueous discharge from the eye, a model supported by converging lines of evidence from the laboratory and from clinical observation.

Laboratory Evidence: Evidence from light, scanning and transmission electron microscopy demonstrates a highly compliant, pressure sensitive trabecular meshwork (TM). Evidence from the dissecting and light microscope as well as from SEM, TEM and tracer studies demonstrates valve-like structures spanning Schlemm's canal (SC). Together these structures contain the functional elements of a pump.

Clinical Evidence: Evidence from the operating microscope demonstrates valve-like structures in SC. Gonioscopic evidence demonstrates pulsatile aqueous flow into SC, and from SC into collec
tor channels. Slitlamp examination demonstrates pulsatile aqueous flow from aqueous veins into episcleral veins.

Structural Component Relationships

Trabecular Meshwork: In this model the primary resistance to pressure in the TM is at the inner wall of SC. The entire TM participates in modulating the degree of distention of SC inner wall endothelium in response to IOP fluctuations. (Structural features that permit TM modulation of SC inner wall distention are as follows: Trabecular lamellae lie in parallel sheets and the larger trabecular lamellae near the AC are anchored at their ends to scleral spur and Schwalbe's line. Intertrabecular cell processes anchor the trabecular lamellae to one another. Smaller lamellae nearer SC attach to the larger lamellae primarily by means of these interlamellar cell processes. Juxtacanalicular cells link the TM to SC inner wall endothelium by numerous processes that attach in one direction to the smaller trabecular lamellae and in the other to SC inner wall endothelium. SC inner wall is thus anchored to the entire underlying TM through a three-dimensional force distributing organization of cellular attachments. The anchoring arrangement allows the TM tissues to distend in response to rising IOP and to undergo TM induced elastic recoil/contraction in response to falling IOP)

Schlemm's Canal Valves: Valves arise from the inner wall endothelium of SC, initially attaining a funnel shape that changes to a cylindrical shape as the structures course across the canal. After crossing SC, the valves consistently attach to the external wall of the canal. Both the lumen & walls of the valves experience pressure- induced shape change.

Dynamics of Tissue Movement & Aqueous Flow

IOP-induced Trabecular Meshwork Movement: IOP constantly oscillates with the ocular pulse, blinking and eye movement. As IOP increases, pressure from the AC exerts its effect directly on SC inner wall endothelium. SC endothelium responds by distending into SC lumen reducing the lumen size. When IOP decreases, stored energy in the trabecular lamellae, the juxtacanalicular cells and SC inner wall endothelial cells causes the tissues to return to their prior shape, both enlarging SC and reducing pressure in the canal lumen.

IOP-induced Aqueous Movement Through SC Valves: As IOP rises during systole, the TM distends into SC and the funnel-shaped region of the valves enlarges and fills with aqueous. When IOP falls, the TM rebounds enlarging SC lumen while reducing SC pressure. When resulting pressure in SC surrounding the cylindrical portion of the structures is low, aqueous flows from the funnel into the cylindrical portion of the valves. The next IOP rise forces a bolus of aqueous from the cylindrical portion of the valves into SC. The aqueous valves thus act as a one-way mechanism for mechanical pumping of aqueous.

IOP-induced Aqueous Movement into Collector Channels, Aqueous Veins and Episcleral Veins: As IOP rises, the TM moves outward into SC. Pressure in SC rises as a result of the TM movement into SC. The TM movement and accompanying pressure rise in SC forces aqueous into the collector channels, ultimately causing a pulsatile discharge of aqueous into the aqueous veins and episcleral veins.

Relevant References

1. Ascher, K.W., Aqueous veins. Am. J. Ophth., 1942. 25(January): p. 31.
2. Goldmann, H., Abfluss des Kammerwassers beim Menschen. Ophthalmologica, 1946. 111(Feb.-Mar.): p. 146.
3. Goldmann, H., Weitere Mitteilung über den Abfluss des Kammerwassers beim Menschen. Ophthalmologica, 1946. 112(December): p. 344.
4. Vries, S., De zichtbare Afvoer von het Kamerwater. Amsterdam, Drukkerij Kinsbergen, 1947.
5. Kleinert, H., Der sichtbare Abfluss des Kammerwassers in den epiibulbären Venen. II. Mitteilung. Die pulsiereden Kammerwassergefässe. von Graefes Arch. Ophth., 1952. 152: p. 587.
6. Johnstone, M.A. and W.M. Grant, Pressure-dependent changes in structure of the aqueous outflow system in human and monkey eyes. Am. J. Ophthalmol, 1973. 75: p. 365-383.
7. Grierson, I. and W.R. Lee, Junctions between the cells of the trabecular meshwork. Albrecht Von Graefes Arch Klin Exp Ophthalmol, 1974. 192(2): p. 89-104.
8. Johnstone, M.A., Pressure-dependent changes in configuration of the endothelial tubules of Schlemm's canal in the cynomolgus monkey (Macaca Iris). Am. J. Ophthalmol, 1974. 78.
9. Grierson, I. and W.R. Lee, The fine structure of the trabecular meshwork at graded levels of intraocular pressure. (1) Pressure effects within the near-physiological range (8-30 mmHg). Exp Eye Res, 1975. 20(6): p. 505-21.
10. Grierson, I., et al., Associations between the cells of the walls of Schlemm's canal. Albrecht Von Graefes Arch Klin Exp Ophthalmol, 1978. 208(1-3): p. 33-47.
11. Johnstone, M.A., Pressure-dependent changes in nuclei and the process origins of the endothelial cells lining Schlemm's canal. Invest. Ophthalmol. & Vis. Sci., 1979. 18(1): p. 44-51.
12. Johnstone, M.A., Glaucoma and the aqueous outflow channels. Transactions of the Pacific Coast OtoOphthalmological Society, 1979. 60: p. 153.
13. Johnstone, M.A., D. Tanner, and B. Chau, Endothelial tubular channels in Schlemm's canal. Invest. Ophthalmol. & Vis. Sci., 1980. 19: p. 123.
14. Johnstone, M.A., The morphology of the aqueous outflow channels, in Glaucoma: Applied Pharmacology in Medical Treatment, S.M. Drance, Editor. 1984, Grune & Stratton: New York. p. 87-109.
15. Smit, B.A. and M.A. Johnstone, Effects of viscocanalostomy on the histology of Schlemm's canal in primate eyes. Invest. Ophthalmol. & Vis. Sci., 2000. 41: p. S578.