Ramesh C. Tripathi

Mechanism of the aqueous outflow across the trabecular wall of Schlemm's canal

Structural and ultrastructural analysis of the endothelial lining of Schlemms canal in normal and experimentally perfused eyes of primates revealed that normally the vacuolated endothelial cells provide the pathway for the aqueous outflow into the canal. It is postulated that the formation of the vacuoles in the endothelial lining of the trabecular wall is a dynamic process and that the morphogenesis of the vacuoles is related to the inherent peculiar property of these cells, which in turn is partly dependent on the hydrostatic pressure gradient across the trabecular wall and other factors, such as the composition of the aqueous humour, etc. The existence of the intracellular channels is regarded as a most significant stage in the vacuolation cycle which provides an easier escape for the aqueous humour; this may also allow in certain circumstances a passage for particulate matter, blood corpuscles and other wandering cells. The possibility that the endothelial vacuolation cycle may also be responsible for the maintenance of the intraocular pressure is suggested.

Vacuolar transcellular channels as a drainage pathway for cerebrospinal fluid

1. Based on our ultrastructural investigations in monkeys, we report here a new concept as to the physiological mechanism of drainage of cerebrospinal fluid (c.s.f.) which would seem to bridge the gap between the two apparently opposing views of ‘closed’ and ‘open’ system.

Ultrastructure of schlemm's canal in relation to aqueous outflow

The structure of Schlemms canal in Rhesus monkey eyes has been studied by light and electron microscopy to ascertain its relationship to the exit pathway of the aqueous. Special emphasis has been placed upon the fine structure of the trabecular wall. The trabecular wall of Schlemms canal in the normotensive eye of primates is characterized by a thin but continuous endothelial lining showing cytoplasmic vacuoles and large globular nuclei protruding into the lumen of Schlemms canal. The ultrastructural morphology of the three components of this wall, namely, the endothelial lining, basement membrane and endothelial meshwork is described and discussed. Thin serial sections of vacuolated endothelial cells, studied by electron microscopy, show an open pathway through a proportion of them. The possible functional significance of these pathways for aqueous percolation in normotensive eyes is discussed. A preliminary report of the findings in the eyes from a case of chronic simple glaucoma is given. There was almost total absence of vacuolated cells in the endothelial lining of the trabecular wall of Schlemms canal in both eyes. Although at present it is not certain whether this finding truly reflects conditions existing in life, it is one that might have considerable significance in the pathogenesis of this disease. From the functional standpoint, anatomical division into the anterior (outer) and posterior (inner) walls of Schlemms canal is misleading and better avoided. More suitable terms would be the “corneoscleral” and “trabecular” walls.

Ultrastructure of the exit pathway of the aqueous in lower mammals: (A preliminary report on the “angular aqueous plexus”)

Abstract A preliminary study of the structural and ultrastructural morphology of the angular channels concerned with the drainage of the aqueous in lower mammals ( Rodentia, Ungulata and Carnivora ), designated in this report as the “angular aqueous plexus”, revealed that they have a continuous endothelial lining, many cells of which show giant vacuoles. Ultrastructural analysis of the vacuolated endothelial cells suggests the morphogenesis of the vacuoles to be similar to that of the vacuoles in the endothelial cells lining the canal of Schlemm in primate eyes. It would seem most likely that this dynamic process of endothelial vacuolation is a fundamental mechanism for the aqueous outflow in many animal eyes.

The mechanism of aqueous outflow in lower mammals

Abstract Ultrastructural studies of the angular aqueous plexuses in lower mammals revealed that normally they were lined by a continuous endothelium, adjacent cells being joined by occluding zonules. Many endothelial cells contained giant vacuoles. Serial sections showed that the majority of these vacuoles have openings towards their basal aspect and only a small proportion had openings on both surfaces thereby constituting a transcellular channel. Apart from traces of flocculent material in occasional vacuoles, almost all vacuoles were electron optically empty. Following in vivo intracameral perfusion with an electron dense tracer (Thorotrast, particle size approximately 10 nm), the majority of vacuoles in the endothelial cells lining the aqueous plexus were readily filled with the tracer element. No leakage of this material was seen across the endothelial cell junctions. Serial sections further revealed that the transcellular channels formed by the vacuoles provided a pathway for the tracer element and wandering cells across the endothelial barrier of the aqueous plexus. It is concluded that the outflow of aqueous humour across the intact endothelial barrier of the aqueous plexus is achieved by the dynamic process of endothelial vacuolation. In this process, the role of many factors, including the unique property of the cell and the pressure gradient across the endothelial barrier of the aqueous plexus, is briefly discussed. From the present work, it is clear that the mechanism of aqueous outflow in lower mammals is essentially similar to that occurring in primate eyes ( Tripathi, 1971a ). This evidence, together with the work in progress, favours our hypothesis advanced earlier that endothelial vacuolation is a fundamental mechanism for the outflow of the aqueous humour in the eyes of many vertebrates.

Theoretical considerations on the mechanism of the aqueous outflow

Abstract Two model systems for aqueous outflow across the trabecular wall of Schlemms canal are considered assuming respectively: (i) vacuolar transport by ‘ macropinocytosis ’; and (ii) flow through permanent transcellular channels. The implications of each are compared. An alternative hypothesis is suggested according to which the main outflow takes place through transcellular channels which are formed as a transient stage in the “life-history” of a vacuole. Using parameters reported in the literature it appears that the observed rates of aqueous outflow could be accounted for if this transient stage occupied some 1 50th of the total “life-cycle” of each vacuole. This hypothesis would explain both the pressure-dependence of outflow and the presence of intracamerally injected colloidal material in closed vacuoles.

Perivascular and Intervascular Reticular Fibers of the Retina

We demonstrated the existence in the retina of an argyrophilic perivascular membrane comparable in all respects to that which exists throughout the vascular system. We compared our findings by light microscopy with those previously reported. We identified, by electron microscopy, the normal general location of the network between the basal laminae of the glial and vessel cells and demonstrated its selective staining with silver methenamine. These perivascular fibers, intercapillary strands, and bridges develop in relation to the process of retinal vascularization and the potential for forming these fibers is reactivated in senility and disease, as in diabetes. We summarized the evidence by concluding that these fibers are most likely composed of reticulin; it appears provable that in the past there may have been some confusion with neural fibers.

The argyrophilic mosaic of the internal limiting membrane of the retina

Abstract It has been known for many years that when the surface of the retina is stained with silver a characteristic mosaic pattern appears. Several authors have offered explanation for its origin but no unanimity of opinion exists. We have re-investigated this problem using the additional techniques of electron microscopy and staining with ruthenium red. We conclude that the argyrophilic mosaic pattern is due to the selective uptake of silver at the cell borders of the Muller and accessory glial cells. Viewed from the surface these silver lines are seen in depth and stand out in contrast to the lighter silver deposition on the internal limiting membrane. This intercellular pattern is closely comparable to that seen in silver staining of many cellular membranes. Positive staining with ruthenium red shows a similar distribution in the retina to that of silver; possible explanations for these findings are briefly considered.