Exp. Eye Res. (1975) 20, 173-188
Abstracts Abstracts of papers October 1974.
Comparative D. F. C'OLE,
presented
at the
15th
Aspects of Aqueous Humour
Meeting
of the Association
Formation
and Dynamics
for
Eye
Reserwch,
13-16
London
Although much work has been carried out on aqueous humour dynamics in mammals the information relating to lower vertebrates is relatively scanty, except for a few physiological studies on aqueous flow and intraocular pressure in birds (cf. Cole, 1974). In order to obtain an overall vienof the pattern in vertebrates we must therefore rely upon a functional interpretation of morphological observations and on studies of the chemical composition of the aqueous humour. On reviewing the literature it is evident that many submammalian vertebrates share t)he following characteristics with the more commonly investigated laboratory mammals: (1) the presence of what, on morphological grounds, may be regarded as a “secretory type“ of eplthelium m the ciliary region; (2) morphological characteristics of a system providing for bulk outflow (cf. Tripathi. 1974a and 11: (3) an aqueous humour composition which differs significantly from that of a simple dialysnte or ult,rafiltrate of plasma ; (4) the fact that inhibitors of active transport, such as ouabain and acetazolamide, modify t be composition of the aqueous humour, cause ultrastructural changes in the ciliary epithelium or lower the intraocular pressure. On the basis of these criteria it is suggested that active transport across the blood-aqueous barrier plays a significant role in aqueous formation and dynamics throughout the whole vertebrate phylum and that. the secretSion of aqueous humour is not. as Duke-Elder (1958) suggested. confinrd to the terrestial species. REFEREXCES Cole.
D. F. Vol. 5. Duke-Elder, Tripat,hi, R. Tripathi, R. In The
(1974). Comparative aspects of the intraocular fluids. Academic Press, London, p. 71. IV. S. (1958). System of OphthalmoZogy. Vol. 1, p. 267. C. (1974a). Exp. Eye Rec. 20 [article below). C. (1974b). Comparative physiology and anatomy of Eye (Ed. Davson, H.). Vol. 5, p. 163. Academic Press,
Comparative K. (‘. TRIPATHI.
Aspects of the Oufflow
In The Eye. H. Kimpton, the aqueous London.
(Ed.
Davson,
H.\.
London. outflow
pat’hw-ay-.
of Aqueous Humour
London
Although there are wide variations in the mode of life and habitat of the various vertebrates, the close similarity in the ontogeny and basic features of their eyes is most remarkable. The anterior chamber shows wide variations in its size, shape and depth, the latter being further influenced by the dynamic alterations in the dioptric mechanism of the eye. In all species, this cavity primarily acts as a reservoir for a clear watery fluid, the aqueous humour, which through its hydrodynamics is responsible for maintaining the normal intraocular pressure and this, in conjunction with t,he fibrous tunics of the eye, provides the stability of ocular dimensions in the performance of visual function. The comparat,ive morphology and physiology of the aqueous drainage pathway in vertebrate eyes is discussed under two main headings : (a) The accessory drainage routes includes uveoscleral drainage, diffusion across iris vessels, posterior drainage (through the vitreous into the retina and optic nerve) and transcornsal flux. (b) The “conventional” drainage pathway is responsible for the bulk drainage of the aqueous humour from the anterior chamber into the channels located in the angular region. In the adult eye, the morphological organization of the irido-cornea1 angle and the angular meshwork varies from species to species. The formation of the ciliary cleft, a wedge-shaped separation of the ciliary body into two leaves, as seen in most amphibians, reptiles, birds and lower placentals, seems largely related to the evolution and degree of development of the ciliary 173
17-i
hBS’I’l?X(:TS
musculature. For practical purposes, the ciliar,y cLeft can be regarded as a peripheral cast cbnsicrl 01 the anterior chamber. In primates, however, as the ciliary musculature is highly evolved and forms a compact structure, the ciliary cleft is obliterated. The extent to which the structural organization of the cellular and fibrous components of the angular meshwork and the presence of a mucinour substance in this region and/or in the aqueous humour may account for the resistance to aqueous outflow is discussed. As suggested by our electron-microscopical studies, there would seem to be t,hroughout the. vertebrates a structural basis for the bulk drainage of aqueous humour. In order to designate 2% morphological and functional significance to the structure immediatley concerned with the outfo;\ of aqueous from the angular meshwork, we have proposed an etymological nomenclature “angular aqueous plexus/sinus” (specifically known as the canal of Schlemm in primates). The plexus/sinus is generally located on the inner aspect of t.he t,ransitional zone of the cornea-sclera and is supported towards the anterior chamber aspect by t.he reticnIated/trabeculated fibrocellular tissue of the It forms a complete circumferential structure in higher primates. birds and angular meshwork. many reptiles, but is present only in the dorsal and ventral segments of the angular region in amphibians and only in a localized ventral segment of many fishes. The final drainage of aqueous humour into the venous system is catered for by the intraand epi-scleral plexuses of veins, and in some instances also by the supraciliary and wprachoroidal plexuses of veins. which dirert,iy communicate externally with the angular aqueous plexus/sinus. Although certain variations arv found in the location, limit and genera1 shape of the angular plexus/sinus in different species. there are close similarities in their ultrastructural morphology. The endothelial lining of the plexus/sinus forms a continuous membrane adjacent t,ells being joined by ‘tight’ junct’ions. Tracer studies suggest that the latter offer a barrier t,o the pa,ssage of molecules of colloidal dimensions. Although the micropinocytotic vesicles in the endothelial c-lln do appear to provide a basis for clearance, normally their contribution is very small and they alone cannot account for the rapidity with which the aqueous humour and large particulate matter leave t.he anterior chamber and can be recovered in the exit channels. A unique feature of many lining cells is the presence of large membranous infoldings termrd giant vacuoles. On the basis of our studies of normal and perfused eyes, it is post.ulated that the vacuoles are stages in the formation of a system of transcellular pores which allow the bulk outflow of aqueous humour down a pressure gradient. Experimental and ultrastructural evidence suggests that these transcellular channels probably act as one-way valves, i.e. allow the passage of aqueous humour from the anterior chamber to the lumen of the aqueous plexus/sinus far more easily than in the reverse direction. This interpretation, therefore, entails that’ the endothelial vacuolation cycle in providing the requisite number of t’ranscellular pores across the endothelial barrier at any given time is an important factor in controlling the outflow of aqueous humoar and in tht maintenance of intraocular pressure. The genera1 conclusion to emerge is that except for minor variations, there is a st,ructural basis for the bulk drainage of aqueous humour throughout the vertebrate phylum. The nornral presencr of giant vacuoles in the endothelial lining of the aqueous plexus/sinus in species as diverse a3 dogfish, pigeon and man would suggest that the bulk outflow of aqueous humour by t.he dynamic system of vacuolar transcellular pores is a fundamental biological mechanism, not previounlp realized, and of enormous antiquity. (Supported by a grant from the Medical Research Council.)
Tripathi, Tripathi, Tripathi, York Tripathi, Tripathi, Tripathi,
Adrenergic
REFERENCES R. C. (1971). Ex~,. Eye Res. 11, 116. R. C. (1971). Ez~. Eye Res. 12, 311. R. C. 1974). In The Eye (Ed. Davson, H.). Vol. 5, pp. 163-356. and London. R. C. and Tripathi, B. J. (1972). Exp. Eye Res. 14, 73. R. C. and Tripathi, B. J. (1973). Exp. Eye Res. 15,409 and 426. R. C. and Tripathi, B. J. (1974). J. P&sioZ. (London) 139, 195.
Tachyphylaxis
in Animal
Academic
Press,
New
and Human Eyes
31. E. LANGHAM, Baltimore A common difficulty encountered with the use of automatic agonists is the development of tolerance. This results from the integrated effect of multiple analications of drugs that are administered so frequently that the influe&e of one application &es not disappear befo% the next is administered; this is tachyphylaxis. The conscious rabbit has been found to be a suitable animal mode1 to study adrenergic tachyphylaxis in the eye. One drop of adrenergic agonist is applied topically to one eye of a conscious
Abstracts Abstracts of papers October 1974.
Comparative D. F. C'OLE,
presented
at the
15th
Aspects of Aqueous Humour
Meeting
of the Association
Formation
and Dynamics
for
Eye
Reserwch,
13-16
London
Although much work has been carried out on aqueous humour dynamics in mammals the information relating to lower vertebrates is relatively scanty, except for a few physiological studies on aqueous flow and intraocular pressure in birds (cf. Cole, 1974). In order to obtain an overall vienof the pattern in vertebrates we must therefore rely upon a functional interpretation of morphological observations and on studies of the chemical composition of the aqueous humour. On reviewing the literature it is evident that many submammalian vertebrates share t)he following characteristics with the more commonly investigated laboratory mammals: (1) the presence of what, on morphological grounds, may be regarded as a “secretory type“ of eplthelium m the ciliary region; (2) morphological characteristics of a system providing for bulk outflow (cf. Tripathi. 1974a and 11: (3) an aqueous humour composition which differs significantly from that of a simple dialysnte or ult,rafiltrate of plasma ; (4) the fact that inhibitors of active transport, such as ouabain and acetazolamide, modify t be composition of the aqueous humour, cause ultrastructural changes in the ciliary epithelium or lower the intraocular pressure. On the basis of these criteria it is suggested that active transport across the blood-aqueous barrier plays a significant role in aqueous formation and dynamics throughout the whole vertebrate phylum and that. the secretSion of aqueous humour is not. as Duke-Elder (1958) suggested. confinrd to the terrestial species. REFEREXCES Cole.
D. F. Vol. 5. Duke-Elder, Tripat,hi, R. Tripathi, R. In The
(1974). Comparative aspects of the intraocular fluids. Academic Press, London, p. 71. IV. S. (1958). System of OphthalmoZogy. Vol. 1, p. 267. C. (1974a). Exp. Eye Rec. 20 [article below). C. (1974b). Comparative physiology and anatomy of Eye (Ed. Davson, H.). Vol. 5, p. 163. Academic Press,
Comparative K. (‘. TRIPATHI.
Aspects of the Oufflow
In The Eye. H. Kimpton, the aqueous London.
(Ed.
Davson,
H.\.
London. outflow
pat’hw-ay-.
of Aqueous Humour
London
Although there are wide variations in the mode of life and habitat of the various vertebrates, the close similarity in the ontogeny and basic features of their eyes is most remarkable. The anterior chamber shows wide variations in its size, shape and depth, the latter being further influenced by the dynamic alterations in the dioptric mechanism of the eye. In all species, this cavity primarily acts as a reservoir for a clear watery fluid, the aqueous humour, which through its hydrodynamics is responsible for maintaining the normal intraocular pressure and this, in conjunction with t,he fibrous tunics of the eye, provides the stability of ocular dimensions in the performance of visual function. The comparat,ive morphology and physiology of the aqueous drainage pathway in vertebrate eyes is discussed under two main headings : (a) The accessory drainage routes includes uveoscleral drainage, diffusion across iris vessels, posterior drainage (through the vitreous into the retina and optic nerve) and transcornsal flux. (b) The “conventional” drainage pathway is responsible for the bulk drainage of the aqueous humour from the anterior chamber into the channels located in the angular region. In the adult eye, the morphological organization of the irido-cornea1 angle and the angular meshwork varies from species to species. The formation of the ciliary cleft, a wedge-shaped separation of the ciliary body into two leaves, as seen in most amphibians, reptiles, birds and lower placentals, seems largely related to the evolution and degree of development of the ciliary 173
17-i
hBS’I’l?X(:TS
musculature. For practical purposes, the ciliar,y cLeft can be regarded as a peripheral cast cbnsicrl 01 the anterior chamber. In primates, however, as the ciliary musculature is highly evolved and forms a compact structure, the ciliary cleft is obliterated. The extent to which the structural organization of the cellular and fibrous components of the angular meshwork and the presence of a mucinour substance in this region and/or in the aqueous humour may account for the resistance to aqueous outflow is discussed. As suggested by our electron-microscopical studies, there would seem to be t,hroughout the. vertebrates a structural basis for the bulk drainage of aqueous humour. In order to designate 2% morphological and functional significance to the structure immediatley concerned with the outfo;\ of aqueous from the angular meshwork, we have proposed an etymological nomenclature “angular aqueous plexus/sinus” (specifically known as the canal of Schlemm in primates). The plexus/sinus is generally located on the inner aspect of t.he t,ransitional zone of the cornea-sclera and is supported towards the anterior chamber aspect by t.he reticnIated/trabeculated fibrocellular tissue of the It forms a complete circumferential structure in higher primates. birds and angular meshwork. many reptiles, but is present only in the dorsal and ventral segments of the angular region in amphibians and only in a localized ventral segment of many fishes. The final drainage of aqueous humour into the venous system is catered for by the intraand epi-scleral plexuses of veins, and in some instances also by the supraciliary and wprachoroidal plexuses of veins. which dirert,iy communicate externally with the angular aqueous plexus/sinus. Although certain variations arv found in the location, limit and genera1 shape of the angular plexus/sinus in different species. there are close similarities in their ultrastructural morphology. The endothelial lining of the plexus/sinus forms a continuous membrane adjacent t,ells being joined by ‘tight’ junct’ions. Tracer studies suggest that the latter offer a barrier t,o the pa,ssage of molecules of colloidal dimensions. Although the micropinocytotic vesicles in the endothelial c-lln do appear to provide a basis for clearance, normally their contribution is very small and they alone cannot account for the rapidity with which the aqueous humour and large particulate matter leave t.he anterior chamber and can be recovered in the exit channels. A unique feature of many lining cells is the presence of large membranous infoldings termrd giant vacuoles. On the basis of our studies of normal and perfused eyes, it is post.ulated that the vacuoles are stages in the formation of a system of transcellular pores which allow the bulk outflow of aqueous humour down a pressure gradient. Experimental and ultrastructural evidence suggests that these transcellular channels probably act as one-way valves, i.e. allow the passage of aqueous humour from the anterior chamber to the lumen of the aqueous plexus/sinus far more easily than in the reverse direction. This interpretation, therefore, entails that’ the endothelial vacuolation cycle in providing the requisite number of t’ranscellular pores across the endothelial barrier at any given time is an important factor in controlling the outflow of aqueous humoar and in tht maintenance of intraocular pressure. The genera1 conclusion to emerge is that except for minor variations, there is a st,ructural basis for the bulk drainage of aqueous humour throughout the vertebrate phylum. The nornral presencr of giant vacuoles in the endothelial lining of the aqueous plexus/sinus in species as diverse a3 dogfish, pigeon and man would suggest that the bulk outflow of aqueous humour by t.he dynamic system of vacuolar transcellular pores is a fundamental biological mechanism, not previounlp realized, and of enormous antiquity. (Supported by a grant from the Medical Research Council.)
Tripathi, Tripathi, Tripathi, York Tripathi, Tripathi, Tripathi,
Adrenergic
REFERENCES R. C. (1971). Ex~,. Eye Res. 11, 116. R. C. (1971). Ez~. Eye Res. 12, 311. R. C. 1974). In The Eye (Ed. Davson, H.). Vol. 5, pp. 163-356. and London. R. C. and Tripathi, B. J. (1972). Exp. Eye Res. 14, 73. R. C. and Tripathi, B. J. (1973). Exp. Eye Res. 15,409 and 426. R. C. and Tripathi, B. J. (1974). J. P&sioZ. (London) 139, 195.
Tachyphylaxis
in Animal
Academic
Press,
New
and Human Eyes
31. E. LANGHAM, Baltimore A common difficulty encountered with the use of automatic agonists is the development of tolerance. This results from the integrated effect of multiple analications of drugs that are administered so frequently that the influe&e of one application &es not disappear befo% the next is administered; this is tachyphylaxis. The conscious rabbit has been found to be a suitable animal mode1 to study adrenergic tachyphylaxis in the eye. One drop of adrenergic agonist is applied topically to one eye of a conscious
