Synthesis and composition of glycosaminoglycans by explant cultures of human ciliary body and ciliary processes in serum-containing and serum-free defined media

Improved ultrastructural preservation of rat ciliary body after high pressure freezing and freeze substitution: a perspective view based upon comparison with tissue processed according to a conventional protocol or by osmium tetroxide/microwave fixation

Conventional fixation of the delicate, highly folded rat ciliary body and its iridial extension, as well as of vitreal structures, is associated with the induction of a number of artifacts, thus limiting the reliability of morphological interpretations. Improved ultrastructural preservation may be achieved by microwave heating in combination with osmium tetroxide fixation. This protocol, although simple and cheap, yields results, particularly with respect to the extracellular matrix compartment between inner and outer ciliary epithelial cells, which are not greatly inferior to those obtained by implementing the sophisticated high pressure freezing and freeze substitution technique. The latter affords good to very good ultrastructural preservation of epithelium and stromal components, such as blood vessels, neural elements, smooth muscle cells, fibrocytes, and free cells, up to a depth of 50-100 microns from the tissue surface. Its superiority over osmium tetroxide/microwave fixation is revealed in the cytoplasmic, intraorganellar, and vitreal matrix compartments, which incur no obvious losses.

Formation and release of vesicles from the basal surfaces of rat eye non-pigmented ciliary epithelial cells: a novel secretory mechanism?

When rat ciliary body is processed by high pressure freezing and freeze substitution, numerous membrane-bound vesicle profiles are seen in the vitreous associated with the pars plana and in the valleys between the ciliary processes. They consist of a homogeneously distributed fine granular matrix and varying numbers of ribosome-like structures. The mechanism by which these vesicles are secreted appears to follow an apocrine-type pattern, albeit at the basal cell surface. Matrix material accumulates between the basal plasma membrane of non-pigmented ciliary epithelial cells and a cortical layer of cytoskeletal components; the blebs thus formed protrude through a discontinuity in the basal lamina and, by a progressive narrowing of the neck region, are eventually pinched off, giving rise to free vesicles. Under conventional aqueous chemical fixation conditions, most of these vesicles are washed away or their contents solubilized and extracted, which accounts for their not having been identified hitherto as genuine morphological structures. They are nonetheless apparent, albeit in reduced numbers and mostly empty. Such vesicles are also observed in tissue processed according to several other chemical fixation techniques, namely, conventional fixation in the presence of the cationic dye ruthenium hexamine trichloride, simultaneous glutaraldehyde/osmium tetroxide fixation, and microwave fixation. In the latter instance, comparable vesicle preservation to that obtained by high pressure freezing/freeze substitution may be achieved if fixation is followed by cryoprotection, plunge freezing, and freeze substitution instead of conventional post-fixation and dehydration procedures.

Outflow facility studies in the perfused human ocular anterior segment

We have recently developed a tissue model of the human aqueous outflow pathway involving placement of the eviscerated anterior corneoscleral shell, [with lens and uveal tissue removed but trabecular meshwork (TM) attached] onto a specialized perfusion apparatus. The TM and associated outflow tissues are perfused with culture medium at a physiologically-relevant perfusion pressure in a 5% CO2 environment at 37 degrees C. Under these conditions, the perfused outflow tissues are similar for several days, to the human and/or subhuman primate outflow system in vivo with regard to morphology as well as several functional parameters. Measured facility of outflow (0.271 +/- 0.018 microliters min-1 mmHg-1, n = 79) is similar to facility values obtained by tonography in living human beings. Moreover, outflow facility decreases in a linear fashion with increased perfusion pressure by 1.4% mmHg-1. Finally the removal of the TM results in a 41% decrease in measured outflow resistance. The ability to study viable human outflow tissue for at least several days and the opportunity to establish a model which serves as an alternative to animal testing, point to the potential importance of this technique in investigating the biology of the aqueous outflow system.

In vitro cultures of trabecular meshwork cells of the human eye as a model system for the study of cellular aging

Cells from the human trabecular meshwork providing a drainage system for the outflow of aqueous humour in the eye were isolated and propagated in monolayer culture. Following serial subcultivation of the primary cultures, there was a gradual decline in the fraction of dividing cells with increasing population doubling level (PDL) resulting finally in growth cessation and disintegration of these 'senescent' cultures. The number of population doublings was at most 20. Senescent cultures revealed reduced glycosaminoglycan synthesis rates (as measured by [14C]glucosamine incorporation) with a relative decrease of hyaluronic acid and increase of heparan sulfate. Medium-supplied (exogenous) hyaluronic acid enhanced hyaluronic acid synthesis of trabecular meshwork cells cultured in a defined, serum-free medium. Ascorbic acid (25-200 micrograms/ml), which is found in very high concentration in the ocular aqueous humour, stimulated hyaluronic acid synthesis of confluent cultures, also. The functional significance of decreased hyaluronic acid (and elevated heparan sulfate) synthesis in the process of cellular aging in vitro (and in vivo?), as well as the importance of hyaluronic acid for the structural integrity and functional activity of the trabecular meshwork were discussed.

Exogenous glycosaminoglycans stimulate hyaluronic acid synthesis by cultured human trabecular-meshwork cells

Addition of hyaluronic acid (50-200 micrograms ml-1) to the defined, serum-free media of cultured human trabecular-meshwork cells resulted in an increase of glycosaminoglycan (GAG) synthesis as measured by the incorporation of [14C]glucosamine. Lesser stimulatory effects were exerted by dermatan sulfate and chondroitin-4- or -6-sulfate. Nearly 90% of the labeled GAGs were found to be exerted into the medium and ea. 10% were associated with the cell layer. Mainly hyaluronic acid synthesis was stimulated by the exogenous GAGs. Analysis of the GAG-pattern revealed that exogenous hyaluronic acid stimulated hyaluronic acid synthesis (positive feedback), while exogenous dermatan sulfate and chondroitin sulfate had additional effects on chondroitin sulfate synthesis. Cell growth of these cultures, which exhibited a limited proliferative capacity (ca. 18 population doublings during their life span) was not affected by the GAG treatment. Thus, exogenous hyaluronic acid and to a lower degree dermatan sulfate or chondroitin sulfate appeared to interfere with the GAG-metabolism of these human trabecular-meshwork cells in culture.

Synthesis and migration of 3H-fucose-labeled glycoproteins in the ciliary epithelium of the eye: effects of microtubule-disrupting drugs

3H-fucose was injected intravenously or intravitreously into albino rats. After time intervals of 10, 40, and 50 min, 1, 1.5, and 4 hr, 1, 3, and 7 days, and 1, 2, and 4 weeks after injection, the animals were sacrificed by intracardiac perfusion with glutaraldehyde. Samples of the ciliary body were prepared for light and electron microscope radioautography. Light microscope autoradiographs showed that the cells of both the inner and outer layers of ciliary epithelium actively incorporated 3H-fucose label in a reaction that peaked in intensity at 4 hr after injection, and then progressively declined. Electron microscope radioautographs revealed that, at early time intervals, most of the label was localized to the Golgi apparatus. With time, the plasma membrane of both cell types became increasingly labeled, and accounted for 60-70% of the total silver grains at 4 hr after injection. Adjacent to the basal cell surface of the inner layer cells, the fibers of the zonula became increasingly labeled from 1.5 hr onwards, providing strong evidence that these cells secrete glycoproteins to the zonula. When vinblastine was administered 30 min before 3H-fucose injection, followed by sacrifice 1.5 hr later, a much larger proportion of label remained localized to the Golgi apparatus than in controls, and the plasma membrane and zonula were much less labeled. These results suggest that, as documented in other cell types, microtubules may play a role in the intracellular transport of membrane and secretory glycoproteins in these cells.

Factors influencing glycosaminoglycan synthesis by calf trabecular meshwork cell cultures

When cell or explant cultures were established from excised calf aqueous outflow pathway tissue and subsequently labeled with radioactive precursors of glycosaminoglycans [( 3H]glucosamine and [35S]sulfate), hyaluronic acid was the predominant glycosaminoglycan produced initially. As the cultures reached confluence, sulfated glycosaminoglycans became more prominent products. Further investigation revealed that the cultured trabecular cells could synthesize all of the glycosaminoglycans found in isolated calf trabecular tissue once a critical cell density had been attained. Increasing concentrations of fetal bovine serum in the culture medium stimulated glycosaminoglycan accumulation without altering the quantitative distribution of the various glycosaminoglycan products. Treatment of confluent monolayers with enzymes capable of degrading extracellular matrix molecules caused marked changes in the distribution of glycosaminoglycans produced by calf trabecular-cell cultures. The results suggest that calf trabecular cell cultures can be used to study the dynamics of extracellular matrix production and turnover under controlled experimental conditions.

Histoautoradiographic and biochemical studies on human and monkey trabecular meshwork and ciliary body in short-term explant culture

Explants of two types of chamber angle tissue derived from five human autopsy eyes and five freshly enucleated monkey eyes were kept under tissue culture conditions for 1-3 days and then incubated with labelled 14C-glucosamine for 40-48 h. In all specimens an intense labelling was seen histoautoradiographically within the area of the trabecular meshwork, especially in the cribriform layer and the outer wall of Schlemm's canal. Large quantities of silver grains were also found at the tips of the ciliary processes where the nonpigmented epithelium showed the most intense labelling. Measuring the total radioactivity of the two types of specimens in the tissue-localized and the medium-released glycosaminoglycans (GAGs), we found that the type 1 specimens containing only trabecular meshwork work and corneosclera "excrete" relatively more GAGs into the medium than the type 2 specimens consisting of corneosclera, trabecular meshwork and ciliary body. Biochemical analysis of these GAGs revealed that 57%-70% of the tissue GAGs were hyaluronic acid, 16%-33% heparan sulphate and 12%-22% various types of chondroitin sulphate. The findings show that both the ciliary epithelium and the trabecular meshwork possesses the ability to synthesize large amounts of GAGs, probably in the form of proteoglycans. The differences between the in vitro behaviour of type 1 and type 2 specimens indicate a functional interrelationship between the ciliary body and the trabecular meshwork, at least with regard to the GAG metabolism.