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

Heparan sulfate deficiency leads to Peters anomaly in mice by disturbing neural crest TGF-beta2 signaling

During human embryogenesis, neural crest cells migrate to the anterior chamber of the eye and then differentiate into the inner layers of the cornea, the iridocorneal angle, and the anterior portion of the iris. When proper development does not occur, this causes iridocorneal angle dysgenesis and intraocular pressure (IOP) elevation, which ultimately results in developmental glaucoma. Here, we show that heparan sulfate (HS) deficiency in mouse neural crest cells causes anterior chamber dysgenesis, including corneal endothelium defects, corneal stroma hypoplasia, and iridocorneal angle dysgenesis. These dysfunctions are phenotypes of the human developmental glaucoma, Peters anomaly. In the neural crest cells of mice embryos, disruption of the gene encoding exostosin 1 (Ext1), which is an indispensable enzyme for HS synthesis, resulted in disturbed TGF-beta2 signaling. This led to reduced phosphorylation of Smad2 and downregulated expression of forkhead box C1 (Foxc1) and paired-like homeodomain transcription factor 2 (Pitx2), transcription factors that have been identified as the causative genes for developmental glaucoma. Furthermore, impaired interactions between HS and TGF-beta2 induced developmental glaucoma, which was manifested as an IOP elevation caused by iridocorneal angle dysgenesis. These findings suggest that HS is necessary for neural crest cells to form the anterior chamber via TGF-beta2 signaling. Disturbances of HS synthesis might therefore contribute to the pathology of developmental glaucoma.

The critical role of the conjunctiva in glaucoma filtration surgery

This review considers the critical role of the conjunctiva in determining the success or failure of glaucoma filtration surgery. Glaucoma filtration surgery can be defined as an attempt to lower intraocular pressure (IOP) by the surgical formation of an artificial drainage pathway from the anterior chamber to the subconjunctival space. Many types of glaucoma filtration surgery have been developed since the first attempts almost 180 years ago. The wide range of new techniques and devices currently under investigation is testament to the limitations of current techniques and the need for improved therapeutic outcomes. Whilst great attention has been paid to surgical techniques and devices to create the drainage pathway, relatively little attention has been given to address the question of why drainage from such artificial pathways is often problematic. This is in contrast to normal drainage pathways which last a lifetime. Furthermore, the consequences of potential changes in aqueous humour properties induced by glaucoma filtration surgery have not been sufficiently addressed. The mechanisms by which aqueous fluid is drained from the subconjunctival space after filtration surgery have also received relatively little attention. We propose that factors such as the degree of tissue damage during surgery, the surrounding tissue reaction to any surgical implant, and the degree of disruption of normal aqueous properties, are all factors which influence the successful formation of long term drainage channels from the conjunctiva, and that these channels are the key to successful filtration surgery. In recent years it has been suggested that the rate of fluid drainage from the subconjunctival space is actually the determining factor in the resultant IOP reduction. Improved knowledge of aqueous humour induced changes in such drainage pathways has the potential to significantly improve the surgical management of glaucoma. We describe for the first time a novel type of drainage surgery which attempts to minimise surgical trauma to the overlying conjunctiva. The rationale is that a healthy conjunctiva allows drainage channels to form and less opportunity for inflammation and scar tissue formation which are a frequent cause of failure in glaucoma filtration surgery. Successful drainage over extended periods of time has been demonstrated in monkey and rabbit eyes. Long lasting drainage pathways were clearly associated with the presence of lymphatic drainage pathways. A new philosophy in glaucoma drainage surgery is proposed in which minimisation of surgical trauma to the conjunctiva and the encouragement of the development of conjunctival drainage pathways, particularly lymphatic pathways, are central pillars to a successful outcome in glaucoma filtration surgery.

Extracellular matrix in the trabecular meshwork

The extracellular matrix (ECM) of the trabecular meshwork (TM) is thought to be important in regulating intraocular pressure (IOP) in both normal and glaucomatous eyes. IOP is regulated primarily by a fluid resistance to aqueous humor outflow. However, neither the exact site nor the identity of the normal resistance to aqueous humor outflow has been established. Whether the site and nature of the increased outflow resistance, which is associated with open-angle glaucoma, is the same or different from the normal resistance is also unclear. The ECMs of the TM beams, juxtacanalicular region (JCT) and Schlemm's canal (SC) inner wall are comprised of fibrillar and non-fibrillar collagens, elastin-containing microfibrils, matricellular and structural organizing proteins, glycosaminoglycans (GAGs) and proteoglycans. Both basement membranes and stromal ECM are present in the TM beams and JCT region. Cell adhesion proteins, cell surface ECM receptors and associated binding proteins are also present in the beams, JCT and SC inner wall region. The outflow pathway ECM is relatively dynamic, undergoing constant turnover and remodeling. Regulated changes in enzymes responsible for ECM degradation and biosynthetic replacement are observed. IOP homeostasis, triggered by pressure changes or mechanical stretching of the TM, appears to involve ECM turnover. Several cytokines, growth factors and drugs, which affect the outflow resistance, change ECM component expression, mRNA alternative splicing, cellular cytoskeletal organization or all of these. Changes in ECM associated with open-angle glaucoma have been identified.

Biochemical studies on the secretion of glycoproteins by isolated ciliary body of rabbits

The contribution of the ciliary body to the origin of vitreous proteins was investigated in rabbits by incubating explants of this eye component under novel conditions. At the end of incubations for up to 21 h, the tissues were processed histologically and were shown to be in an excellent state of morphological preservation. When radioactive amino acids and fucose were added to the culture medium, protein and glycoprotein synthesis and secretion were detected using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) plus fluorography. The origin of these secretory products was traced by autoradiography to the ciliary epithelium. When samples of vitreous bodies - injected intravitreally with the same radioactive precursors - were run beside samples of the tissue culture media, comigration of at least 8 radioactively labelled bands including the one previously identified as transferrin was detected. This indicates that some vitreous proteins may be secreted by the ciliary body and that cultures of explants of ciliary body-iris are useful tools for studies on vitreous protein secretion.

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.

Glycosaminoglycans of human trabecular meshwork in perfusion organ culture

The synthetic profile of glycosaminoglycans (GAGs) of human trabecular meshwork in perfusion organ culture was studied in a series of 34 human eyes. The anterior segments of these eyes were cultured for periods of two to 28 days and received medium containing 3H-glucosamine and 35S-sulfate during the final 48 hours of culture. The meshwork was then dissected and the GAGs isolated and subjected to sequential enzyme digestion. Active labelling of hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparan sulfate was found in all time periods. Eyes cultured seven and 14 days had similar incorporation profiles to "fresh" eyes (cultured 48 hours to allow for labelling). Eyes cultured 21 days showed an increase in dermatan sulfate labelling and a slight decrease in keratan sulfate labelling when compared with "fresh" eyes. Light microscopic autoradiography confirmed the trabecular meshwork incorporation of the radiolabelled precursors at all time periods. Thus, the trabecular meshwork remains metabolically active and GAG synthetic profiles remain reasonably similar to fresh eyes for up to three weeks in a perfusion organ culture system. This system may serve as a model for future studies of human trabecular meshwork GAGs.

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.

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.

Age-related changes in the human outflow apparatus

Tissue from 53 human eyes was used for qualitative and quantitative morphological study (light microscopy and electron microscopy) of age-related changes in the human outflow apparatus. The specimen source was surgical procedures. Qualitative observations showed that the general configuration of the trabecular meshwork changed gradually with age, from a long wedge shape to a shorter, more rhomboidal form. The scleral spur became more prominent, the uveal meshwork more compact, and localized canal closure increased in incidence. Intercellular cytoplasmic processes between neighboring trabecular endothelial cells were less evident and areas of trabecular denudation were frequently observed in older eyes. Quantitative light microscopic studies showed a progressive thickening of the trabeculae, and the numbers of giant vacuoles in the lining endothelium of Schlemm's canal decreasing significantly with age. Morphometric analysis of the cribriform layer (transmission electron microscopy) showed that there was a change in the character of the extracellular materials with an increase in the deposition of electron dense plaques and a decrease in the ground substances from the fifth decade onwards.

Histoautoradiographic and electron microscopic studies on short-term explant cultures of the glaucomatous trabecular meshwork

Explant cultures of 15 trabeculectomy specimens derived from glaucomatous eyes were studied, using electron microscopy and histoautoradiography. The morphology of the trabecular meshwork was monitored for 3-14 days after explanation. In 4 cases the trabeculectomy specimens were cut in 2-3 pieces each and then incubated with 14C-glucosamine at different times during the observation period (2-10 days). The cells of the cribriform layer and the outer corneoscleral trabeculae become "activated" as soon as 3-4 days after explantation, developing a large amount of endoplasmic reticulum, Golgi material, and numerous mitochondria. Histoautoradiographically, intense labeling of the trabecular meshwork, especially the cribriform layer, was found after incubation with 14C-glucosamine. We assume that under tissue culture conditions the "activated" cells of the glaucomatous meshwork produce various types of glycosaminoglycans (GAGs) but, based on the experiments alone, no final conclusions can be made in this respect.

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

Freshly isolated ciliary body explants and tips of ciliary processes derived from human eyes were cultured in serum-containing or serum-free defined medium. These cultures synthesized tissue-bound and medium-released ("excreted") glycosaminoglycans (GAGs), as evidenced by the incorporation of 14C-glucosamine and enzymatic characterization of these labelled GAGs (hyaluronic acid, heparan sulphate, chondroitin sulphate, dermatan sulphate). The GAG synthesis and excretion rate was enhanced by serum. It is suggested that ciliary epithelium performs this function of GAG synthesis also under in vivo conditions.