In vitro studies on the trabecular meshwork of the primate eye

Establishment of human trabecular meshwork cell cultures using nontransplantable corneoscleral rims

Human trabecular meshwork (hTM) cell isolation in academic settings utilizes the motile nature of these cells, allowing them to migrate away from the explant and proliferate on distal regions of the culture substrate. Corneoscleral rims used for transplantation are a potential source of explants for the establishment of hTM cell cultures. However, cell isolation and the initiation of primary cell cultures from ocular tissues stored in Optisol-GS medium for an extended period of time (>6 days) has proven difficult, since Optisol-GS remarkably reduces cell viability and cellularity. Therefore, explants obtained from ocular tissues stored in Optisol-GS do not often provide adequate cell yield to initiate primary cell cultures if conventional culture techniques are used. Therefore, the majority of the research on primary hTM cell isolation has been accomplished using donor tissue obtained within 72 h postmortem. The goal of this study was to develop an hTM cell isolation procedure from nontransplantable ocular materials, utilizing the anchorage dependency of TM cells. This procedure yielded functionally viable cells, eficiently dissociated from the trabecular meshwork. Isolated cells demonstrated typical hTM cell characteristics including monolayer formation, contact inhibition, phagocytosis, and responses to glucocorticoid exposure. To the best of our knowledge, this is the first time an expired explant has been utilized in the successful isolation of hTM cells. Our results clearly demonstrate the advantage of increasing the anchor points of hTM cells for enhanced cell migration out from the explants, which have limited cell proliferative capacity.

Consensus recommendations for trabecular meshwork cell isolation, characterization and culture

Cultured trabecular meshwork (TM) cells are a valuable model system to study the cellular mechanisms involved in the regulation of conventional outflow resistance and thus intraocular pressure; and their dysfunction resulting in ocular hypertension. In this review, we describe the standard procedures used for the isolation of TM cells from several animal species including humans, and the methods used to validate their identity. Having a set of standard practices for TM cells will increase the scientific rigor when used as a model, and enable other researchers to replicate and build upon previous findings.

Life under pressure: The role of ocular cribriform cells in preventing glaucoma

Primary open-angle glaucoma is a multifactorial blinding disease often impacting the two pressure-sensitive regions of the eye: the conventional outflow pathway and the optic nerve head (ONH). The connective tissues that span these two openings in the globe are the trabecular meshwork of the conventional outflow pathway and the lamina cribrosa of the ONH. Resident cribiform cells of these two regions are responsible for actively remodeling and maintaining their connective tissues. In glaucoma, aberrant maintenance of the juxtacanalicular tissues (JCT) of the conventional outflow pathway results in ocular hypertension and pathological remodeling of the lamina cribrosa results in ONH cupping, damaging retinal ganglion cell axons. Interestingly, cells cultured from the lamina cribrosa and the JCT of the trabecular meshwork have similarities regarding gene expression, protein production, plus cellular responses to growth factors and mechanical stimuli. This review compares and contrasts the current knowledge of these two cell types, whose health is critical for protecting the eye from glaucomatous changes. In response to pressure gradients across their respective cribiform tissues, the goal is to better understand and differentiate healthy from pathological behavior of these two cell types.

The many faces of the trabecular meshwork cell

With the combined purpose of facilitating useful vision over a lifetime, a number of ocular cells have evolved specialized features not found elsewhere in the body. The trabecular meshwork (TM) cell at the irido-corneal angle, which is a key regulator of intraocular pressure, is no exception. Examination of cells in culture isolated from the human TM has shown that they are unique in many ways, displaying characteristic features of several different cell types. Thus, these neural crest derived cells display expression patterns and behaviors typical of endothelia, fibroblasts, smooth muscle and macrophages, owing to the multiple roles and two distinct environments where they operate to maintain intraocular pressure homeostasis. In most individuals, TM cells function normally over a lifetime in the face of persistent stressors, including phagocytic, oxidative, mechanical and metabolic stress. Study of TM cells isolated from ocular hypertensive eyes has shown a compromised ability to perform their daily duties. This review highlights the many responsibilities of the TM cell and its challenges, progress in our understanding of TM biology over the past 30 years, as well as discusses unanswered questions about TM dysfunction that results in IOP dysregulation and glaucoma.

Cathepsin B is up-regulated and mediates extracellular matrix degradation in trabecular meshwork cells following phagocytic challenge

Cells in the trabecular meshwork (TM), a tissue responsible for draining aqueous humor out of the eye, are known to be highly phagocytic. Phagocytic activity in TM cells is thought to play an important role in outflow pathway physiology. However, the molecular mechanisms triggered by phagocytosis in TM cells are unknown. Here we investigated the effects of chronic phagocytic stress on lysosomal function using different phagocytic ligands (E. coli, carboxylated beads, collagen I-coated beads, and pigment). Lysotracker red co-localization and electron micrographs showed the maturation of E. coli- and collagen I-coated beads-containing phagosomes into phagolysosomes. Maturation of phagosomes into phagolysosomes was not observed with carboxylated beads or pigment particles. In addition, phagocytosis of E. coli and collagen I-coated beads led to increased lysosomal mass, and the specific up-regulation and activity of cathepsin B (CTSB). Higher levels of membrane-bound and secreted CTSB were also detected. Moreover, in vivo zymography showed the intralysosomal degradation of ECM components associated with active CTSB, as well as an overall increased gelatinolytic activity in phagocytically challenged TM cells. This increased gelatinolytic activity with phagocytosis was partially blocked with an intracellular CTSB inhibitor. Altogether, these results suggest a potential role of phagocytosis in outflow pathway tissue homeostasis through the up-regulation and/or proteolytic activation of extracellular matrix remodeling genes.

Trabecular Meshwork and Uveoscleral Outflow Models

PURPOSE

To describe current laboratory models used to study aqueous outflow. Because development of primary open angle glaucoma (POAG) is rare in animals, no animal model of POAG is available for laboratory study on the pathogenesis of this condition. This review will discuss the alternative models used in the laboratory to study aqueous outflow.

METHODS

This is a brief review of published techniques, with commentary upon advantages and limitations of each.

RESULTS

Whole eyes and anterior segments are useful in assessing aqueous outflow resistance and anatomical changes in the meshwork. They have a limited duration of viability. Cell cultures are less expensive, easier to obtain, provide more cells than in intact meshwork, and are excellent in studying cellular physiology. The artificial environment of cell culture, however, including the plastic dish or artificial substrate and also the requirement of serum supplements, may limit applicability. Modern understanding of cells, their interaction with the extracellular matrix, and sensitivity to the composition of culture media suggest that these laboratory models should be updated to more closely mimic the in vivo situation.

CONCLUSION

Outflow models are a useful beginning in the study of cellular physiology, cell-extracellular matrix interactions, and reactions of the whole meshwork. All have limitations, however; the development of an animal model of human POAG would be an ideal solution to many of the current limitations.

The effect of organ culture on human trabecular meshwork

The effect of organ culture on the trabecular meshwork was studied in a series of human eyes using a perfusion culture system. One eye of a pair was cultured while the fellow eye was immediately fixed in glutaraldehyde. Culture periods ranged from 2 to 28 days, during which a mean intraocular pressure of 27 mmHg +/- 15 was maintained. The perfused culture medium appeared to leave the eye via Schlemm's canal and collector channels, mimicking the in vivo situation. The trabecular meshwork was well maintained with this system in 21 of 25 eyes. Cells remained in position on trabecular lamellae, cellular organelles usually remained normal, and the lamellae remained intact. Culture-induced changes were noted, with some cells developing intracytoplasmic lipid vacuoles and other cells developing swollen mitochondria. In addition, scattered focal cell necrosis was observed, most often in the uveal meshwork. Meshwork cellularity (nuclei:solid tissue) was determined with an image analysis system; there was an average cell loss of 20-40% in the cultured eyes as compared to their respective fellow control eyes. Overall, organ culture appears to maintain the human trabecular meshwork for at least 28 days, allowing controlled experimental studies to be performed.

The morphological features of bovine meshwork cells in vitro and their synthetic activities

The morphological characteristics of 3rd-passage cultured bovine meshwork cells were investigated, as were some of their synthetic activities. Growing meshwork cells had the ultrastructural characteristics of metabolically active cells whereas postconfluent cells formed gap junctions at their lateral borders and were closer in their fine structure to meshwork cells in vivo. The incorporation of 3H proline and 3H glucosamine was significantly greater in growing than postconfluent cultures. The normal bovine outflow system was rich in type I collagen but type V collagen and fibronectin was also evident. The 3rd-passage cells were shown to synthesize type I collagen, type V collagen and fibronectin. However, ultrastructural studies of the extracellular matrix which was produced in vitro demonstrated the presence of small fibrils with no distinctive banding pattern, unlike the larger collagen fibrils with a 640 A banding pattern seen in vivo.

Species differences in the responses of the eye to irritation and trauma: a hypothesis of divergence in ocular defense mechanisms, and the choice of experimental animals for eye research

Information published during the past century, especially the last decade, has identified pronounced species differences, not only in the morphological organization of ocular structures, but also in the functional responses of the eyes of different mammals to experimental and surgical procedures, as well as to drugs and autacoids. For the most part, these differences have been regarded as peculiarities or weakness rather than as fundamental evolutionary adaptations optimally suited to the environment and behavior of each species. This paper proposes a working hypothesis of evolutionary divergence in ocular defense mechanisms, based on some of the known morphological and functional differences among mammals, and discusses the implications of these differences with regard to the choice of appropriate animals for use as models in different areas of ophthalmic research.

Human trabecular meshwork in primary culture: a morphological and autoradiographic study

The growth and behaviour of human trabecular meshwork cells in primary culture was studied by light microscopy, time-lapse cinephotomicrography, autoradiography and electron microscopy. In all 925 explants of trabecular tissue from 132 eyes (68 +/- 14 years) were set up, and 20% produced extensive monolayers. Failure to produce primary outgrowths did not increase substantially until the eyes were over 5 days post-mortem. The latent period prior to growth was anything from a few days to 4 weeks; only then did meshwork cells label with [3H]-thymidine. The cells which grew incorporated large amounts of [3H]-leucine and this served as a useful marker to identify migratory cells. In addition they had the ultrastructural features of metabolically active cells including many mitochondria, a well-developed Golgi apparatus, abundant rough endoplasmic reticulum and prominent lysosomes. It was considered that the cells in primary culture had many features in common with 'activated' rather than normal meshwork cells.

Structural changes of human and monkey trabecular meshwork following in vitro cultivation

The entire chamber angle tissue of ten monkey eyes and nine normal human eyes was cultivated in organ cultures for 1-10 days and then investigated by electron microscopy. We found that the uveal and corneoscleral trabecular cells often degenerate as early as 2-3 days after explantation, whereas the cells of the cribriform region proliferate and show an increasing number of cell organelles (mitochondria, endoplasmic reticulum and Golgi complexes). Regarding the behaviour in vitro, we distinguished three different cell populations in the trabecular meshwork with probably different functions: (1) the endothelial cells of Schlemm's canal, (2) the trabecular cells of the cribriform region and (3) the uveal or corneoscleral trabecular cells.

Synthesis and composition of glycosaminoglycans by cultured human trabecular meshwork cells

A human trabecular meshwork cell line with a limited number of population doublings was established in monolayer culture. All cultures produced hyaluronic acid, heparan sulfate, chondroitin sulfate, and dermatan sulfate. Following [14C]-glucosamine incorporation into proliferating (phase II) cultures, 70%--80% of the medium glycosaminoglycan label was found in hyaluronic acid and 8%--14% in heparin sulfate. About 60% of the cell-bound activity was associated with hyaluronic acid and 30% with heparan sulfate. Long-term cultivation under nondividing ("senescent") conditions resulted in a change of the pattern of synthesized and excreted (medium) [14C]-glucosamine-labeled glycosaminoglycans with a relative decrease of hyaluronic acid and a relative increase of heparan sulfate.

Preliminary observations on human trabecular meshwork cells in vitro

This report presents our preliminary observations on the trabecular meshwork from human eyes up to 5 days post-mortem in tissue culture. Satisfactory primary cultures were obtained from about 20% of the 423 explants which were investigated. The period prior to growth was from 4 days to 4 weeks and from the appearance of the initial outgrowth it took 25 to 30 days to reach maximum cellular spread within the culture chambers. The progress of the explant and the spreading of the trabecular meshwork cells was monitored by phase-contrast microscopy, time-lapse cinephotomicrography, light microscopy, transmission electron microscopy, scanning electron microscopy and autoradiography (using tritiated thymidine). On the basis of their ultrastructural appearance the cultured meshwork cells seems to be metabolically active. Their cytoplasm contained abundant rough endoplasmic reticulum, many mitochondria, a well developed Golgi apparatus and many coated and uncoated micropinosomes. However, even in short-term culture the trabecular meshwork cells had adapted to the artificial environment of our system and no longer resembled "normal" trabecular meshwork cells as seen in vivo. Since trabecular meshwork cells can quickly adapt their morphology in a culture environment and because the adult human meshwork contains a significant population of non-trabecular cells, the value of long term culture as a means of investigating the cellular activity of the normal and glaucomatous outflow system must be open to question.