Electron microscopic studies on reactive changes of the trabecular meshwork in human eyes after microsurgery

Aqueous outflow - A continuum from trabecular meshwork to episcleral veins

In glaucoma, lowered intraocular pressure (IOP) confers neuroprotection. Elevated IOP characterizes glaucoma and arises from impaired aqueous humor (AH) outflow. Increased resistance in the trabecular meshwork (TM), a filter-like structure essential to regulate AH outflow, may result in the impaired outflow. Flow through the 360° circumference of TM structures may be non-uniform, divided into high and low flow regions, termed as segmental. After flowing through the TM, AH enters Schlemm's canal (SC), which expresses both blood and lymphatic markers; AH then passes into collector channel entrances (CCE) along the SC external well. From the CCE, AH enters a deep scleral plexus (DSP) of vessels that typically run parallel to SC. From the DSP, intrascleral collector vessels run radially to the scleral surface to connect with AH containing vessels called aqueous veins to discharge AH to blood-containing episcleral veins. However, the molecular mechanisms that maintain homeostatic properties of endothelial cells along the pathways are not well understood. How these molecular events change during aging and in glaucoma pathology remain unresolved. In this review, we propose mechanistic possibilities to explain the continuum of AH outflow control, which originates at the TM and extends through collector channels to the episcleral veins.

Trabecular bypass stents decrease intraocular pressure in cultured human anterior segments

PURPOSE

To determine the effect on intraocular pressure (IOP) of bypassing the trabecular meshwork in cultured human anterior segments.

DESIGN

Prospective laboratory investigation using normal human eyes obtained at autopsy.

METHODS

Anterior segments from 21 eyes were placed in perfusion culture, and trabecular bypass stents were inserted through the trabecular meshwork, with the lumen of the tube opening into Schlemm's canal. Eyes received from one to four stents, placed equidistant apart. In eyes receiving one or two stents, additional stents were later added to a maximum of four per eye.

RESULTS

Intraocular pressure was lowered after placement of a single stent, from 21.4 +/- 3.8 mm Hg to 12.4 +/- 4.2 (P < .001). This corresponded to an 84% increase in facility of outflow. Eyes receiving more than one stent had final IOP of 11.9 +/- 3.7 mm Hg. Nine eyes had sequential addition of stents, and seven of these had a further decrease of IOP (13.6 +/- 4.1 to 10.0 +/- 4.3; P = .02). Excision of the entire meshwork, between stents, dropped IOP to 6.3 +/- 3.2 mm Hg, indicating some residual meshwork or canal resistance remained even after placement of three stents.

CONCLUSIONS

Bypass of the trabecular meshwork lowers IOP in cultured human anterior segments. One stent produced the greatest change in pressure. The sequential addition of more stents further lowered pressure in seven of nine eyes. This technique holds promise as a new clinical surgery for glaucoma.

How does nonpenetrating glaucoma surgery work? Aqueous outflow resistance and glaucoma surgery

Histologic, experimental, and theoretical studies of the aqueous outflow pathways point toward the juxtacanalicular region and inner wall of Schlemm's canal as the likely site of aqueous outflow resistance in the normal eye. At least 50% of the aqueous outflow resistance in the normal eye and the bulk of the pathologically increased resistance in the glaucomatous eye resides in the trabecular meshwork and the inner wall of Schlemm's canal. The uveoscleral, or uveovortex, pathway, which accounts for perhaps 10% of the aqueous drainage in the healthy aged human eye, can become a major accessory route for aqueous drainage after pharmacologic treatment. Surgeries designed to incise or remove the abnormal trabecular meshwork of glaucoma address the pathologic problem of the disease. Surgeries that unroof Schlemm's canal or expand the canal, such as viscocanalostomy, probably cause inadvertent ruptures of the inner wall and juxtacanalicular tissue, thus relieving the abnormal outflow resistance of glaucoma. This review is a summary of current thought on the pathophysiology of aqueous outflow resistance in glaucoma and, in light of this, provides an interpretation of the mechanism of pressure reduction created by these new surgeries.

Repair in the rabbit outflow system

An in vivo study was conducted to study repair processes in the injured rabbit outflow system. A uniform injury was produced by raising intraocular pressure (IOP) manometrically to 70 mmHg for 1 h. The recovery process, which was followed clinically for 8 weeks and morphologically for 6 weeks, led to the re-establishment of normal meshwork architecture within this period. The morphological studies included light microscopy, autoradiography and electron microscopy. The initial lesion consisted of large deficits in the meshwork with breakdown of cell-to-cell connections, loss of extracellular materials and disruption of the vessels of the aqueous plexus. There was a significant lowering of IOP in the first week of recovery, which thereafter climbed back to normal. Also in the first week the meshwork became infiltrated with inflammatory cells which cleared by 4 weeks. There was some meshwork cell death by either necrosis or apoptosis. The majority of meshwork cells became activated within the first few days and remained activated for at least the first 2 weeks. Tritiated proline incorporation was maximal between 1 and 2 weeks. Tritiated thymidine labelling was seen throughout, but only after the inflammation subsided was it clear that meshwork cells in all regions of the meshwork were proliferating. Our study provided no evidence that normal meshwork cells have a basal proliferative turnover level. Our injury model involved complete repair of the outflow tissues and that required meshwork cells to become activated, mobilise, undertake synthetic activity and proliferate. This is the first example, other than argon laser trabeculoplasty, where meshwork cells in vivo have been induced to divide. Possible therapeutic implications for glaucoma are discussed.

Immunohistochemical monitoring of the effect of a synthetic fibronectin-like peptide (Arg-Gly-Asp) on the age-related changes in the isolated human corneoscleral tissue of glaucomatous eyes

Fibronectin, an adhesion glycoprotein has been detected and localized in samples of the trabecular meshwork from eight normotensive and 30 glaucomatous human eyes of various ages by means of the indirect immunoperoxidase staining technique. Fibronectin concentration in the trabecular meshwork tissue was evaluated by morphometric analysis. Deposits of the adhesion glycoprotein fibronectin were shown to be spread in the ocular drainage outflow system from patients along with progressive primary open-angle glaucoma (POAG). The fibronectin level quantitatively evaluated in serial cross-sections of trabecular meshwork, appeared to be increased during ageing and more rapidly in the event of POAG development. The active amino acid sequence in fibronectin is an arginine-glycine-aspartic acid tripeptide (Arg-Gly-Asp) and it was shown that the synthetic Arg-Gly-Asp peptide specifically inhibited the adhesive function of fibronectin in trabecular meshwork samples when incubated for 30 min at a concentration of 1-2 mg/ml. The peptide concentration necessary for a 50% decrease of the maximal fibronectin level in the trabecular meshwork specimen derived from patients with moderately advanced POAG stage, was about 1 mg/ml. Immunohistochemical staining exhibited a fainter fibronectin staining in trabecular tissues including the external trabecular layers and subendothelial region of Schlemm's canal, in samples incubated with the synthetic peptide compared with the same tissue explants before peptide treatment. It may be concluded that the adhesion control system is likely to play an important role in development and maintenance of tissue architecture and specialization of the normal human trabecular meshwork.

Chemoattractants produced by ocular cells induce trabecular meshwork cell migration

Media conditioned by bovine corneal endothelium and scleral fibroblasts were found to induce the migration of bovine trabecular meshwork cells. The migrational activity had both chemokinetic (stimulated random motion) and chemotactic (directional migration) components. That produced by corneal endothelial cells had a markedly more potent effect. The relevance of these in vitro studies is discussed. Biochemically stimulated migration and loss of meshwork cells may be involved in the pathogenesis of glaucoma.

Biomechanics of echothiophate-induced anatomic changes in monkey aqueous outflow system

Cynomolgus monkeys underwent long-term topical treatment with echothiophate, echothiophate + atropine, or control solution. Echothiophate-treated eyes exhibited increased intraocular pressure, collapse and densification of the trabecular meshwork with accumulation of extracellular material in the cribriform region, alterations in the shape and orientation of Schlemm's canal and the ciliary muscle, and discontinuity between ciliary muscle bundles and trabecular beams. Atropine or ciliary muscle disinsertion with subsequent scar formation supporting the mesh posteriorly at least partially prevented these alterations. Only sometimes did discontinuing echothiophate treatment restore normal anatomy. Collectively, these findings indicate that the pathophysiology of structural alterations in the outflow apparatus induced by echothiophate is mediated at least in part by an anterior segment muscarinic receptor, involves mechanical factors and underperfusion of the meshwork, and does not involve any direct toxic effect of echothiophate.

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.

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.

In vitro studies on the trabecular meshwork of the primate eye

Small pieces of the chamber angle region of a vervet eye containing only a lamella of the corneosclera and the trabecular meshwork were cultured in vitro and studied by light and electron microscopy at various intervals. The explanted trabecular meshwork underwent a rapid dedifferentiation. The activation of the trabecular cells is demonstrated by an increasing amount of encoplasmic reticulum, ribosomes and mitochondria. They are capable of phagocytosis for detritus and pigment granules. In the 45-day-tissue culture the trabecular cells revealed many lipid inclusions. In the culture the trabecular lamellae gradually dissolve, whereas the basement membranes and the elastic fibers remain unchanged for the longest. An increase or change in the clusters of lattice or curly collagen is not seen.