A morphologic and morphometric analysis of the aqueous outflow system of the developing cat eye

Feline Glaucoma

Feline glaucoma is best categorized as either secondary, congenital and anterior segment dysgenesis associated, or primary. More than 90% of all feline glaucoma develops secondary to uveitis or intraocular neoplasia. The uveitis is usually idiopathic and assumed to be immune-mediated, whereas lymphosarcoma and diffuse iridal melanoma account for many of the intraocular neoplastic-induced glaucoma in cats. Several topical and systemic therapies are useful in the control of the inflammation and elevated intraocular pressures associated with feline glaucoma. Enucleation remains the recommended therapy for blind glaucomatous feline eyes. Enucleated globes from cats with chronic glaucoma should be submitted to an appropriate laboratory for histologic confirmation of the type of glaucoma.

Trabecular meshwork morphogenesis: A comparative analysis of wildtype and anterior segment dysgenesis mouse models

The trabecular meshwork (TM), a tissue residing in the iridocorneal angle of the eye, is the primary site of aqueous humor outflow and often develops abnormally in children with anterior segment dysgenesis (ASD). However, the cellular mechanisms underlying both normal and pathophysiological TM formation are poorly understood. Here, we improve the characterization of TM development via morphological and molecular analyses. We first assessed the TM of wild-type C57BL/6J mice at multiple time points throughout development (E15.5-P21). The morphology of TM cells, rate of cell division, presence of apoptotic cell death, and age of onset of an established TM marker (αSMA) were each assessed in the developing iridocorneal angle. We discovered that TM cells are identifiable histologically at P1, which coincided with both the onset of αSMA expression and a significant decrease in TM precursor cell proliferation. Significant apoptotic cell death was not detected during TM development. These findings were then used to assess two mouse models of ASD. Jag1 and Bmp4 heterozygous null mice display ASD phenotypes in the adult, including TM hypoplasia and corneal adherence to the iris. We further discovered that both mutants exhibited similar patterns of developmental TM dysgenesis at P1, P5, and P10. Our data indicate that P1 is an important time point in TM development and that TM dysgenesis in Jag1 and Bmp4 heterozygous null mice likely results from impaired TM cell migration and/or differentiation.

The post-natal development of intraocular pressure in normal domestic cats (Felis catus) and in feline congenital glaucoma

Intraocular pressure (IOP) is the most consistent risk factor for progressive vision loss in glaucoma. Cats with recessively inherited feline congenital glaucoma (FCG) exhibit elevated IOP with gradual, painless progression of glaucoma similar to humans and are studied as a model of glaucoma in humans and animals. Here, post-natal development of IOP was characterized in normal domestic cats and in cats with FCG caused by a homozygous LTBP2 mutation. Rebound tonometry (TonoVet^®, ICare Oy, Finland) was used to measure IOP non-invasively, 2-3 times weekly in 63 FCG and 33 normal kittens, of both sexes, from eyelid opening until 3-6 months of age. IOPs in the left and right eyes of both FCG and normal kittens were compared by paired t-test and linear regression. One-way ANOVA and Tukey-Kramer post-tests were used to compare IOP of cats grouped by age and disease status. A p-value <0.05 was considered significant. In the second week of life, mean IOP was 7.16 mmHg (SD = 1.3) in normal kittens and 8.72 mmHg (SD = 1.4) in kittens with FCG. Mean IOP at age 10 weeks was significantly higher in FCG (19.8 mmHg; 95% CI = 17.7, 21.9  mmHg) than in normal kittens (13.2 mmHg; 95% CI = 11.9, 14.5  mmHg). At 3 months of age, IOP in normal cats reached adult values while IOP in FCG cats continued to increase through at least six months of age. These results provide ranges for normal IOP values in young kittens and confirm that IOP is significantly higher than normal by 10wks of age in this spontaneous feline glaucoma model.

A Mutation in LTBP2 Causes Congenital Glaucoma in Domestic Cats (Felis catus)

The glaucomas are a group of diseases characterized by optic nerve damage that together represent a leading cause of blindness in the human population and in domestic animals. Here we report a mutation in LTBP2 that causes primary congenital glaucoma (PCG) in domestic cats. We identified a spontaneous form of PCG in cats and established a breeding colony segregating for PCG consistent with fully penetrant, autosomal recessive inheritance of the trait. Elevated intraocular pressure, globe enlargement and elongated ciliary processes were consistently observed in all affected cats by 8 weeks of age. Varying degrees of optic nerve damage resulted by 6 months of age. Although subtle lens zonular instability was a common feature in this cohort, pronounced ectopia lentis was identified in less than 10% of cats examined. Thus, glaucoma in this pedigree is attributed to histologically confirmed arrest in the early post-natal development of the aqueous humor outflow pathways in the anterior segment of the eyes of affected animals. Using a candidate gene approach, significant linkage was established on cat chromosome B3 (LOD 18.38, θ = 0.00) using tightly linked short tandem repeat (STR) loci to the candidate gene, LTBP2. A 4 base-pair insertion was identified in exon 8 of LTBP2 in affected individuals that generates a frame shift that completely alters the downstream open reading frame and eliminates functional domains. Thus, we describe the first spontaneous and highly penetrant non-rodent model of PCG identifying a valuable animal model for primary glaucoma that closely resembles the human disease, providing valuable insights into mechanisms underlying the disease and a valuable animal model for testing therapies.

Feline glaucoma--a comprehensive review

Cats with glaucoma typically present late in the course of disease. It is likely that glaucoma in cats is under-diagnosed due to its insidious onset and gradual progression, as well as limitations of some commonly used tonometers in this species. Treatment of glaucoma in feline patients presents a clinical challenge, particularly as glaucoma is often secondary to other disease processes in cats. In this review, we consider the clinical features, pathophysiology, and classification of the feline glaucomas and provide current evidence to direct selection of appropriate treatment strategies for feline glaucoma patients.

Prostaglandin pathway gene therapy for sustained reduction of intraocular pressure

Cyclooxygenase-2 (COX-2) is a rate-limiting enzyme in prostaglandin (PG) biosynthesis. In the eye, loss of COX-2 expression in aqueous humor-secreting cells has been associated with primary open-angle glaucoma (POAG). Reduction of intraocular pressure (IOP) is the main treatment goal in this disease. We used lentiviral vectors to stably express COX-2 and other PG biosynthesis and response transgenes in the ciliary body epithelium and trabecular meshwork (TM), the ocular suborgans that produce aqueous humor and regulate its outflow, respectively. We show that robust ectopic COX-2 expression and PG production require COX-2 complementary DNA (cDNA) sequence optimization. When COX-2 expression was coupled with a similarly optimized synthetic PGF2alpha receptor transgene to enable downstream signaling, gene therapy produced substantial and sustained reductions in IOP in a large animal model, the domestic cat. This study provides the first gene therapy for correcting the main cause of glaucoma.

Immunohistochemical characterization of dendritic cells and macrophages in the aqueous outflow pathways of the rat eye

Immunohistochemical studies were performed to determine the distribution, phenotype and ontogeny of macrophages and dendritic cells (DCs) in the aqueous humour outflow pathways of the rat eye. Optimal fixation and indirect immunoperoxidase techniques were employed in conjunction with a panel of mAbs on tangential frozen sections of ocular tissues from a total of 37 Wistar Furth rats aged 12-13 days (n = 8), 3 weeks (n = 12), 7 weeks (n = 5) and 15 weeks (n = 12). The density of immunopositive cells was scored qualitatively. A moderate to low density of Ia+ cells with a dendritic morphology were observed in the trabecular meshwork. DCs were also identified in the suprachoroidal space and in the connective tissues of nerves and vessels piercing the sclera, i.e. in association with non-conventional aqueous outflow pathways. The phenotypical and morphological characteristics of these cells would indicate that they may potentially act as antigen presenting cells (APCs). Non-dendritic pleomorphic cells with a macrophage phenotype were also identified in the trabecular meshwork, and bipolar or elongated cells with a macrophage phenotype were a noticeable feature in the perivascular region of collector channels and the limbal episcleral veins. Some macrophage and DC-like cells were observed in intimate association with limbal mast cells. Theories on the mechanisms of Anterior Chamber Associated Immune Deviation (ACAID) have assumed APCs are largely absent from the tissues lining the anterior chamber. Our findings of a low but moderate density of putative APCs in the conventional and non-conventional aqueous humour outflow pathways are discussed in relation to the various theories of ACAID.

A quantitative study of the prenatal development of the aqueous outflow system in the human eye

There is a multiplicity of theories on the development and differentiation of the human iridocorneal angle. Many have arisen from the need to explain the pathogenesis of primary infantile (congenital) glaucoma or other related developmental disturbances. The present study, which is the first light and electron microscopical morphometric analysis of human fetal eyes, was designed to determine the nature of the changes in various tissue components during trabecular anlage formation and differentiation (between 12 and 22 weeks of gestation). The results demonstrate that the trabecular anlage doubles in cross-sectional area during this period. However, when circumferential growth is considered there is in fact a threefold increase in anlage volume. Cell density in sections decreases over the 10-week period, as does the number of cells per unit volume. However, employment of stereological analysis together with corrections for volume changes revealed a two to threefold increase in absolute number of cells per eye (2.8 x 10(5) to 7.5 x 10(5)). Relative and absolute volume data on the four major constituents of the trabecular anlage show that extracellular matrix increases in the most significant and predictable fashion (360% in absolute volume), with the increase in intertrabecular spaces being more variable (200%). The findings are discussed in relation to previous theories of trabecular meshwork development.

Glaucoma in the dog and cat

Conjunctivitis, mydriasis, corneal edema, lens luxation, buphthalmos, and blindness are caused by elevated IOP in the glaucomas. Primary glaucomas with a bilateral potential for development are noted in two cats and several dog breeds, with secondary glaucomas caused by uveitis and neoplasia common in the cat and dog. Tonometric evaluation is essential for the early diagnosis and management of glaucoma. Medical therapy of topical demecarium bromide and timolol maleate, with systemic dichlorphenamide, are recommended for general glaucoma maintenance therapy in dogs and cats. Transcleral cyclophotocoagulation will surgically reduce IOP in cases in which maximum medication has been reached.

Anterior segment evaluation of infants with retinopathy of prematurity

To understand the mechanisms of glaucoma in retinopathy of prematurity (ROP), anterior segment evaluation is essential. The authors prospectively examined the anterior segment of 27 eyes of 17 premature infants with stages IV and V ROP. Twenty-six eyes received no previous surgery or treatment. Schiøtz and applanation tonometry were performed. Structural evaluation of each anterior segment was conducted by biomicroscopy and Koeppe gonioscopy. In the 26 eyes, angle closure of greater than 180 degrees was noted in 3 (12%). The authors noted prominent Schwalbe's line in 4 eyes (15%), high iris convexity in 15 (58%), hypopigmentation of the iris root in 19 (73%), translucent matrix in the angles ("Barkan's-type" membrane) in 18 (69%), posterior synechiae in 16 (62%), visible iris or angle vessels in 12 (46%), and pigment clumping in the angle recess in 12 (46%). This study identified structural abnormalities in the anterior segment of ROP infants, including pathologic changes and anatomic features that could have a developmental origin.

Human fetal iridocorneal angle: a light and scanning electron microscopic study

The iridocorneal angle and inner layers of the trabecular meshwork in human fetal eyes were studied by scanning electron microscopy. Tissue from 32 eyes of 17 fetuses with a gestational age from 12 to 22 weeks were investigated in order to determine the morphological changes in the cellular lining of the anterior chamber angle recess during development. The findings indicate that, although hexagonal corneal endothelial profiles extend almost to the angle apex in a few of the younger eyes examined (12-14 weeks), the lining is always perforated by a few discrete intercellular gaps (2-6 microns diameter). As development progresses it becomes clearer that the maturing meshwork is lined by uveal trabecular endothelial cells which are morphologically distinguishable from corneal endothelium. The frequency and size of the gaps between the inner uveal trabecular endothelial cells increase and are well developed by 18-20 weeks, clearly providing a route of communication between the fetal anterior chamber and the developing intercellular spaces in the primitive trabecular tissue. The implications of these observations on the 'Barkan's membrane' theory of congenital glaucoma are discussed.

A morphological study of the inner surface of the anterior chamber angle in pre and postnatal human eyes

The morphological changes in the inner surface of the human anterior chamber angle during pre and postnatal development were studied by light microscopy, scanning and transmission electron microscopy. Seventeen human foetal eyes (12-22 weeks) and ten infant and juvenile eyes (2.5 months--8 years) were investigated with the aim of establishing whether the trabecular meshwork is covered by an uninterrupted membrane at any stage in development. In 12-14 week old foetal eyes cuboidal corneal endothelial cells cover the inner surface of the anterior half to one third of the trabecular anlage. In this region there is a transition to more flattened uveal trabecular endothelial cells. Only rarely did corneal endothelial cells extend completely to the angle apex. The transition zone between corneal and uveal trabecular endothelial cells becomes located more anteriorly as development progresses. Intercellular gaps, which occur between uveal trabecular endothelial cells as early as 12-14 weeks in development, enlarge and become more frequent during development thus providing a route of communication between the anterior chamber and the developing intertrabecular spaces or channels. In the first months of life only a few cord-like uveal trabeculae orientated predominantly in a meridional direction overlie the more lamellate corneoscleral trabeculae. By the second year typical uveal trabeculae are more prominent and form a web-like arrangement. This is accompanied by a gradual decrease in the frequency of the cytoplasmic extensions of uveal trabecular endothelial cells which circumscribe the intratrabecular gaps. The timing of this remodelling and maturation appears to be remarkably variable between individuals. The implications of these findings on the theories of the pathogenesis of congenital glaucoma are discussed.

Establishment of calf trabecular meshwork cell cultures

Trabecular meshwork cell cultures have been grown from excised calf aqueous outflow pathway tissue using two different methods. In one method, tissue explants were placed directly in culture dishes and primary cultures were established by those cells migrating from the tissue. In the second method, a cell suspension was generated by incubating trabecular tissue strips with 0.1% pronase, and the washed cells were used to seed primary cultures. The cell cultures generated by each method appeared to progress through morphological and biochemical changes characteristic of the cell type, although at different respective rates. Comparisons of the cell morphologies, biosynthetic activities, and growth characteristics of the respective subcultures suggested that similar cell populations were obtained by each method. These studies further suggested that sufficient quantities of calf trabecular cells could be grown in culture to permit biochemical and pharmacological studies of trabecular-cell metabolism.