Anterior Segment Development Research Articles

Morphogenesis of the anterior segment in the zebrafish eye.

BackgroundThe ocular anterior segment is critical for focusing incoming light onto the neural retina and for regulating intraocular pressure. It is comprised of the cornea, lens, iris, ciliary body, and highly specialized tissue at the iridocorneal angle. During development, cells from diverse embryonic lineages interact to form the anterior segment. Abnormal migration, proliferation, differentiation, or survival of these cells contribute to diseases of the anterior segment such as corneal dystrophy, lens cataract, and glaucoma. Zebrafish represent a powerful model organism for investigating the genetics and cell biology of development and disease. To lay the foundation for genetic studies of anterior segment development, we have described the morphogenesis of this structure in zebrafish.ResultsAs in other vertebrates, the zebrafish anterior segment derives from diverse origins including surface ectoderm, periocular mesenchyme, and neuroepithelium. Similarly, the relative timing of tissue differentiation in the anterior segment is also conserved with other vertebrates. However, several morphogenic features of the zebrafish anterior segment differ with those of higher vertebrates. These include lens delamination as opposed to invagination, lack of iris muscles and ciliary folds, and altered organization in the iridocorneal angle. In addition, substantial dorsal-ventral differences exist within the zebrafish anterior segment.ConclusionCumulatively, our anatomical findings provide a reference point to utilize zebrafish for genetic studies into the mechanisms of development and maintenance of the anterior segment.

High Myopia Associated With Retinopathy of Prematurity Is Primarily Lenticular

To determine which of the eye's refractive components are responsible for the high myopia in retinopathy of prematurity (ROP), as compared with highly myopic eyes in full-term patients. The study included 53 highly myopic eyes in 34 patients with a history of ROP, and 66 highly myopic eyes in 37 full-term patients. Measurements included refraction, keratometry, and A-scan values for axial length, lens thickness, lens position, anterior chamber depth, anterior segment depth, and lens power calculations. Comparisons were also made with published age-matched, full-term normal controls. Refractions ranged from a spherical-equivalent of -5.0 to -20.75, and from -5.0 to -22.0 diopters in ROP and full-term eyes, respectively. For ROP eyes, increasing myopia was most associated with lens thickness and lens power ( P < 0.001), with lesser contributions from corneal steepness, axial length, and a more forward position of the lens's center. For the eyes with myopia in full-term patients, increasing myopia was highly associated with axial length ( P < 0.001), with smaller contributions from increased lens thickness and lens power. ROP eyes had a lens-thickness/anterior-chamber-depth ratio almost 50% higher than FT and normative eyes. Anterior segment depth was remarkably similar in all eyes studied. High myopia associated with ROP appears pathophysiologically distinct from high myopia in full-term patients. The increased lens thickness seen in ROP eyes was accompanied by shallower anterior chamber depth and maintenance of the anterior segment depth, similar to the normal neonatal eye, suggesting a mechanism of altered anterior segment development in ROP leading to high myopia.

Anterior segment development relevant to glaucoma

Development of the ocular anterior segment involves a series of inductive interactions between neural ectoderm, surface ectoderm and periocular mesenchyme. The timing of these events is well established but less is known about the molecular mechanisms involved. Various genes that participate in these processes have been identified. As the roles of more genes are determined, developmental pathways and networks will emerge. Here, we focus on recent advances made using mouse models. We summarize key morphological events in formation of anterior chamber structures, including the aqueous humor drainage structures that are involved in intraocular pressure (IOP) regulation and glaucoma. We discuss the developmental roles of genes that associate with abnormal anterior segment development and elevated IOP or glaucoma (including Bmp4, Cyp1b1, Foxc1, Foxc2, Pitx2, Lmx1b and Tyr ) and how some of these genes may fit into developmental networks.

A dominant mutation within the DNA-binding domain of the bZIP transcription factor Maf causes murine cataract and results in selective alteration in DNA binding

The murine autosomal dominant cataract mutants created in mutagenesis experiments have proven to be a powerful resource for modelling the biological processes involved in cataractogenesis. We report a mutant which in the heterozygous state exhibits mild pulverulent cataract named 'opaque flecks in lens', symbol Ofl. By molecular mapping, followed by a candidate gene approach, the mutant was shown to be allelic with a knockout of the bZIP transcription factor, Maf. Homozygotes for Ofl and for Maf null mutations are similar but a new effect, renal tubular nephritis, was found in Ofl homozygotes surviving beyond 4 weeks, which may contribute to early lethality. Sequencing identified the mutation as a G-->A change, leading to the amino-acid substitution mutation R291Q in the basic region of the DNA-binding domain. Since mice heterozygous for knockouts of Maf show no cataracts, this suggests that the Ofl R291Q mutant protein has a dominant effect. We have demonstrated that this mutation results in a selective alteration in DNA binding affinities to target oligonucleotides containing variations in the core CRE and TRE elements. This implies that arginine 291 is important for core element binding and suggests that the mutant protein may exert a differential downstream effect amongst its binding targets. The cataracts seen in Ofl heterozygotes and human MAF mutations are similar to one another, implying that Ofl may be a model of human pulverulent cortical cataract. Furthermore, when bred onto a different genetic background Ofl heterozygotes also show anterior segment abnormalities. The Ofl mutant therefore provides a valuable model system for the study of Maf, and its interacting factors, in normal and abnormal lens and anterior segment development.

Disruption of anterior segment development by TGF-beta1 overexpression in the eyes of transgenic mice.

Previous experiments showed that transgenic mice expressing a secreted self-activating transforming growth factor (TGF) -beta1 did not show a phenotype in the lens and cornea until postnatal day 21, when anterior subcapsular cataracts, sporadic thickening of the corneal stroma, and thinning of the corneal epithelium were noted (Srinivasan et al., 1998). To examine the effects of higher concentrations of TGF-beta1 on the lens and cornea, we constructed transgenic mice harboring the strong, lens-specific chicken betaB1-crystallin promoter driving an activated porcine TGF-beta1 gene. In contrast to the earlier study, the transgenic mice had microphthalmic eyes with closed eyelids. Already at embryonic day (E) 13.5, the future cornea of the transgenic mice was threefold thicker than that of wild-type littermates due to increased proliferation of corneal stromal mesenchyme cells. Staining of fibronectin and thrombospondin-1 was increased in periocular mesenchyme. At E17.5, the thickened transgenic corneal stroma was vascularized and densely populated by abundant star-shaped, neural cell adhesion molecule-positive cells of mesenchymal appearance surrounded by irregular swirls of collagen and extracellular matrix. The corneal endothelium, anterior chamber, and stroma of iris/ciliary body did not develop, and the transgenic cornea was opaque. Fibronectin, perlecan, and thrombospondin-1 were elevated, whereas type VI collagen decreased in the transgenic corneal stroma. Stromal mesenchyme cells expressed alpha-smooth muscle actin as did lens epithelial cells and cells of the retinal pigmented epithelium. By E17.5, lens fiber cells underwent apoptotic cell death that was followed by apoptosis of the entire anterior lens epithelium between E18.5 and birth. Posteriorly, the vitreous humor was essentially absent; however, the retina appeared relatively normal. Thus, excess TGF-beta1, a mitogen for embryonic corneal mesenchyme, severely disrupts corneal and lens differentiation. Our findings profoundly contrast with the mild eye phenotype observed with presumably lower levels of ectopic TGF-beta and illustrate the complexity of TGF-beta utilization and the importance of dose when assessing the effects of this growth factor.

The Homeobox Gene Six3 Is a Potential Regulator of Anterior Segment Formation in the Chick Eye

The anterior segment of the vertebrate eye consists of highly organized and specialized ocular tissues critical for normal vision. The periocular mesenchyme, originating from the neural crest, contributes extensively to the anterior segment. During chick eye morphogenesis, the homeobox gene Six3 is expressed in a subset of periocular mesenchymal cells and in differentiating anterior segment tissues. Retrovirus-mediated misexpression of Six3 causes eye anterior segment malformation, including corneal protrusion and opacification, ciliary body and iris hypoplasia, and trabecular meshwork dysgenesis. Histological and molecular marker analyses demonstrate that Six3 misexpression disrupts the integrity of the corneal endothelium and the expression of extracellular matrix components critical for corneal transparency. Six3 misexpression also leads to a reduction of the periocular mesenchymal cell population expressing Lmx1b, Pitx2, and Pax6, transcription factors critical for eye anterior segment morphogenesis. Moreover, elevated levels of Six3 attenuate proliferation of periocular mesenchymal cells in vitro and differentiating anterior segment tissues in vivo. These results suggest that, in addition to its function in eye primordium determination, Six3 plays a role in regulating the development of the vertebrate eye anterior segment.

Genetic basis of glaucoma.

The application of molecular genetic techniques to the study of glaucoma has accelerated greatly during the past few years. In addition to localizing and identifying genes for specific types of glaucoma, researchers have begun to characterize the gene products and investigate molecular mechanisms involved in glaucoma. Much research has been focused on the gene expression, protein processing, and mutations of MYOC/TIGR, which is associated with both juvenile-and adult-onset primary open angle glaucoma. Investigations of other glaucoma-related genes, such as PITX2, FOXC1, and CYP1B1, are enabling a better understanding of anterior segment development and its relation to glaucoma.

Domain disruption and mutation of the bZIP transcription factor, MAF, associated with cataract, ocular anterior segment dysgenesis and coloboma.

Human congenital cataract and ocular anterior segment dysgenesis both demonstrate extensive genetic and phenotypic heterogeneity. We identified a family where ocular developmental abnormalities (cataract, anterior segment dysgenesis and microphthalmia) co-segregated with a translocation, t(5;16)(p15.3;q23.2), in both balanced and unbalanced forms. We hypothesized that this altered the expression of a gene of developmental significance in the human lens and ocular anterior segment. Cloning the 16q23.2 breakpoint demonstrated that it transected the genomic-control domain of MAF, a basic region leucine zipper (bZIP) transcription factor, first identified as an oncogene, which is expressed in vertebrate lens development and regulates the expression of the eye lens crystallins. The homozygous null mutant Maf mouse embryo demonstrates defective lens formation and microphthalmia. Through mutation screening of a panel of patients with hereditary congenital cataract we identified a mutation in MAF in a three-generation family with cataract, microcornea and iris coloboma. The mutation results in the substitution of an evolutionarily highly conserved arginine with a proline at residue 288 (R288P) in the basic region of the DNA-binding domain of MAF. Our findings further implicate MAF/Maf in mammalian lens development and highlight the role of the lens in anterior segment development. The 16q23.2 breakpoint transects the common fragile site, FRA16D, providing a molecular demonstration of a germline break in a common fragile site.

Haploinsufficient Bmp4 ocular phenotypes include anterior segment dysgenesis with elevated intraocular pressure

BackgroundGlaucoma is a blinding disease usually associated with high intraocular pressure (IOP). In some families, abnormal anterior segment development contributes to glaucoma. The genes causing anterior segment dysgenesis and glaucoma in most of these families are not identified and the affected developmental processes are poorly understood. Bone morphogenetic proteins (BMPs) participate in various developmental processes. We tested the importance of Bmp4 gene dosage for ocular development and developmental glaucoma.ResultsBmp4+/- mice have anterior segment abnormalities including malformed, absent or blocked trabecular meshwork and Schlemm's canal drainage structures. Mice with severe drainage structure abnormalities, over 80% or more of their angle's extent, have elevated IOP. The penetrance and severity of abnormalities is strongly influenced by genetic background, being most severe on the C57BL/6J background and absent on some other backgrounds. On the C57BL/6J background there is also persistence of the hyaloid vasculature, diminished numbers of inner retinal cells, and absence of the optic nerve.ConclusionsWe demonstrate that heterozygous deficiency of BMP4 results in anterior segment dysgenesis and elevated IOP. The abnormalities are similar to those in human patients with developmental glaucoma. Thus, BMP4 is a strong candidate to contribute to Axenfeld-Rieger anomaly and other developmental conditions associated with human glaucoma. BMP4 also participates in posterior segment development and wild-type levels are usually critical for optic nerve development on the C57BL/6J background. Bmp4+/- mice are useful for studying various components of ocular development, and may allow identification of strain specific modifiers affecting a variety of ocular phenotypes.

Haploinsufficiency of the transcription factors FOXC1 and FOXC2 results in aberrant ocular development.

Anterior segment developmental disorders, including Axenfeld-Rieger anomaly (ARA), variably associate with harmfully elevated intraocular pressure (IOP), which causes glaucoma. Clinically observed dysgenesis does not correlate with IOP, however, and the etiology of glaucoma development is not understood. The forkhead transcription factor genes Foxc1 (formerly Mf1 ) and Foxc2 (formerly Mfh1 ) are expressed in the mesenchyme from which the ocular drainage structures derive. Mutations in the human homolog of Foxc1, FKHL7, cause dominant anterior segment defects and glaucoma in various families. We show that Foxc1 (+/-)mice have anterior segment abnormalities similar to those reported in human patients. These abnormalities include small or absent Schlemm's canal, aberrantly developed trabecular meshwork, iris hypoplasia, severely eccentric pupils and displaced Schwalbe's line. The penetrance of clinically obvious abnormalities varies with genetic background. In some affected eyes, collagen bundles were half normal diameter, or collagen and elastic tissue were very sparse. Thus, abnormalities in extracellular matrix synthesis or organization may contribute to development of the ocular phenotypes. Despite the abnormalities in ocular drainage structures in Foxc1 (+/-)mice, IOP was normal in almost all mice analyzed, on all genetic backgrounds and at all ages. Similar abnormalities were found in Foxc2 (+/-)mice, but no disease-associated mutations were identified in the human homolog FKHL14 in 32 ARA patients. Foxc1 (+/-)and Foxc2 (+/-)mice are useful models for studying anterior segment development and its anomalies, and may allow identification of genes that interact with Foxc1 and Foxc2 (or FKHL7 and FKHL14 ) to produce a phenotype with elevated IOP and glaucoma.

Lmx1b, a LIM homeodomain class transcription factor, is necessary for normal development of multiple tissues in the anterior segment of the murine eye

Proper development of the anterior segment of the mammalian eye is critical for normal ocular function. Indeed, several congenital syndromes associated with anterior segment anomalies can lead to impaired vision and glaucoma. One such syndrome is nail patella syndrome (NPS), caused by haploinsufficiency for the LIM-homeodomain transcription factor LMX1B. Although mutations in LMX1B cosegregate with NPS, whether these mutations cause the glaucoma associated with NPS is not known. Here, we provide evidence that the LIM-homeodomain transcription factor lmx1b is an essential regulator of murine anterior segment development. Mice that are homozygous for a targeted mutation of lmx1b display iris and ciliary body hypoplasia, and cornea stromal defects. In addition, two cDNAs normally downregulated in presumptive cornea, mf1 and mfh1, exhibit persistent expression, while keratocan, a keratin sulfate proteoglycan expressed by keratocytes, is not detected in mutant corneas. Moreover, ultrastructural examination of homozygous mutants indicates that corneal collagen fibrillogenesis is perturbed. Taken together, our studies suggest a developmental etiology for glaucoma in NPS patients and highlight lmx1b as an essential regulator of anterior segment morphogenesis and patterning. genesis 26:15-25, 2000.

Anterior segment dysgenesis in mosaic Turner syndrome

AIMS/BACKGROUNDFemales with Turner syndrome commonly exhibit ophthalmological abnormalities, although there is little information in the literature documenting findings specific to Turner syndrome mosaics. Ophthalmic findings are described in four patients...

Ring Stromal Opacity in Lens-Corneal Adhesion

Severe congenital corneal clouding may be associated with abnormal anterior segment development (dysgenesis). Often, congenital corneal opacities preclude visualization of intraocular anatomy. In this report, five eyes of four patients with congenital corneal clouding and lens-corneal adhesion (LCA) are described. All had a ring- or doughnut-shaped haze within the stroma surrounding a central clear corneal zone delineating the adhesion. Recognition of LCA is pivotal in planning surgical therapy for eyes with congenital corneal clouding. As a clue to recognition of this anomaly, the ring-shaped stromal opacity may aid in diagnosis of this phenomenon and in avoidance of surgical complications.

Role of the Neural Crest in Anterior Segment Development and Disease

Certain eye and associated systemic developmental anomalies are apparently related by virture of a common neural crest origin. The development of the anterior segment is extremely complex and is dependent upon the presence or absence of certain local factors (including extracellular matrices and glycoproteins), inductors, receptors, and specific time sequencing. Understanding anterior segment anomalies and their systemic associations requires an understanding of neural crest proliferation and migration patterns; and they may be unified under the designation of neurocristopathies. Goldenhar's syndrome, not previously considered a neurocristopathy, may be considered one on the basis of the relationship between clinical findings and neural crest embryology.

Ophthalmic changes from age of 10 to 18 years. A longitudinal study of sequels to low birth weight. III. Ultrasound oculometry and keratometry of anterior eye segment.

Anterior eye segment features were investigated longitudinally by ultrasound and keratometry in 70 low-birth-weight subjects (LBW less than 2000 g) and 67 full-term controls, at the ages of 10 and 18 years. Concerning the changes during adolescence, a slight significant deepening of anterior chamber and lens position is found (about 0.1 mm), while lens thickness, corneal curvature radius and corneal astigmatism have remained stable. Differences between BW-groups indicate a permanent influence of LBW on ocular development, as evident from a presumed early environmental (LBW) arrest of corneal growth, given by a significantly lower value of corneal curvature radius, just as zonular slackness due to a smaller suspension ring may explain a thicker lens. Further a LBW disturbance of correlation between refraction and anterior eye segment parameters is suggested. Besides the documented occasional posterior eye segment damage of LBW (retinopathy of prematurity), the present study indicates a more general restraining influence also on anterior eye segment development.