Optic cup morphogenesis (OCM) generates the basic structure of the vertebrate eye. Although it is commonly depicted as a series of epithelial sheet folding events, this does not represent an empirically supported model. Here, we combine four-dimensional imaging with custom cell tracking software and photoactivatable fluorophore labeling to determine the cellular dynamics underlying OCM in zebrafish. Although cell division contributes to growth, we find it dispensable for eye formation. OCM depends instead on a complex set of cell movements coordinated between the prospective neural retina, retinal pigmented epithelium (RPE) and lens. Optic vesicle evagination persists for longer than expected; cells move in a pinwheel pattern during optic vesicle elongation and retinal precursors involute around the rim of the invaginating optic cup. We identify unanticipated movements, particularly of central and peripheral retina, RPE and lens. From cell tracking data, we generate retina, RPE and lens subdomain fate maps, which reveal novel adjacencies that might determine corresponding developmental signaling events. Finally, we find that similar movements also occur during chick eye morphogenesis, suggesting that the underlying choreography is conserved among vertebrates.
Zebrafish; Eye; Morphogenesis; Cell tracking; Retina; Retinal pigmented epithelium (RPE); Lens; Fate map
How the eye forms has been one of the fundamental issues in developmental biology. The retinal anlage first appears as the optic vesicle (OV) evaginating from the forebrain. Subsequently, its distal portion invaginates to form the two-walled optic cup, which develops into the outer pigmented and inner neurosensory layers of the retina. Recent work has shown that this optic-cup morphogenesis proceeds as a self-organizing activity without any extrinsic molecules. However, intrinsic factors that regulate this process have not been elucidated. Here we show that a LIM-homeobox gene, Lhx1, normally expressed in the proximal region of the nascent OV, induces a second neurosensory retina formation from the outer pigmented retina when overexpressed in the chicken OV. Lhx2, another LIM-homeobox gene supposed to be involved in early OV formation, could not substitute this function of Lhx1, while Lhx5, closely related to Lhx1, could replace it. Conversely, knockdown of Lhx1 expression by RNA interference resulted in the formation of a small or pigmented vesicle. These results suggest that the proximal region demarcated by Lhx1 expression permits OV development, eventually dividing the two retinal domains.
Lhx1; Lhx5; Optic vesicle; LIM-homeobox; Chicken; Eye development; Neural retina
Organogenesis of the eye is a multi-step process that starts with the formation of optic vesicles followed by invagination of the distal domain of the vesicles and the overlying lens placode resulting in morphogenesis of the optic cup. The late optic vesicle becomes patterned into distinct ocular tissues; the neural retina, retinal pigment epithelium (RPE) and optic stalk. Multiple congenital eye disorders, including anophthalmia or microphthalmia, aniridia, coloboma and retinal dysplasia, stem from disruptions in embryonic eye development. Thus, it is critical to understand the mechanisms that lead to initial specification and differentiation of ocular tissues. An accumulating number of studies demonstrate that a complex interplay between inductive signals provided by tissue-tissue interactions and cell-intrinsic factors is critical to ensure proper specification of ocular tissues as well as maintenance of RPE cell fate. While several of the extrinsic and intrinsic determinants have been identified, we are just at the beginning to understand how these signals are integrated. In addition, we know very little about the actual output of these interactions. In this chapter, we provide an update of the mechanisms controlling early steps of eye development in vertebrates, with emphasis on optic vesicle evagination, specification of neural retina and RPE at the optic vesicle stage, the process of invagination during morphogenesis of the optic cup and maintenance of the RPE cell fate.
Wnt2b is normally expressed at the optic cup lip and is implicated in ciliary body induction. The lens has often been considered an organizer for the anterior eye, but recent studies demonstrate that the anterior cell fates are correctly specified in the absence of the lens. This study uses Wnt2b as a marker to reveal the mechanism behind the specification of the anterior domain of the optic cup.
Developing chick embryos were used as a model system. Eyes were microsurgically manipulated to assess the role of the lens in the development of the anterior optic cup. Eyes were molecularly manipulated, using fibroblast growth factor expressing replication-incompetent retrovirus, introduced into the retinal pigmented epithelium (RPE) domain. Ectopic fibroblast growth factor transformed the RPE into nonpigmented epithelium (NPE; ciliary body). As the virus does not spread, discrete borders between RPE and NPE were experimentally created. Wnt2b expression was assessed after surgical and molecular manipulation.
Contrary to expectations, we found that the lens is not able to induce Wnt2b expression in optic cup tissue: When the optic cup lip is experimentally misspecified such that it no longer contains the juxtaposition of pigmented and nonpigmented tissue, Wnt2b is not expressed. In addition, if the prelens ectoderm is removed from the optic vesicle before morphogenesis, the resulting lensless optic cup expresses Wnt2b even though it was not in contact with lens tissue. We also show that ectopic lenses do not induce Wnt2b in optic cup tissue. The ciliary body/anterior eye domain is specified at the border of RPE and the NPE of the ciliary body. During development, this border is normally found at the optic cup lip. We can manipulate tissue specification using retroviral-mediated gene transfer, and create ectopic borders between nonpigmented and pigmented tissue. At such borders, Wnt2b is ectopically expressed in the absence of lens contact. Finally, we describe a role for the lens in maintenance of Wnt2b expression and demonstrate support for this in two ways: First, we show that if the lens is removed from the formed optic cup, endogenous Wnt2b expression is specifically lost from the optic cup lip; and second, we show that while ectopic Wnt2b expression is initially found in the majority of ectopic borders, as eye development proceeds ectopic expression is maintained only in those borders that are close to the lens.
Taken together, the results provide support for a model in which the anterior optic cup domain, as described in part by Wnt2b expression, is specified through the elaboration of a border within the optic neuroepithelium rather than through interactions with the surrounding environment.
The bending of cell sheets plays a major role in multicellular embryonic morphogenesis. Recent advances are leading to a deeper understanding of how the biophysical properties and the force-producing behaviors of cells are regulated, and how these forces are integrated across cell sheets during bending. We review work that shows that the dynamic balance of apical versus basolateral cortical tension controls specific aspects of invagination of epithelial sheets, and recent evidence that tissue expansion by growth contributes to neural retinal invagination in a stem cell-derived, self-organizing system. Of special interest is the detailed analysis of the type B inversion in Volvox reported in BMC Biology by Höhn and Hallmann, as this is a system that promises to be particularly instructive in understanding morphogenesis of any monolayered spheroid system.
See research article: http://www.biomedcentral.com/1741-7007/9/89
Optic cups are a structural feature of diverse eyes, from simple pit eyes to camera eyes of vertebrates and cephalopods. We used the planarian prototypic eye as a model to study the genetic control of optic cup formation and regeneration. We identified two genes encoding transcription factors, sp6-9 and dlx, that were expressed in the eye specifically in the optic cup and not the photoreceptor neurons. RNAi of these genes prevented formation of visible optic cups during regeneration. Planarian regeneration requires an adult proliferative cell population with stem cell-like properties called the neoblasts. We found that optic cup formation occurred only after migration of progressively differentiating progenitor cells from the neoblast population. The eye regeneration defect caused by dlx and sp6-9 RNAi can be explained by a failure to generate these early optic cup progenitors. Dlx and Sp6-9 genes function as a module during the development of diverse animal appendages, including vertebrate and insect limbs. Our work reveals a novel function for this gene pair in the development of a fundamental eye component, and it utilizes these genes to demonstrate a mechanism for total organ regeneration in which extensive cell movement separates new cell specification from organ morphogenesis.
Some invertebrates, such as planarians and Hydra, can regenerate fully after amputations that remove large parts of the body. We investigated how cells in the body of planarians provide new cells for eye regeneration after complete head removal. Planarians possess highly potent regenerative cells (neoblasts) in a compartment inside the worm, and these cells must be present in a body fragment for it to regenerate. We identify a pair of transcription factors, sp6-9 and dlx, that are expressed in the optic cup, and use expression of these genes as markers to demonstrate that lineage restriction of eye cells during regeneration begins within the neoblast compartment. dlx and sp6-9 are essential for formation of optic cup progenitors, and inhibition of these genes with RNA interference results in eyes that lack optic cups after regeneration. During eye development in both flies and vertebrates, progenitors form within a patterned epithelium. Interestingly, planarian eye precursors only aggregate once they have stopped cycling and undergone extensive migration. At this stage they already express markers of the terminally differentiated state. Therefore, we identify a mechanism for eye formation during regeneration and a novel function for a conserved gene pair in eye regeneration.
In this study we have characterized the ocular defects in the recessive zebrafish mutant blowout that presents with a variably penetrant coloboma phenotype. blowout mutants develop unilateral or bilateral colobomas and as a result, the retina and retinal pigmented epithelium are not contained within the optic cup. Colobomas result from defects in optic stalk morphogenesis whereby the optic stalk extends into the retina and impedes the lateral edges of the choroid fissure from meeting and fusing. The expression domain of the proximal optic vesicle marker pax2a is expanded in blowout at the expense of the distal optic vesicle marker pax6, suggesting that the initial patterning of the optic vesicle into proximal and distal territories is disrupted in blowout. Later aspects of distal optic cup formation (i.e. retina development) are normal in blowout mutants, however. Positional cloning of blowout identified a nonsense mutation in patched1, a negative regulator of the Hedgehog pathway, as the underlying cause of the blowout phenotype. Expanded domains of expression of the Hedgehog target genes patched1 and patched2 were observed in blowout, consistent with a loss of Patched1 function and upregulation of Hedgehog pathway activity. Moreover, colobomas in blowout could be suppressed by pharmacologically inhibiting the Hedgehog pathway with cyclopamine, and maximal rescue occurred when embryos were exposed to cyclopamine between 5.5 and 13 hours post fertilization. These observations highlight the critical role that Hedgehog pathway activity plays in mediating patterning of the proximal/distal axis of the optic vesicle during the early phases of eye development and they provide genetic confirmation for the integral role that patched1-mediated negative regulation of Hedgehog signaling plays during vertebrate eye development.
zebrafish; optic vesicle; coloboma; Hedgehog; Patched1
The optic cup is created through invagination of the optic vesicle. The morphogenetic rearrangement creates a double-layered cup, with a hinge (the Optic Cup Lip) where the epithelium bends back upon itself. Shortly after the optic cup forms, it is thought to be sub-divided into separate lineages: i) pigmented epithelium in the outer layer; ii) presumptive iris and ciliary body at the most anterior aspect of the inner layer; and iii) presumptive neural retina in the remainder of the inner layer. We test the native developmental potential of the anterior cup to determine if it normally contributes to the retina.
Vital dye and green fluorescent protein (GFP) expressing replication-incompetent retroviral vectors were used to label cells in the nascent optic cup and follow their direct progeny throughout development. Label was applied to either the optic cup lip (n=40), or to the domain just posterior to the lip (n=20). Retroviral labeling is a permanent lineage marker and enabled the analysis of advanced stages of development.
Labeling within the optic cup gave rise to labeled progeny in the posterior optic cup that differentiated as neural retina (20 of 20). In contrast, labeling cells in the optic cup lip gave rise to progeny of labeled cells arrayed in a linear progression, from the lip into the neural retina (36 of 40). Label was retained in cells at the optic cup lip, regardless of age at examination. In older embryos, labeled progeny delaminated from the optic cup lip to differentiate as muscle of the pupillary margin.
The data show that the cells at the optic cup lip are a common progenitor population for pigmented epithelium, anterior eye tissues (ciliary body, iris, and pupillary muscle) and retinal neurons. The findings are supportive of an interpretation where the optic cup lip is a specialized niche containing a multipotent progenitor population.
Tbx2 is a T-box transcription factor gene that is dynamically expressed in the presumptive retina during optic vesicle invagination. Several findings implicate Tbx2 in cell cycle regulation, including its overexpression in tumours and regulation of proliferation during heart development. We investigated the role of Tbx2 in optic cup formation by analysing mice with a targeted homozygous mutation in Tbx2. Loss of Tbx2 caused a reduced presumptive retinal volume due to increased apoptosis, and a delay in ventral optic vesicle invagination leading to the formation of small and abnormally shaped optic cups. Tbx2 is essential for maintenance, but not induction of expression of the dorsal retinal determinant, Tbx5, and acts downstream of Bmp4, a dorsally expressed gene implicated in human microphthalmia. The small retina showed a hypocellular ventral region, loss of Fgf15, normally expressed in proliferating central retinal cells, and increased numbers of mitotic cells in the dorsal region, indicating that Tbx2 is required for normal growth and development across the D-V axis. Dorsal expression of potential regulators of retinal growth, Cyp1b1 and Cx43, and the topographic guidance molecule ephrinb2, was increased, and intra-retinal axons were disorganised resulting in a failure of optic nerve formation. Our data provide evidence that Tbx2 is required for proper optic cup formation and plays a critical early role in regulating regional retinal growth and the acquisition of shape during optic vesicle invagination.
A three-dimensional model of the left acetabulum with inserted threaded cup has been generated, based on the finite element method, to calculate stress patterns in the standing phase during walking. In this study, a hemispherical cup with sharp threads, a parabolic cup with flat threads and a conical cup with sharp threads were analysed and compared. Stress patterns in both implant components and adjacent bony structures were calculated in a directly postoperative situation. The different cups were found to induce different stress patterns, deformations and shifting tendencies. The inlays deform notably and show characteristic rotational movement patterns together with the shell. The inclination angle increases in the hemispherical cup and decreases in the parabolic cup. The conical cup levers outward almost parallel to the bone stock by approximately 0.05 mm. The pole surfaces of the various cups – especially the very convex area next to the threads – induce increased compressive stress in the superior section of the acetabular base. This is increased by a factor of three in the conical cup in comparison to the hemispherical cup and less so in comparison to the parabolic cup. This study illustrates that three-dimensional stress calculations are suitable for procuring additional biomechanical information to augment clinical studies, for evaluating implants and for establishing stability prognoses, especially for newly developed prototypes.
Pax2 is essential for the development of the urogenital system, neural tube, otic vesicle, optic cup and optic tract. Within the visual system, a loss-of-function leads to lack of choroid fissure closure (known as a coloboma), a loss of optic nerve astrocytes, and anomalous axonal pathfinding at the optic chiasm. This study is directed at determining the effects of ectopic Pax2 expression in the chick ventral optic cup past the normal developmental period when Pax2 is found. In ovo electroporation of Pax2 into the chick ventral optic cup results in the formation of colobomas, a condition typically associated with a loss of Pax2 expression. While the overexpression of Pax2 appears to phenocopy a loss of Pax2, the mechanism of the failure of choroid fissure closure is associated with a cell fate switch from ventral retina and retinal pigmented epithelium (RPE) to an astrocyte fate. Further, ectopic expression of Pax2 in RPE appears to have non-cell autonomous effects on adjacent RPE, creating an ectopic neural retina in place of the RPE.
Retina; Pax2; Chick; Optic Cup; Coloboma; Ectopic retina
The accuracy and reproducibility of the optic nerve head analyser (ONHA) and the Heidelberg retina tomograph (HRT) were compared and the performance of the HRT in measuring fundus elevations was evaluated. The coefficient of variation of three repeated measurements in a model eye and in volunteers and the relative error in a model eye was calculated. With ONHA measurements the pooled coefficient of variation in volunteers was 9.3% in measuring cup areas and 8.4% in measuring the cup volume. In a model eye the pooled coefficient of variation was 7.6% for the parameter 'cup area' and 9.9% for the parameter 'cup volume'. The pooled relative error in the model eye was 6.6% for the parameter 'cup area' and 5.1% for the parameter 'cup volume'. With HRT measurements in volunteers the pooled coefficient of variation of both the parameters 'volume below contour' and 'volume below surface' was 6.9%. In the model eye the pooled coefficient of variation was 2.4% for the 'volume below contour' and 4.1% for the parameter 'volume below surface'. The pooled relative error in the model eye was 11.3% for the 'volume below contour' and 11% for the 'volume below surface'. The pooled relative error in measuring retinal elevations in the model eye was 3.8%. The coefficient of variation was 3.5%. The accuracies of the HRT and ONHA were similar. However, as the ONHA 'cup volume' is unreliable in patients because of the design of the ONHA whereas the HRT volume parameters are reliable it seems reasonable to assume that the HRT is superior to the ONHA. Only the HRT is capable of quantifying retinal elevations.
The majority of vertebrate species have a layer of hyaline cartilage within the fibrous sclera giving an extra degree of support to the eyeball. In chicks, this is seen as a cuplike structure throughout the scleral layer. However, the mechanisms that control the development of scleral cartilage are largely unknown.
Here we have studied the phases of scleral cartilage development and characterised expression profiles of genes activated during the cartilage differentiation programme. CART1 and SOX9, the earliest markers of pre-committed cartilage, are expressed in the mesenchyme surrounding the optic cup. Later AGGRECAN, a matrix protein expressed during chondrocyte differentiation, is also expressed. The expression of these genes is lost following early removal of the optic cup, suggesting a role for this tissue in inducing scleral cartilage. By grafting young retinal pigment epithelium (RPE) and retina into cranial mesenchyme in vivo, it was found that RPE alone has the ability to induce cartilage formation.
There are some exceptions within the vertebrates where scleral cartilage is not present; one such example is the placental mammals. However, we found that the cartilage differentiation pathway is initiated in mice as seen by the expression of Cart1 and Sox9, but expression of the later cartilage marker Aggrecan is weak. Furthermore, cartilage forms in mouse peri-ocular mesenchyme micromass culture. This suggests that the process halts in vivo before full differentiation into cartilage, but that murine scleral mesenchyme has retained the potential to make cartilage in vitro.
RA, Wnts and Bmps have been linked to the cartilage development process and are expressed within the developing RPE. We find that RA may have a role in early scleral cartilage development but is not likely to be the main factor involved.
These data reveal the course of scleral cartilage formation and highlight the key role that the optic cup plays in this process. The driving element within the optic cup is almost certainly the retinal pigmented epithelium.
Eye; Development; Sclera; Cartilage; RPE; Neural crest
FGF signaling is critical in the development of the vertebrate retina, which differentiates in a wave-like pattern similar to that found in the Drosophila eye. In this study, we have investigated the mechanism of FGF signaling in vertebrate eye development by identifying Shp2, a protein tyrosine phosphatase, as a novel factor in orchestrating retinal morphogenesis. Using a series of Shp2 conditional mutants, we have shown that Shp2 is specifically required for the initiation of retinal neurogenesis but not for the maintenance of the retinal differentiation program. By mosaic deletion of Shp2, we further demonstrated that Shp2 ablation did not prevent the spreading of the retinal differentiation wave. Shp2 instead controls the patterning of the optic vesicle by regulating the retinal progenitor factors and cell proliferation. In ex vivo culture models and genetic rescue experiments, we showed that Shp2 acts downstream to FGF signaling in retinal development and that it can be functionally substituted by activated Ras signaling. Taken together, these results demonstrate that Shp2 mediates FGF-Ras signaling to control retinal progenitor cell fate.
Retinal Ganglion Cell; retinal; Phosphatase; Differentiation; Kinase; ERK
To develop a fully automated algorithm (AP) to perform a volumetric measure of the optic disc using conventional stereoscopic optic nerve head (ONH) photographs, and to compare algorithm-produced parameters with manual photogrammetry (MP), scanning laser ophthalmoscope (SLO) and optical coherence tomography (OCT) measurements.
One hundred twenty-two stereoscopic optic disc photographs (61 subjects) were analyzed. Disc area, rim area, cup area, cup/disc area ratio, vertical cup/disc ratio, rim volume and cup volume were automatically computed by the algorithm. Latent variable measurement error models were used to assess measurement reproducibility for the four techniques.
AP had better reproducibility for disc area and cup volume and worse reproducibility for cup/disc area ratio and vertical cup/disc ratio, when the measurements were compared to the MP, SLO and OCT methods.
AP provides a useful technique for an objective quantitative assessment of 3D ONH structures.
PURPOSE: To develop a fully automated algorithm (AP) to perform a volumetric measure of the optic disc using conventional stereoscopic optic nerve head (ONH) photographs, and to compare algorithm-produced parameters with manual photogrammetry (MP), scanning laser ophthalmoscope (SLO) and optical coherence tomography (OCT) measurements. METHODS: One hundred twenty-two stereoscopic optic disc photographs (61 subjects) were analyzed. Disc area, rim area, cup area, cup/disc area ratio, vertical cup/disc ratio, rim volume and cup volume were automatically computed by the algorithm. Latent variable measurement error models were used to assess measurement reproducibility for the four techniques. RESULTS: AP had better reproducibility for disc area and cup volume and worse reproducibility for cup/disc area ratio and vertical cup/disc ratio, when the measurements were compared to the MP, SLO and OCT methods. CONCLUSION: AP provides a useful technique for an objective quantitative assessment of 3D ONH structures.
(100.0100) Image processing; (100.2960) Image Analysis; (100.6890) Three-dimensional image processing
Vertebrate retinal pigment epithelium (RPE) cells are derived from the multipotent optic neuroepithelium, develop in close proximity to the retina, and are indispensible for eye organogenesis and vision. Recent advances in our understanding of RPE development provide evidence for how critical signaling factors operating in dorso-ventral and distal-proximal gradients interact with key transcription factors to specify three distinct domains in the budding optic neuroepithelium: the distal future retina, the proximal future optic stalk/optic nerve, and the dorsal future RPE. Concomitantly with domain specification, the eye primordium progresses from a vesicle to a cup, RPE pigmentation extends towards the ventral side, and the future ciliary body and iris form from the margin zone between RPE and retina. While much has been learned about the molecular networks controlling RPE cell specification, key questions concerning the cell proliferative parameters in RPE and the subsequent morphogenetic events still need to be addressed in greater detail.
Mitf/Otx/Chx10/Pax6/activin/sonic; hedgehog/fibroblast; growth; factor/neuroepithelial; domain specification/evolution
The purpose of this study was to investigate the correlations between microcirculation in the optic disc, average peripapillary retinal nerve fiber layer thickness cupping parameters, and visual field defects in glaucoma patients with the generalized enlargement disc type.
A total of 38 eyes from 38 glaucoma patients with the generalized enlargement disc type were included. The microcirculation of the optic nerve head was examined with laser speckle flow graphy, and the mean blur rate in all areas, in vessel area, and in tissue area were calculated using the laser speckle flow graphy analyzer software. Average peripapillary retinal nerve fiber layer thickness was measured using Stratus optical coherence tomography, and cupping parameters were accessed using the Heidelberg retina tomograph. The mean deviation in the Humphrey field analyzer (30-2 SITA standard) was analyzed. The correlation between these parameters was evaluated using the Spearman rank correlation coefficient.
The correlation coefficient of mean blur rate in all optic disc area to the average peripapillary retinal nerve fiber layer thickness, vertical C/D, and mean deviation were r = 0.7546 (P < 0.0001), r = −0.6208 (P < 0.0001), and r = 0.6010 (P = 0.0001), respectively. The mean blur rate in tissue area of the optic disc showed r = 0.7305 (P < 0.0001), r = −0.6438 (P < 0.0001), and r = 0.6338 (P < 0.0001).
We found that the mean blur rate in the optic disc was significantly correlated with the average peripapillary retinal nerve fiber layer thickness, vertical C/D, and mean deviation in patients with the generalized enlargement disc type of glaucoma. In particular, the mean blur rate in tissue area was more highly correlated than the vessel area with other results of examination in glaucoma patients with the generalized enlargement disc type.
ocular blood flow; optic disc type; laser speckle flowgraphy; function; structure
Apical constriction facilitates epithelial sheet bending and invagination during morphogenesis1, 2. Apical constriction is conventionally thought to be driven by the continuous purse-string-like contraction of a circumferential actin and Non-Muscle Myosin-II (myosin) belt underlying adherens junctions3–7. However, it is unclear whether other force-generating mechanisms can drive this process. Here, we use real-time imaging and quantitative image analysis of Drosophila gastrulation to show that apical constriction of ventral furrow cells is pulsed. Repeated constrictions, which are asynchronous between neighboring cells, are interrupted by pauses in which the constricted state of the cell apex is maintained. In contrast to the purse-string model, constriction pulses are powered by actin-myosin network contractions that occur at the medial apical cortex and pull discrete adherens junction sites inward. The transcription factors Twist and Snail differentially regulate pulsed constriction. Expression of snail initiates actin-myosin network contractions while expression of twist stabilizes the constricted state of the cell apex. Our results suggest a new model for apical constriction in which a cortical actin-myosin cytoskeleton functions as a developmentally controlled sub-cellular ratchet to incrementally reduce apical area.
The pathophysiologic events in optic nerve axons have recently been recognized as crucial to an understanding of clinically significant acquired alterations in the ophthalmoscopic appearance of the optic disc. Stasis and related abnormalities of axonal transport appear to explain most aspects of optic nerve head swelling, including optic disc drusen and retinal cottonwool spots. Loss of axoplasm and axonal death can be invoked to interpret optic disc pallor, thinning and narrowing of rim tissue, changes in the size and outline of the optic cup, laminar dots, atrophy of the retinal nerve fiber layer, and acquired demyelination and myelination of the retinal nerve fiber layer. It is speculated that the axons may also play a role in the mechanical support of the lamina cribrosa in resisting the pressure gradient across the pars scleralis of the optic nerve head. Axons and their associated glial cells may be involved in those cases where "reversibility" of cupping of the optic disc has been reported. The structure, physiology, and experimental pathologic findings of the optic nerve head have been reviewed. Many aspects concerning the final anatomic appearance of the optic nerve head have been explained. However, many questions remain concerning the intermediate mechanisms by which increased intracranial pressure retards the various components of axonal transport in papilledema and by which increased IOP causes axonal loss in glaucoma. Investigation of the molecular biology of axonal constituents and their responses to abnormalities in their physical and chemical milieu could extend our understanding of the events that result from mechanical compression and local ischemia. Moreover, we have identified a need to further explore the role of axons in the pathophysiology of optic disc cupping.
Background/aim: In three dimensional optic disc tomography a reference plane is required to calculate optic disc rim or cup values. The position of the reference plane often depends on the retinal thickness at the temporal disc margin. Originally it was assumed that the retinal thickness at the temporal disc margin is independent of age. The aim of the study was to check this hypothesis using optical coherence tomography, and additionally to determine the reproducibility of OCT measurements in this area.
Methods: 100 eyes of 100 healthy volunteers were included in this study. Three OCT scans were performed on each eye. The scans were aligned vertically and placed at the temporal edge of the optic disc. For each eye, the thickness of the whole retina as well as the thickness of the retinal nerve fibre layer were calculated together with their coefficients of variation. Thereafter retinal thickness and nerve fibre layer thickness were correlated with age.
Results: The mean retinal thickness was 249 (SD 22) μm. The mean nerve fibre layer thickness was 109 (22) μm. The mean coefficients of variation were 3.5% (total retinal thickness) and 8.0% (nerve fibre layer thickness). Both the total retinal thickness and the nerve fibre layer thickness were significantly correlated with age (total retina: y = 269.5 − 0.53 × x; R2 = 0.133; p = 0.0002, nerve fibre layer: y = 126.8 − 0.44 × x; R2 = 0.094; p<0.0019.
Conclusions: Using OCT scans the total retinal thickness can be calculated with high reproducibility (coefficient of variation = 3.5%). The reproducibility of nerve fibre layer thickness measurements is clearly lower (coefficient of variation = 8.0%). Both the total retinal thickness and the nerve fibre layer thickness significantly decrease with age. This influence of the age related decrease in RNFL/retinal thickness on the reference plane, however, is negligible.
Heidelberg retina tomograph; optical coherence tomography; retinal nerve fibre layer; retinal thickness
Retinal degeneration due to genetic, diabetic and age-related disease is the most common cause of blindness in the developed world. Blindness occurs through the loss of the light-sensing photoreceptors; to restore vision, it would be necessary to introduce alternative photosensitive components into the eye. The recent development of an electronic prosthesis placed beneath the severely diseased retina has shown that subretinal stimulation may restore some visual function in blind patients. This proves that residual retinal circuits can be reawakened after photoreceptor loss and defines a goal for stem-cell-based therapy to replace photoreceptors. Advances in reprogramming adult cells have shown how it may be possible to generate autologous stem cells for transplantation without the need for an embryo donor. The recent success in culturing a whole optic cup in vitro has shown how large numbers of photoreceptors might be generated from embryonic stem cells. Taken together, these threads of discovery provide the basis for optimism for the development of a stem-cell-based strategy for the treatment of retinal blindness.
age-related macular degeneration; retinitis pigmentosa; retinal degeneration; embryonic stem cell; induced pluripotent stem cell; transplantation
The formation and invagination of the optic stalk coincides with the migration of cranial neural crest (CNC) cells, and a growing body of data reveals that the optic stalk and CNC cells communicate to lay the foundations for periocular and craniofacial development. Following migration, the interaction between the developing eye and surrounding periocular mesenchyme (POM) continues, leading to induction of transcriptional regulatory cascades that regulate craniofacial morphogenesis. Studies in chick, mice and zebrafish have revealed a remarkable level of genetic and mechanistic conservation, affirming the power of each animal model to shed light on the broader morphogenic process. This review will focus on the role of the developing eye in orchestrating craniofacial morphogenesis, utilizing morphogenic gradients, paracrine signaling, and transcriptional regulatory cascades to establish an evolutionarily-conserved facial architecture. We propose that in addition to the forebrain, the eye functions during early craniofacial morphogenesis as a key organizer of facial development, independent of its role in vision.
Neural Crest; Orbit; Extraocular Muscles; PitX2; Retinoic Acid; Cavefish; Blind Eel; Zebrafish; Periocular Mesenchyme; Retina
Aim: To study optic disc topographic parameters in normal cynomolgus monkeys by Heidelberg retina tomograph (HRT).
Methods: 12 optic disc topographic parameters were investigated in 36 normal eyes in 18 male monkeys. Mean (SD) and interocular differences were obtained for each parameter from three independent measurements made during a 1 week period. Correlations among the topographic parameters were analysed, too.
Results: No significant differences between right and left eyes were detected for any topographic parameters. Disc area, rim area, and height variation contour showed smaller right-left differences than other parameters. The coefficients of variation for rim area, height variation contour, rim volume, mean cup depth, maximum cup depth, mean retinal nerve fibre layer (RNFL) thickness, and RNFL cross section area were less than 10% (for rim area, less than 5%). Rim area and height variation contour showed relatively weak interrelations and neither showed a correlation with disc area.
Conclusion: For evaluating time related changes in the optic disc by HRT in monkeys, rim area and height variation contour might be useful parameters because coefficients of variation and right-left differences were lower than for other parameters and because these parameters showed weak interrelations and no correlation with disc area.
cynomolgus monkey; Heidelberg retina tomograph; optic disc; topographic parameter
Recent studies revealed that the Wnt receptor Frizzled-5 (Fzd5) is required for eye and retina development in zebrafish and Xenopus, however, its role during mammalian eye development is unknown. In the mouse embryo, Fzd5 is prominently expressed in the pituitary, distal optic vesicle and optic stalk, then later in the progenitor zone of the developing retina. To elucidate the role of Fzd5 during eye development, we analyzed embryos with a germline disruption of the Fzd5 gene at E10.25, just before embryos die due to defects in yolk sac angiogenesis. We observed severe defects in optic cup morphogenesis and lens development. However, in embryos with conditional inactivation of Fzd5 using Six3-Cre we observed no obvious early eye defects. Analysis of Axin2 mRNA expression and TCF/LEF-responsive reporter activation demonstrate that Fzd5 does not regulate the Wnt/β-catenin pathway in the eye. Thus, the function of Fzd5 during eye development appears to be species-dependent.
mouse; eye; optic cup; morphogenesis; retina; lens; wnt; Frizzled; pituitary