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Annals of Anatomy = Anatomischer... Jul 2020Most organs of the human body are supplied with a dense network of blood and lymphatic vessels. However, some tissues are either hypovascular or completely devoid of... (Review)
Review
PURPOSE
Most organs of the human body are supplied with a dense network of blood and lymphatic vessels. However, some tissues are either hypovascular or completely devoid of vessels for proper function, such as the ocular tissues sclera and cornea, cartilage and tendons. Since many pathological conditions are affecting the human sclera, this review is focussing on the lymphangiogenic and hemangiogenic privilege in the human sclera.
METHODS
This article gives an overview of the current literature based on a PubMed search as well as observations and experience from clinical practice.
RESULTS
The healthy human sclera is the outer covering layer of the eye globe consisting mainly of collagenous extracellular matrix and fibroblasts. Physiologically, the sclera shows only a superficial network of blood vessels and a lack of lymphatic vessels. This vascular privilege is actively regulated by balancing anti- and proangiogenic factors expressed by cells within the sclera. In pathological situations, such as open globe injuries or ciliary body melanomas with extraocular extension, lymphatic vessels can secondarily invade the sclera and the inner eye. This mechanism most likely is important for tumor cell metastasis, wound healing, immunologic defense against intruding microorganism, and autoimmune reactions against intraocular antigens.
CONCLUSIONS
The human sclera is characterized by a tightly regulated vascular network that can be compromised in pathological situations, such as injuries or intraocular tumors affecting healing outcomes Therefore, the molecular and cellular mechanisms underlying wound healing following surgical interventions deserve further attention, in order to devise more effective therapeutic strategies.
Topics: Eye Neoplasms; Humans; Lymphangiogenesis; Lymphatic Vessels; Macrophages; Sclera
PubMed: 32120002
DOI: 10.1016/j.aanat.2020.151485 -
Progress in Retinal and Eye Research Jan 2020As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the... (Review)
Review
As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the cornea, the sclera must also provide stable mechanical support to vulnerable internal ocular structures such as the retina and optic nerve head. Moreover, it must achieve this under complex, dynamic loading conditions imposed by eye movements and fluid pressures. Recent years have seen significant advances in our knowledge of scleral biomechanics, its modulation with ageing and disease, and their relationship to the hierarchical structure of the collagen-rich scleral extracellular matrix (ECM) and its resident cells. This review focuses on notable recent structural and biomechanical studies, setting their findings in the context of the wider scleral literature. It reviews recent progress in the development of scattering and bioimaging methods to resolve scleral ECM structure at multiple scales. In vivo and ex vivo experimental methods to characterise scleral biomechanics are explored, along with computational techniques that combine structural and biomechanical data to simulate ocular behaviour and extract tissue material properties. Studies into alterations of scleral structure and biomechanics in myopia and glaucoma are presented, and their results reconciled with associated findings on changes in the ageing eye. Finally, new developments in scleral surgery and emerging minimally invasive therapies are highlighted that could offer new hope in the fight against escalating scleral-related vision disorder worldwide.
Topics: Aging; Animals; Biomechanical Phenomena; Glaucoma; Humans; Myopia; Sclera
PubMed: 31412277
DOI: 10.1016/j.preteyeres.2019.100773 -
Klinische Monatsblatter Fur... Aug 2021Evaluation of the three currently most common techniques for intraocular lens (IOL) sclera fixation: (1) Prolene suture with Hoffman sclera pocket (2) four-point GoreTex... (Review)
Review
BACKGROUND
Evaluation of the three currently most common techniques for intraocular lens (IOL) sclera fixation: (1) Prolene suture with Hoffman sclera pocket (2) four-point GoreTex suture technique (3) sutureless flanged intrascleral IOL fixation with double-needle ("Yamane") technique.
MATERIAL AND METHODS
Retrospective, clinical case series (chart review) at the Department of Ophthalmology, Ludwig-Maximilians-University (LMU), Munich, Germany. Enrolled in the study were 51 patients with 55 eyes. Best-corrected visual acuity (BCVA); manifest refraction (OR); corneal tomography (central corneal thickness, CCT); biometry; central macular thickness (CMT) by optical coherence tomography (OCT); intraocular pressure (IOP); and IOL type and IOL power were recorded and compared prior to and 3 - 12 months post IOL sclera fixation surgery. Pre- and postsurgery difference analysis was performed by Wilcoxon rank sum testing (z).
RESULTS
Intrascleral fixation by GoreTex suture was performed in 14 (25.5%) eyes, by Prolene suture in 19 (34.5%,) and by Yamane technique in 22 (40.0%) eyes. Within the 3 - 12 months follow-up post scleral fixation, a total of 2 (14.3%) eyes from the GoreTex, 3 (15.8%) from the Prolene and 1 (4.5%) eye from the Yamane group required refixation. Pre- and post-surgery analysis revealed a statistically significant difference in the total patient population BCVA (exact Wilcoxon test: z = - 3.202; p = 0.001; n = 55) and the Yamane subgroup (exact Wilcoxon test: z = - 2.068; p = 0.001; n = 22). The GoreTex (n = 14) and Prolene (n = 19) subgroups revealed no statistically significant differences versus preoperative baseline. Across groups, there was no statistically significant difference in IOP, CMT, and CCT. No retinal complications were observed, neither intraoperatively nor during follow-up.
CONCLUSION
The volume of IOL revision surgery is increasing. Often, the only option left for visual rehabilitation is scleral IOL fixation. All three scleral fixation techniques studied demonstrated a good safety profile with no statistically significant impact on IOP, CMT, CCT, but with a notable revision rate. Visual rehabilitation to preoperative baseline levels (GoreTex [n = 14] and Prolene [n = 19]) and a statistically significant increase in visual acuity (total cohort [n = 55] and Yamane [n = 22]) seems possible. Unlike iris fixation, scleral fixation is surgically more complex and the surgeon must master a steeper learning curve.
Topics: Humans; Lens Implantation, Intraocular; Lenses, Intraocular; Retrospective Studies; Sclera; Visual Acuity
PubMed: 33853190
DOI: 10.1055/a-1333-3199 -
PloS One 2021Species vary widely in the conspicuousness of their eye morphology and this could influence gaze perception. Eyes with conspicuous morphology can enhance gaze perception...
Species vary widely in the conspicuousness of their eye morphology and this could influence gaze perception. Eyes with conspicuous morphology can enhance gaze perception while eyes with camouflaged morphology may hinder gaze perception. While evidence suggests that conspicuous eye morphology enhances gaze perception, little is known about how environmental conditions affect this interaction. Thus, we investigated whether environmental light conditions affect gaze perception. Human subjects (Homo sapiens) were instructed to find direct-gaze faces within arrays of averted-gaze faces or to find averted-gaze faces within arrays of directed-gaze faces. The faces were displayed under conditions simulating nighttime or daytime conditions. Furthermore, the faces had naturally-colored sclera (white) or modified sclera (same color as the iris). Participants were fastest and most accurate in detecting faces during the daytime and nighttime conditions when the sclera were naturally-colored. Participants were worst at detecting faces with modified sclera during the nighttime conditions. These results suggest that eyes with conspicuous morphology enhance gaze perception during both daytime and nighttime conditions.
Topics: Adult; Female; Fixation, Ocular; Humans; Male; Pigmentation; Sclera; Time Factors
PubMed: 33780503
DOI: 10.1371/journal.pone.0249137 -
Progress in Retinal and Eye Research May 2003Myopia is one of the most prevalent ocular conditions and is the result of a mismatch between the power of the eye and axial length of the eye. As a result images of... (Review)
Review
Myopia is one of the most prevalent ocular conditions and is the result of a mismatch between the power of the eye and axial length of the eye. As a result images of distant objects are brought to a focus in front of the retina resulting in blurred vision. In the vast majority of cases the structural cause of myopia is an excessive axial length of the eye, or more specifically the vitreous chamber depth. In about 2% of the general population, the degree of myopia is above 6 dioptres (D) and is termed high myopia. The prevalence of sight-threatening ocular pathology is markedly increased in eyes with high degrees of myopia ( > -6 D). This results from the excessive axial elongation of the eye which, by necessity, must involve the outer coat of the eye, the sclera. Consequently, high myopia is reported as a leading cause of registered blindness and partial sight. Current theories of refractive development acknowledge the pivotal role of the sclera in the control of eye size and the development of myopia. This review considers the major biochemical mechanisms that underlie the normal development of the mammalian sclera and how the scleral structure influences the rate of eye growth during development. The review will characterise the aberrant mechanisms of scleral remodelling which underlie the development of myopia. In describing these mechanisms we highlight how certain critical events in both the early and later stages of myopia development lead to scleral thinning, the loss of scleral tissue, the weakening of the scleral mechanical properties and, ultimately, to the development of posterior staphyloma. This review aims to build on existing models to illustrate that the prevention of aberrant scleral remodelling must be the goal of any long-term therapy for the amelioration of the permanent vision loss associated with high myopia.
Topics: Animals; Animals, Newborn; Extracellular Matrix; Fibroblasts; Humans; Myopia; Refraction, Ocular; Sclera; Vision, Ocular
PubMed: 12852489
DOI: 10.1016/s1350-9462(02)00063-0 -
Current Eye Research Jun 2020: Corneal collagen cross-linking by ultraviolet light activation of riboflavin has been used clinically to enhance corneal stiffness. We sought to determine if...
: Corneal collagen cross-linking by ultraviolet light activation of riboflavin has been used clinically to enhance corneal stiffness. We sought to determine if cross-linking differentially affects scleral regions.: Adjacent, parallel strips of sclera were cut from superolateral, superomedial, inferolateral, and inferomedial quadrants of posterior and equatorial sclera of 12 human cadaver eyes. One of each pair served as control while the other was cross-linked by immersion in 0.1% riboflavin and 365 nm exposure at 6 mW/cm irradiance for 30 min. Behavior of strips was characterized using a microtensile load cell. Preloaded strips were imaged using orthogonally mounted cameras and optical coherence tomography to determine specimen dimensions including cross-sectional area. Tension was measured during 0.1 mm/s constant rate elongation.: Young's modulus (YM), the slope of the relationship relating tensile stress to strain, was calculated at 8% strain, and increased significantly after cross-linking ( < .001). In posterior sclera, mean (± standard error of mean, SEM) YM is increased in the superolateral, superomedial, inferolateral, and inferomedial quadrants by 46 ± 15%, 32 ± 11%, 67 ± 20%, and 53 ± 11%, respectively. In equatorial sclera, YM is increased by 139 ± 43%, 68 ± 27%, 143 ± 92%, and 68 ± 14%, respectively. The YM of pooled equatorial quadrants increased significantly more than that of the pooled posterior quadrants.: Scleral collagen cross-linking by ultraviolet activation of riboflavin differentially increases scleral YM more in the equatorial than posterior sclera, and most in the lateral, equatorial sclera. Cross-linking might be used to arrest progressive myopia or to prevent staphyloma formation.
Topics: Biomechanical Phenomena; Collagen; Cross-Linking Reagents; Elastic Modulus; Humans; Photochemotherapy; Photosensitizing Agents; Riboflavin; Sclera; Tensile Strength; Tomography, Optical Coherence; Ultraviolet Rays
PubMed: 31735063
DOI: 10.1080/02713683.2019.1694157 -
Journal Francais D'ophtalmologie Jan 2018Sclerotic scatter involves the scattering of incident light by the limbal sclera followed by entry of part of the scattered light into the cornea, where some of the... (Review)
Review
Sclerotic scatter involves the scattering of incident light by the limbal sclera followed by entry of part of the scattered light into the cornea, where some of the light travels through total internal reflection to the other side, where it scatters a second time in the limbal sclera. It is then visible in the form of a limbal scleral arc of light. Sclerotic scatter has been used for decades to spot and delineate corneal opacities, which disrupt and scatter the light travelling through total internal reflection. To implement the technique, the slit beam and the binoculars of the slit lamp should be dissociated so that the limbal sclera is illuminated, while the binoculars are centered on the cornea. The technique does not provide any information as to the depth of corneal opacities and therefore needs to be complemented by direct illumination. The second sclerotic scatter may also be used clinically, for instance for diode cycloablation, the posterior part of the arc of light projecting 0.5mm behind the scleral spur. This article aims to describe the phenomenon of sclerotic scatter, explaining how the slit-lamp should be set to use this technique, describing its clinical applications (in the opacified cornea and in the normal sclera), showing that the limbal scleral arc of light of sclerotic scatter may be seen under certain circumstances in daily life with the naked eye and, finally, explaining how the arc of light differs from peripheral light focusing ("Coroneo effect").
Topics: Adaptation, Ocular; Cornea; Corneal Opacity; Humans; Light; Scattering, Radiation; Sclera
PubMed: 29246385
DOI: 10.1016/j.jfo.2017.08.003 -
ACS Biomaterials Science & Engineering Dec 2022Scleras are mainly used for the treatment of glaucoma, eyelid damage, and scleral ulcers. Given that the sclera and cornea collectively constitute the complete external...
Scleras are mainly used for the treatment of glaucoma, eyelid damage, and scleral ulcers. Given that the sclera and cornea collectively constitute the complete external structure of the eyeball and both have the same tissue and cell origin, we attempted to identify scleral materials to treat lamellar and penetrating corneal injuries. Based on research in our center, antigenic components in decellularized porcine sclera (DPS) were removed using a simplified decellularization method, leaving the collagen structure and active components undamaged. DPS preserved the mechanical properties and did not significantly inhibit the proliferation and replication of human corneal epithelial cells. In vivo, the graft epithelium healed well after lamellar and penetrating scleral grafting, and the graft thickness did not change evidently. DPS can resist suture traction during scleral transplantation and maintain anterior chamber stability until day 28 post-operatively, especially in penetrating repairs. No obvious immune rejection of lamellar or penetrating scleral grafts was found 28 days after DPS transplantation. This study shows that DPS could be used as an alternative material for the emergency repair of corneal perforations and lamellar injuries, representing another application of sclera.
Topics: Humans; Swine; Animals; Corneal Perforation; Sclera; Cornea
PubMed: 36454184
DOI: 10.1021/acsbiomaterials.2c00972 -
Micron (Oxford, England : 1993) Jul 2011In this study, the organization of collagen fibrils within the sclera of the eye was investigated using the 7 keV hard X-ray microscope of the Pohang light source and... (Comparative Study)
Comparative Study
In this study, the organization of collagen fibrils within the sclera of the eye was investigated using the 7 keV hard X-ray microscope of the Pohang light source and compared to images from electron and atomic force microscopy. From the captured X-ray images, individual collagen fibrils were observed clearly in a spatial resolution much better than 100 nm, both in longitudinal sections and in transverse sections. Some of the collagen fibrils showed evidence of axial periodicity. In some regions of the samples, we could see cross-bridge like structures between adjacent collagen fibrils. The X-ray microscope also allowed the observation of keratocytes and the lamella structure of the scleral stroma. The X-ray microscope has some unique advantages in the nano-scale imaging of bio-samples relative to other established imaging techniques.
Topics: Animals; Cattle; Collagen; Electron Probe Microanalysis; Image Processing, Computer-Assisted; Sclera
PubMed: 21349730
DOI: 10.1016/j.micron.2011.01.012 -
ELife Mar 2022Hallmark social activities of humans, such as cooperation and cultural learning, involve eye-gaze signaling through joint attentional interaction and ostensive...
Hallmark social activities of humans, such as cooperation and cultural learning, involve eye-gaze signaling through joint attentional interaction and ostensive communication. The gaze-signaling and related cooperative-eye hypotheses posit that humans evolved unique external eye morphologies, including uniformly white sclera (the whites of the eye), to enhance the visibility of eye-gaze for conspecifics. However, experimental evidence is still lacking. This study tested the ability of human and chimpanzee participants to discriminate the eye-gaze directions of human and chimpanzee images in computerized tasks. We varied the level of brightness and size in the stimulus images to examine the robustness of the eye-gaze directional signal against simulated shading and distancing. We found that both humans and chimpanzees discriminated eye-gaze directions of humans better than those of chimpanzees, particularly in visually challenging conditions. Also, participants of both species discriminated the eye-gaze directions of chimpanzees better when the contrast polarity of the chimpanzee eye was reversed compared to when it was normal; namely, when the chimpanzee eye has human-like white sclera and a darker iris. Uniform whiteness in the sclera thus facilitates the visibility of eye-gaze direction even across species. Our findings thus support but also critically update the central premises of the gaze-signaling hypothesis.
Topics: Animals; Eye Movements; Fixation, Ocular; Hominidae; Humans; Pan troglodytes; Sclera
PubMed: 35256053
DOI: 10.7554/eLife.74086