Identification of lymphatics in the ciliary body of the human eye: A novel “uveolymphatic” outflow pathway

Impaired aqueous humor flow from the eye may lead to elevated intraocular pressure and glaucoma. Drainage of aqueous fluid from the eye occurs through established routes that include conventional outflow via the trabecular meshwork, and an unconventional or uveoscleral outflow pathway involving the ciliary body. Based on the assumption that the eye lacks a lymphatic circulation, the possible role of lymphatics in the less well defined uveoscleral pathway has been largely ignored. Advances in lymphatic research have identified specific lymphatic markers such as podoplanin, a transmembrane mucin-type glycoprotein, and lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). Lymphatic channels were identified in the human ciliary body using immunofluorescence with D2-40 antibody for podoplanin, and LYVE-1 antibody. In keeping with the criteria for lymphatic vessels in conjunctiva used as positive control, D2-40 and LYVE-1-positive lymphatic channels in the ciliary body had a distinct lumen, were negative for blood vessel endothelial cell marker CD34, and were surrounded by either discontinuous or no collagen IV-positive basement membrane. Cryo-immunogold electron microscopy confirmed the presence D2-40-immunoreactivity in lymphatic endothelium in the human ciliary body. Fluorescent nanospheres injected into the anterior chamber of the sheep eye were detected in LYVE-1-positive channels of the ciliary body 15, 30, and 45 min following injection. Four hours following intracameral injection, Iodine-125 radio-labeled human serum albumin injected into the sheep eye (n = 5) was drained preferentially into cervical, retropharyngeal, submandibular and preauricular lymph nodes in the head and neck region compared to reference popliteal lymph nodes (P < 0.05). These findings collectively indicate the presence of distinct lymphatic channels in the human ciliary body, and that fluid and solutes flow at least partially through this system. The discovery of a uveolymphatic pathway in the eye is novel and highly relevant to studies of glaucoma and other eye diseases.     

Physiology of aqueous humor outflow resistance: its relation to giant vacuoles

About 2% of the French population over the age of 40 suffers from open angle glaucoma, a disorder for which ocular hypertension is the main risk factor. Improving our understanding of primary open-angle glaucoma physiopathology has been an area of intense research for nearly a century. The main aqueous outflow system in the human eye includes the trabecular meshwork, Schlemm's canal, aqueous veins, and the episcleral veins. Schlemm's canal is bordered by endothelium cells, many of which contain giant vacuoles, structures that are sensitive to intraocular pressure and that therefore can act as markers of active outflow. Giant vacuoles are most often found near collector channels in normal eyes, probably in areas of low downstream pressure. In the glaucoma eye, determining where giant vacuoles form could help localize the site of pathological outflow resistance.     

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

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

The critical role of the conjunctiva in glaucoma filtration surgery

This review considers the critical role of the conjunctiva in determining the success or failure of glaucoma filtration surgery. Glaucoma filtration surgery can be defined as an attempt to lower intraocular pressure (IOP) by the surgical formation of an artificial drainage pathway from the anterior chamber to the subconjunctival space. Many types of glaucoma filtration surgery have been developed since the first attempts almost 180 years ago. The wide range of new techniques and devices currently under investigation is testament to the limitations of current techniques and the need for improved therapeutic outcomes. Whilst great attention has been paid to surgical techniques and devices to create the drainage pathway, relatively little attention has been given to address the question of why drainage from such artificial pathways is often problematic. This is in contrast to normal drainage pathways which last a lifetime. Furthermore, the consequences of potential changes in aqueous humour properties induced by glaucoma filtration surgery have not been sufficiently addressed. The mechanisms by which aqueous fluid is drained from the subconjunctival space after filtration surgery have also received relatively little attention. We propose that factors such as the degree of tissue damage during surgery, the surrounding tissue reaction to any surgical implant, and the degree of disruption of normal aqueous properties, are all factors which influence the successful formation of long term drainage channels from the conjunctiva, and that these channels are the key to successful filtration surgery. In recent years it has been suggested that the rate of fluid drainage from the subconjunctival space is actually the determining factor in the resultant IOP reduction. Improved knowledge of aqueous humour induced changes in such drainage pathways has the potential to significantly improve the surgical management of glaucoma.We describe for the first time a novel type of drainage surgery which attempts to minimise surgical trauma to the overlying conjunctiva. The rationale is that a healthy conjunctiva allows drainage channels to form and less opportunity for inflammation and scar tissue formation which are a frequent cause of failure in glaucoma filtration surgery. Successful drainage over extended periods of time has been demonstrated in monkey and rabbit eyes. Long lasting drainage pathways were clearly associated with the presence of lymphatic drainage pathways. A new philosophy in glaucoma drainage surgery is proposed in which minimisation of surgical trauma to the conjunctiva and the encouragement of the development of conjunctival drainage pathways, particularly lymphatic pathways, are central pillars to a successful outcome in glaucoma filtration surgery.     

New perspectives in aqueous humor secretion and in glaucoma: the ciliary body as a multifunctional neuroendocrine gland

The discovery in the human ocular ciliary body of glaucoma-associated genes (i.e., MYOC, CYP1B1), neuroendocrine processing enzymes, neuroendocrine peptides, steroid-converting enzymes, glutamate transporters, glutamate-metabolizing enzymes, and anti-angiogenic factors requires a reevaluation of its function on aqueous humor secretion, intraocular pressure and its role in glaucoma. The ciliary body should be considered as a multifunctional and interactive tissue. The intrinsic hypotensive and/or hypertensive biological activities of many of the endocrine peptides released by the ciliary epithelium are best explained within the context of a neuroendocrine system, linking the inflow and the outflow of aqueous humor. This interpretation is consistent with physiological and genetic studies indicating that changes altering the inflow affects intraocular pressure. In the proposed endocrine system, regulatory peptides secreted by the ciliary epithelium may subserve multiple functions in the following: inflow and outflow pathways of aqueous humor, ciliary blood flow, the immune privilege status of the anterior segment and the diurnal circadian rhythms of aqueous humor secretion and intraocular pressure. These previously unsuspected and challenging functions of the ciliary epithelium should be considered when assessing the multifactorial events which lead to the pathophysiology of glaucoma affecting the outflow pathways of aqueous humor. This review highlights published, and ongoing studies on authors' labs supporting neuroendocrine, steroidogenic and glutamatergic features of the ciliary epithelium and the endocrine communication between the inflow and outflow pathways of aqueous humor. We also discuss how glaucoma-associated genes expressed in the ciliary body and their mutant proteins could influence intraocular pressure, contributing to the development of glaucoma.     

房水流出通道形态及功能在体评估进展

青光眼病理性高眼压的常见原因是房水流出受阻,因此房水流出通道的形态观察和功能研究是青光眼防治的重点。研究房水流出通道的形态及功能对研究青光眼发病机制、选择手术方式和开发靶向药物均有重要意义。房水流出成像方式中眼前段相干光断层扫描及血管成像、超声生物显微镜活体显像,可对房水流出通道进行结构性评估;而静态和实时房水血管造影为功能性评估。目前尚缺乏房水结构流出特征与功能流量值之间的精确关系研究。使用房水血管造影术可研究与评价青光眼的药物疗效。房水血管造影术有助于引导小梁网靶向的微创青光眼手术以获得更好的降眼压效果。但房水血管造影术有创,目前只能在手术室使用。未来尚需进一步改进评估房水流出通道的方法,以期在临床上通过结构和功能评估相结合,更好地了解房水流出通道和眼前段结构,提高青光眼诊治水平。（国际眼科纵览,2020, 45：397-403）     

Aqueous outflow - A continuum from trabecular meshwork to episcleral veins

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

Aqueous outflow regulation – 21st century concepts

We propose an integrated model of aqueous outflow control that employs a pump-conduit system in this article. Our model exploits accepted physiologic regulatory mechanisms such as those of the arterial, venous, and lymphatic systems. Here, we also provide a framework for developing novel diagnostic and therapeutic strategies to improve glaucoma patient care. In the model, the trabecular meshwork distends and recoils in response to continuous physiologic IOP transients like the ocular pulse, blinking, and eye movement. The elasticity of the trabecular meshwork determines cyclic volume changes in Schlemm's canal (SC). Tube-like SC inlet valves provide aqueous entry into the canal, and outlet valve leaflets at collector channels control aqueous exit from SC. Connections between the pressure-sensing trabecular meshwork and the outlet valve leaflets dynamically control flow from SC. Normal function requires regulation of the trabecular meshwork properties that determine distention and recoil. The aqueous pump-conduit provides short-term pressure control by varying stroke volume in response to pressure changes. Modulating TM constituents that regulate stroke volume provides long-term control. The aqueous outflow pump fails in glaucoma due to the loss of trabecular tissue elastance, as well as alterations in ciliary body tension. These processes lead to SC wall apposition and loss of motion. Visible evidence of pump failure includes a lack of pulsatile aqueous discharge into aqueous veins and reduced ability to reflux blood into SC. These alterations in the functional properties are challenging to monitor clinically. Phase-sensitive OCT now permits noninvasive, quantitative measurement of pulse-dependent TM motion in humans. This proposed conceptual model and related techniques offer a novel framework for understanding mechanisms, improving management, and development of therapeutic options for glaucoma.     

原发性开角型青光眼房水外流通路改变的研究进展

原发性开角型青光眼(POAG)是一种以视神经轴索及相关视网膜神经节细胞丢失为特征的视神经病变.眼压升高是POAG最重要的危险因素.大多数POAG患者眼压升高主要是房水外流阻力异常增高所致.小梁网是产生房水排出阻力的主要部位.目前多数研究者认为POAG患者小梁网功能不良与致炎因子表达、细胞老化、氧化应激损伤及细胞质成分减少等因素有关.小梁网细胞本身及细胞外基质的变化均可以引起房水外流阻力的改变,进而导致眼内压的升高.为了进一步开展对POAG发病机制的研究,有必要就目前有关POAG患者房水外流通路改变的研究进展予以综述,旨在为POAG的深入研究提供参考依据.     

Corticosteroid-induced ocular hypertension and glaucoma: a brief review and update of the literature

PURPOSE OF REVIEW: The purpose of this article is to briefly review the literature of corticosteroid-induced ocular hypertension and glaucoma, its risk factors, the pathophysiology, and treatment options. In particular, literature pertaining to glaucoma in response to intravitreal triamcinolone acetonide will be reviewed. RECENT FINDINGS: Primary open-angle glaucoma, status as a glaucoma suspect, and a family history of glaucoma are risk factors for an ocular hypertensive response with the use of corticosteroid therapy. Recent studies suggest that younger age may also be a risk factor in patients treated via the intravitreal route with corticosteroids. The mechanism of elevated intraocular pressure is increased aqueous outflow resistance owing to an accumulation of extracellular matrix material in the trabecular meshwork. SUMMARY: Corticosteroid-induced ocular hypertension and glaucoma has been recognized for more than 50 years. Knowing the risk factors, prevalence, and pathophysiology can help the clinician prevent, monitor, and treat corticosteroid-induced ocular hypertension and glaucoma.     

激素性青光眼的研究进展

目的自激素性青光眼首次报道以来已有50多年,人们对于激素性青光眼的危险因素、发病机制和防治都有了进一步的了解。原发性开角型青光眼患者及其亲属,高度近视眼患者等对激素治疗引起的眼压增高较敏感。糖皮质激素主要是通过糖皮质激素受体发挥作用,使小梁网的细胞外基质沉积,增加房水流出阻力而导致眼压升高。进一步了解激素性青光眼的危险因素和发病机制可以有助于我们更好的预防和治疗激素性青光眼。     

房水流出通路结构及功能显像的研究进展

房水作为重要眼内容物之一,一直处于动态循环中,若房水流出通路(AHO)中的任一部位受阻,都将导致眼压升高,并伴随青光眼的发生,进而出现视神经损伤,严重影响视功能。而开角型青光眼患者的房角呈开放状态,其眼压升高是房水流出阻力增加所致,因此,直观地观察AHO的结构,评价其功能变化,有助于我们更清晰地了解AHO的阻力所在,进一步阐明开角型青光眼的发病机制。目前对于AHO的显像技术包括光学相干断层扫描(OCT)以及AHO造影术等。本文将就AHO结构及功能显像的研究进展做一综述。     

色素性青光眼的发病机制

色素性青光眼是一类发病机制相对"单纯"的继发性开角型青光眼,眼压升高的主要原因在于大量色素颗粒沉积于小梁网并导致房水流出阻力增加.色素颗粒除了机械性堵塞小梁网间隙,还可导致吞噬色素颗粒超负荷的小梁细胞死亡、裂解,小梁柱暴露并相互融合,小梁网间隙变窄甚至消失.此外,葡萄膜巩膜途径房水动力学改变,复杂的分子遗传以及免疫机制...     

Schlemm管成形术研究进展

Schlemm管成形术（canaloplasty）作为一种新的非穿透性手术越来越引起人们的重视,它通过扩张Schlemm管重建自然房水外流通道以加强房水外流,从而达到降低眼压的目的。Schlemm管成形术手术成功率在原发性开角型青光眼与小梁切除术类似。其优点包括：降眼压作用是非滤过泡依赖性的,术中不需要使用抗代谢药物,因而与滤过泡相关的并发症极低。但这项新技术大范围应用于临床仍有很长的路要走。（国际眼科纵览,2015,39:341.345）     

The physiological and pathophysiological roles of the autophagy lysosomal system in the conventional aqueous humor outflow pathway: More than cellular clean up

During the last few years, the autophagy lysosomal system is emerging as a central cellular pathway with roles in survival, acting as a housekeeper and stress response mechanism. Studies by our and other labs suggest that autophagy might play an essential role in maintaining aqueous humor outflow homeostasis, and that malfunction of autophagy in outflow pathway cells might predispose to ocular hypertension and glaucoma pathogenesis. In this review, we will collect the current knowledge and discuss the molecular mechanisms by which autophagy does or might regulate normal outflow pathway tissue function, and its response to different types of stressors (oxidative stress and mechanical stress). We will also discuss novel roles of autophagy and lysosomal enzymes in modulation of TGFβ signaling and ECM remodeling, and the link between dysregulated autophagy and cellular senescence. We will examine what we have learnt, using pre-clinical animal models about how dysregulated autophagy can contribute to disease and apply that to the current status of autophagy in human glaucoma. Finally, we will consider and discuss the challenges and the potential of autophagy as a therapeutic target for the treatment of ocular hypertension and glaucoma.     

Corneal endothelial permeability and aqueous humor dynamics in normal pressure glaucoma

Corneal endothelial permeability and aqueous humor dynamics were studied in 17 non-treated normal pressure glaucoma patients in order to analyse the relevance of these parameters in the pathophysiology of this disease. Corneal endothelial permeability and aqueous humor flow were measured by fluorophotometry and aqueous outflow facility was determined by tonography. The results were compared with those of 17 healthy controls of similar age. The mean corneal endothelial permeability values and the aqueous flow and outflow facility values of the patients did not differ significantly from those of the healthy controls (P=0.8, P=0.2 and P=0.5, respectively). Normal pressure glaucoma does not affect the corneal endothelial permeability. The aqueous humor dynamics are not primarily involved in the pathophysiology of normal pressure glaucoma.Abbreviations IOP intraocular pressure - NPG normal pressure glaucoma - POAG primary open-angle glaucoma Part of this study was presented at the Annual Meeting of the Association for Eye Research on September 16, 1993 in Granada, Spain     

黏小管成形术研究进展

黏小管成形术作为一种新型非穿透性手术,通过扩张Schlemm管重建自然房水外流通道以加强房水外流,从而达到降眼压目的。黏小管成形术从最初的外路手术到近来改良的内路黏小管成形术（ab-interno canaloplasty,ABiC）,变得更加简单、微创和标准化,二者均对原发性开角型青光眼有治疗效果;其降眼压作用是非滤过泡依赖性的,术中无需使用抗代谢药物,与滤过泡相关的并发症也极罕见。黏小管成形术不论单独手术或联合白内障手术均显示出较好的安全性和有效性,具有较好的应用前景。（国际眼科纵览, 2018,  42:  78-82）     

Aqueous humour outflow imaging: seeing is believing

Elevated intraocular pressure (IOP) is the primary risk factor for blindness in glaucoma. IOP is determined by many factors including aqueous humour production and aqueous humour outflow (AHO), where AHO disturbance represents the primary cause of increased IOP. With the recent development of new IOP lowering drugs and Minimally Invasive Glaucoma Surgeries (MIGS), renewed interest has arisen in shedding light on not only how but where AHO is occurring for the trabecular/conventional, uveoscleral/unconventional, and subconjunctival outflow pathways. Historical studies critical to understanding outflow anatomy will be presented, leading to the development of modern imaging methods. New biological behaviours uncovered by modern imaging methods will be discussed with relevance to glaucoma therapies emphasized.Subject terms: Anatomy, Microscopy     

Rho激酶抑制剂在青光眼治疗中的应用进展

青光眼是以特征性视神经萎缩和视野缺损为共同特征的不可逆性致盲性眼病,高眼压是其最主要的发病机制,而导致高眼压的主要原因是房水传统流出途径中小梁网的病理学改变导致的房水流出阻力增加。Rho激酶抑制剂(ROCKi)是直接作用于小梁网(TM)的降眼压药物,主要通过影响细胞骨架改变TM细胞形态、细胞运动、胞质分裂和平滑肌收缩等,从而增加房水流出、降低眼压,在美国和日本已被批准用于临床; 具有改善视网膜血管灌注、促进视神经再生等作用,可能存在视神经保护作用; 此外,具有减少滤过泡瘢痕化等作用。因此,ROCKi作为新型抗青光眼药物备受关注,本文将针对Rho/Rho激酶信号通路、ROCKi的作用机制及其临床应用展开综述。     

Imaging the aqueous humor outflow pathway in human eyes by three-dimensional micro-computed tomography (3D micro-CT)

The site of outflow resistance leading to elevated intraocular pressure in primary open-angle glaucoma is believed to be located in the region of Schlemm’s canal inner wall endothelium, its basement membrane and the adjacent juxtacanalicular tissue. Evidence also suggests collector channels and intrascleral vessels may have a role in intraocular pressure in both normal and glaucoma eyes. Traditional imaging modalities limit the ability to view both proximal and distal portions of the trabecular outflow pathway as a single unit. In this study, we examined the effectiveness of three-dimensional micro-computed tomography (3D micro-CT) as a potential method to view the trabecular outflow pathway. Two normal human eyes were used: one immersion fixed in 4% paraformaldehyde and one with anterior chamber perfusion at 10 mmHg followed by perfusion fixation in 4% paraformaldehyde/2% glutaraldehyde. Both eyes were postfixed in 1% osmium tetroxide and scanned with 3D micro-CT at 2 μm or 5 μm voxel resolution. In the immersion fixed eye, 24 collector channels were identified with an average orifice size of 27.5 ± 5 μm. In comparison, the perfusion fixed eye had 29 collector channels with a mean orifice size of 40.5 ± 13 μm. Collector channels were not evenly dispersed around the circumference of the eye. There was no significant difference in the length of Schlemm’s canal in the immersed versus the perfused eye (33.2 versus 35.1 mm). Structures, locations and size measurements identified by 3D micro-CT were confirmed by correlative light microscopy. These findings confirm 3D micro-CT can be used effectively for the non-invasive examination of the trabecular meshwork, Schlemm’s canal, collector channels and intrascleral vasculature that comprise the distal outflow pathway. This imaging modality will be useful for non-invasive study of the role of the trabecular outflow pathway as a whole unit.