New insight into lacrimal gland function: Role of the duct epithelium in tear secretion

Tear secretion is a complex process with the involvement of the main and accessory lacrimal glands, corneal and conjunctival epithelial cells and the Meibomian glands. The lacrimal gland is the main source of fluid, electrolytes and proteins in tear fluid. Deficient ion and water secretion results in aqueous deficient dry eye with serious consequences on the integrity of the ocular surface. Functions of acinar cells are widely studied, whereas less information is available about the duct system of the lacrimal gland. Secretory mechanisms of duct epithelium may play an important role in tear production, but only limited studies have tried to elucidate the role of the duct system in tear secretion. Significant progress has been made in the past few years, resulting in new insight into lacrimal gland duct function. New experimental techniques were introduced, which contributed to the exploration of the role of lacrimal gland ducts in more detail. Therefore, the aim of this review is to summarize our present knowledge about the role of ducts in lacrimal gland function and tear secretion, which appears to be the first review with a focus on this topic. Short outline of pancreatic and salivary gland duct functions is also given for the purposes of comparison.     

Detection and localization of the hydrophobic surfactant proteins B and C in human tear fluid and the human lacrimal system

PURPOSE: To evaluate the expression and presence of the surfactant proteins (SP) B and C in the lacrimal apparatus at the ocular surface and in tear fluid. METHODS: Expression of SP-B and SP-C was analyzed by RT-PCR in healthy lacrimal gland, conjunctiva, meibomian gland, accessory lacrimal glands, cornea, and nasolacrimal ducts. The deposition of the hydrophobic proteins SP-B and SP-C was determined by Western blot and immunohistochemistry in healthy tissues, tear fluid, and aqueous humor. RESULTS: The presence of both SP-B and SP-C on mRNA and protein level was evidenced in healthy human lacrimal gland, conjunctiva, cornea, and nasolacrimal ducts. Moreover, both proteins were present in tear fluid but were absent in aqueous humor. Immunohistochemical investigations revealed production of both peptides by acinar epithelial cells of the lacrimal gland and additionally by accessory lacrimal glands of the eyelid as well as epithelial cells of the conjunctiva and nasolacrimal ducts. Immunohistochemically, healthy cornea and goblet cells revealed no reactivity. CONCLUSIONS: Besides the recently detected surfactant-associated proteins SP-A and SP-D, our results show that SP-B and SP-C are also peptides of the tear film, the ocular surface, and the lacrimal apparatus. Based on the current knowledge of lowering surface tension in alveolar lung cells, a similar effect of SP-B and SP-C may be assumed concerning the tear film.     

Lacrimal Gland Microenvironment Changes After Obstruction of Lacrimal Gland Ducts

PurposeTo investigate microenvironment changes of the lacrimal gland after obstruction of lacrimal gland ducts.MethodsThe ducts of rat exorbital lacrimal gland were ligated by sutures for different durations. After that, the sutures in some animals were released, and they were observed for 21 days to evaluate the recovery of the lacrimal gland. Slit lamp and tear secretion test was performed to evaluate ocular surface and lacrimal gland function. The lacrimal gland and cornea were harvested and processed for hematoxylin and eosin staining, oil red O staining, LipidTOX staining, Masson staining, quantitative real time polymerase chain reaction, and immunofluorescence staining.ResultsAfter the lacrimal gland ducts were blocked, tear secretion and the weight of the lacrimal gland were reduced. Incidence of corneal neovascularization increased after seven days. Intraglandular ducts dilated and acini destroyed. Long-term ligation induced fibrosis and lipid accumulation of the lacrimal glands. Inflammatory cell infiltrated and inflammatory factors upregulated. Proliferative and apoptotic cells increased. Structure of myoepithelial cells and basement membrane was destroyed. The p63 expression increased whereas Pax6 expression decreased. After suture release, tear secretion and structure of acini could recover in less than seven days after ligation, with a decrease in inflammatory cell infiltration and fibrosis relief. Apoptotic cells and proliferative cells increased at five days thereafter. The structure of the myoepithelial cells and basement membrane could not recover three days after ligation, and the number of mesenchymal cells increased in ligation after five to 14 days.ConclusionsBlockage of the lacrimal gland ducts results in dystrophy of lacrimal gland acini cells, inflammation, and lipid accumulation of the lacrimal gland microenvironment. Long-term duct blockage will cause irreversible lacrimal gland failure.     

Neural regulation of lacrimal gland secretory processes: Relevance in dry eye diseases

The lacrimal gland is the major contributor to the aqueous layer of the tear film which consists of water, electrolytes and proteins. The amount and composition of this layer is critical for the health, maintenance, and protection of the cells of the cornea and conjunctiva (the ocular surface). Small changes in the concentration of tear electrolytes have been correlated with dry eye syndrome. While the mechanisms of secretion of water, electrolytes and proteins from the lacrimal gland differ, all three are under tight neural control. This allows for a rapid response to meet the needs of the cells of the ocular surface in response to environmental conditions. The neural response consists of the activation of the afferent sensory nerves in the cornea and conjunctiva to stimulate efferent parasympathetic and sympathetic nerves that innervate the lacrimal gland. Neurotransmitters are released from the stimulated parasympathetic and sympathetic nerves that cause secretion of water, electrolytes, and proteins from the lacrimal gland and onto the ocular surface. This review focuses on the neural regulation of lacrimal gland secretion under normal and dry eye conditions.     

A single injection of interleukin-1 induces reversible aqueous-tear deficiency, lacrimal gland inflammation, and acinar and ductal cell proliferation

Emerging studies from our laboratory demonstrate that interleukin-1 (IL-1) family members play a major role in impairing lacrimal gland functions. Here we have extended our investigations to observe the effects of IL-1 on aqueous tear production, lacrimal gland secretion, lacrimal gland histology, and acinar and ductal cell proliferation. We demonstrate that a single injection of IL-1 into the lacrimal glands inhibited neurally- as well as agonist-induced protein secretion resulting in decreased tear output. Meanwhile, IL-1 injection induced a severe, but reversible (7-13 days), inflammatory response that led to destruction of lacrimal gland acinar epithelial cells. Finally, we demonstrate that as the inflammatory response subsided and lacrimal gland secretion and tear production returned to normal levels, there was increased proliferation of acinar and ductal epithelial cells. Our work uncovers novel effects of IL-1 on lacrimal gland functions and the potential regenerative capacity of the mouse lacrimal gland.     

Lacrimal fluid and electrolyte secretion: a review

Lacrimal gland fluid is an important component of the precorneal tear film. The rate of lacrimal gland fluid secretion is controlled primarily by parasympathetic innervation, and it is, apparently, modulated by sympathetic innervation. Lacrimal gland fluid is produced in two stages, secretion of a primary fluid which resembles an isotonic ultrafiltrate of plasma in the acinus-early intercalated duct region, and secretion of a KCl-rich fluid in subsequent ductal elements. Little is known about the electrolyte transport mechanisms of the ductal epithelia. Recent work using a variety of techniques, including tracer flux measurements, intracellular electrical recording, intracellular ion activity measurements, patch clamping, and analytical subcellular fractionation, supports a model for transcellular Cl-secretion in the acinus which involves Cl--selective channels in the apical plasma membrane and an array of Na+/H+ antiporters, Cl-/HCO3-antiporters, K+ channels, and Na,K-ATPase in the basal-lateral plasma membrane.     

干眼症的发病机制

眼表（角膜、结膜、副泪腺和睑板腺）、主泪腺和它们之间的神经连接组成的泪腺功能单位共同发挥对泪液分泌和泪膜形成的调控作用,维护眼表的健康,其中任一环节的损害均可导致泪膜完整性和正常功能的破坏.引起干眼症的起始病因很多,如果这些因素在未引起眼表改变时消失,则泪膜恢复正常;如这些因素不能消失,则引起眼表面的病理改变.虽然干眼症的临床表现类型不同,但其病理生理改变是相似的,炎症是干眼症发病机制中最关键的病理生理改变,而细胞凋亡、神经调节及性激素等也共同参与了干眼症的发病过程.     

Detection of surfactant proteins A and D in human tear fluid and the human lacrimal system

PURPOSE: To evaluate the expression and presence of surfactant protein (SP) A and SP-D in the lacrimal apparatus, at the ocular surface, and in tears in healthy and pathologic states. METHODS: Expression of mRNA for SP-A and SP-D was analyzed by RT-PCR in healthy lacrimal gland, conjunctiva, cornea, and nasolacrimal ducts as well as in a spontaneously immortalized conjunctival epithelial cell line (HCjE; IOBA-NHC) and a SV40-transfected cornea epithelial cell line (HCE). Deposition of SP-A and SP-D was determined by Western blot, dot blot, and immunohistochemistry in healthy tissues, in tears, aqueous humor, and in sections of different corneal abnormalities (keratoconus, herpetic keratitis, and Staphylococcus aureus-based ulceration). Cell lines were stimulated with different cytokines and bacterial components and were analyzed for the production of SP-A and SP-D by immunohistochemistry. RESULTS: The presence of SP-A and SP-D on mRNA and protein levels was evidenced in healthy lacrimal gland, conjunctiva, cornea, and nasolacrimal duct samples. Moreover, both proteins were present in tears but were absent in aqueous humor. Immunohistochemistry revealed the production of both peptides by acinar epithelial cells of the lacrimal gland and epithelial cells of the conjunctiva and nasolacrimal ducts, whereas goblet cells revealed no reactivity. Healthy cornea revealed weak reactivity on epithelial surface cells only. In contrast, SP-A and SP-D revealed strong reactivity in patients with herpetic keratitis and corneal ulceration surrounding lesions and in several immigrated defense cells. Reactivity in corneal epithelium and endothelium was also seen in patients with keratoconus. Cell culture experiments revealed that SP-A and SP-D are produced by both epithelial cell lines without and after stimulation with cytokines and bacterial components. CONCLUSIONS: These results show that SP-A, in addition to SP-D, is a peptide of the tear film. Based on the known direct and indirect antimicrobial effects of collectins, the surfactant-associated proteins A and D seem to be involved in several ocular surface diseases.     

AlphaB-crystallin in lacrimal gland duct and tears

PURPOSE: To investigate alphaB-Crystallin expression and localization in the lacrimal gland and tear fluid. METHODS: Mouse, rat, porcine, monkey and human lacrimal gland samples were immuno-histochemically and immuno-electron-microscopically stained with various antibodies against alphaB-crystallin. Western- and Northern-blotting was performed to demonstrate the presence of alphaB-crystallin mRNA and protein. Human tear fluid was analyzed for the presence of alphaB-crystallin using dot blotting. RESULTS: alphaB-Crystallin is located in the lacrimal gland duct cells but not in the acini. Electron microscopically, the protein was frequently found in apical electron-dense granules of lacrimal duct cells, occasionally also in the duct lumina. Western blotting confirmed the presence of alphaB-crystallin in the lacrimal gland, Northern blot samples revealed the presence of alphaB-crystallin mRNA. In the human tear fluid, alphaB-crystallin was present in all samples investigated. CONCLUSIONS: We demonstrate for the first time that alphaB-crystallin is present in the lacrimal gland. Presence of the protein in apical secretory granules as well as presence in the tear fluid might indicate secretion of alphaB-crystallin into the tear fluid.     

Dysfunctional neural regulation of lacrimal gland secretion and its role in the pathogenesis of dry eye syndromes

Tears are a complex fluid consisting of three layers, each of which is secreted by a different set of tissues or glands. The aqueous portion of the tear film is produced predominantly by the lacrimal gland. Dry eye syndromes are diseases in which the amount and composition of tears are altered, which can lead to ocular surface damage. There are many causes for dry eye syndromes. One such cause is the alteration in the functions of nerves innervating the lacrimal gland and the ocular surface. The autoimmune disease Sjogren syndrome can deleteriously affect the innervation of the lacrimal gland. Damage to the sensory nerves in the ocular surface, specifically the cornea, as a result of refractive surgery and normal aging, prevents the normal reflex arc to the lacrimal gland. Both defects can result in decreased tear secretion and dry eye syndromes. This review will discuss the current information regarding neurally-stimulated protein, water, and electrolyte secretion from the lacrimal gland and delineate how nerve dysfunction resulting from a variety of causes decreases secretion from this gland.     

Research in dry eye: report of the Research Subcommittee of the International Dry Eye WorkShop (2007)

Members of the DEWS Research Subcommittee reviewed research into the basic mechanisms underlying dry eye disease. Evidence was evaluated concerning the tear film, lacrimal gland and accessory lacrimal glands, ocular surface epithelia (including cornea and conjunctiva), meibomian glands, lacrimal duct system and the immune system. Consideration was given to both animal and human research data. Results are presented as a series of information matrices, identifying what is known and providing supporting references. An attempt is made to identify areas for further investigation.     

Tear film osmolarity and ocular surface disease in two rabbit models for keratoconjunctivitis sicca

We report the natural history of keratoconjunctivitis sicca (KCS) in two rabbit models. The first one (full KCS model) was created by closing the lacrimal gland excretory duct, and removing the nictitating membrane and harderian gland. We created the second one (lacrimal gland duct only [LGDO]-KCS model) by closing the lacrimal gland excretory duct. Although tear film osmolarity was abnormally high in both models, it was higher in the full KCS model. Decreases in corneal epithelial glycogen and in conjunctival goblet cell density, and morphological abnormalities of the conjunctiva correlated with increases in tear film osmolarity and duration of disease.     

Effect of inflammation on lacrimal gland function

The lacrimal gland is the main contributor to the aqueous layer of the tear film. It secretes proteins, electrolytes and water, which helps to nourish and protect the ocular surface. Lacrimal gland secretion is primarily under neural control, which is achieved through a neural reflex arc. Stimuli to the ocular surface activate afferent sensory nerves in the cornea and conjunctiva. This in turn activates efferent parasympathetic and sympathetic nerves in the lacrimal gland to stimulate secretion. Sex steroid hormones are also important regulators of lacrimal gland functions. A decrease or lack of lacrimal gland secretion is the leading cause of aqueous tear deficient dry eye syndrome (DES). It has been suggested that DES is an inflammatory disorder that affects the ocular surface and the lacrimal gland. In several pathological instances, the lacrimal gland can become a target of the immune system and show signs of inflammation. This can result from autoimmune diseases (Sj?gren's syndrome), organ transplantation (graft versus host disease), or simply as a result of aging. The hallmarks of lacrimal gland inflammation are the presence of focal lymphocytic infiltrates and increased production of proinflammatory cytokines. The mechanisms leading to lacrimal gland dysfunction are still poorly understood. Apoptosis, production of autoantibodies, hormonal imbalance, alterations in signaling molecules, neural dysfunction, and increased levels of proinflammatory cytokines have been proposed as possible mediators of lacrimal gland insufficiency in disease states.     

The role of the lacrimal functional unit in the pathophysiology of dry eye

The majority of dry eye symptoms are due to a chronic inflammation of the lacrimal functional unit resulting in a loss of tear film integrity and normal function. This leads to a reduction in the ability of the ocular surface to respond to environmental challenges. The underlying cause of tear film dysfunction is the alteration of tear aqueous, mucin, and lipid components. This may result from a systemic autoimmune disease or a local autoimmune event. A lack of systemic androgen support to the lacrimal gland has been shown to be a facilitative factor in the initiation of this type of pathophysiology. Tear secretion is controlled by the lacrimal functional unit consisting of the ocular surface (cornea, conjunctiva, accessory lacrimal glands, and meibomian glands), the main lacrimal gland and the interconnecting innervation. If any portion of this functional unit is compromised, lacrimal gland support to the ocular surface is impeded. Factors such as neurogenic inflammation and T cell involvement in the disease pathogenesis as well as newly developed animal models of ocular surface inflammation are discussed.     

Interaction of EGF family growth factors and neurotransmitters in regulating lacrimal gland secretion

The lacrimal gland is the primary source for the aqueous portion of the tear film. This portion contains water, electrolytes and proteins, which are necessary for the health and maintenance of the cells of the ocular surface. Afferent sensory nerves in the cornea and conjunctiva stimulate efferent parasympathetic and sympathetic nerves in the lacrimal gland. Cholinergic agonists, released from parasympathetic nevres, and norepinephrine, released from sympathetic nerves, are major stimuli of lacrimal gland secretion. These neurotransmitters activate distinct, but overlapping signal transduction pathways leading to lacrimal gland secretion. Other stimuli of lacrimal gland secretion are the EGF family of growth factors. In addition to stimulation of secretion, these growth factors can interact with the cells of the lacrimal gland themselves or with the cells of the ocular surface depending upon the location from which these growth factors are released. This review will focus on the effects of the EGF family of growth factors on the lacrimal gland and their interactions with the pathways stimulated by the neurotransmitters released from nerves.     

Examination of mouse exorbital lacrimal gland after exposure to 2-chloroacetophenone vapor

The compound 2-chloroacetophenone has been used to stimulate tear secretion from human subjects, yet the morphological response of the lacrimal gland to this agent has not been experimentally determined. This study used light and electron microscopic techniques to examine the effect of this agent with time on the morphology of the mouse exorbital lacrimal gland. In brief, exposure to 2-chloroacetophenone vapor causes rapid exocytosis of acinar cell granules as well as vacuolation of the secretory and ductal epithelial cells. Concomitantly, intralobular ductal epithelial cells which are normally cuboidal in shape, enlarge in size and release electron-dense granules into the duct lumen. Within 15 min after exposure, acinar cells show a preponderance of cells containing only pale granules and the ductal epithelium returns to a more cuboidal shape. By 60 min after exposure, the gland is not readily distinguishable from the unexposed, control gland.     

去势雄兔泪液分泌及泪膜稳定性的改变

目的观察去势后雄兔眼表及泪膜的变化,评价雄激素对泪液分泌及泪膜稳定性的影响.方法将16只雄兔随机分为去势(实验)及对照两组,每组8只.实验组分别于去势前和去势后1、2、3、4周、2及3个月行Shirmer试验、泪膜破裂时间(break-up time, BUT)及虎红染色检测;利用化学发光法检测实验组手术前、后血清睾酮水平,并行统计学分析处理.3个月后处死两组兔,取其泪腺、Harder腺、结膜、角膜及角膜缘组织行病理组织学观察.结果实验组Shirmer试验、泪膜破裂时间明显低于正常对照组,且随观察时间的延长差异有显著性(t=9.032,P＜0.01;t=4.747,P＜0.01).虎红染色实验组为阳性,对照组为阴性.实验组术后血清睾酮水平(4.52±0.81) nmol/L明显低于术前(19.76±1.53) nmol/L(t=22.290,P＜0.01).病理学观察,实验兔泪腺上皮细胞胞浆萎缩扁平,腺腔扩大,腺泡泡状黏液消失,PAS染色阳性物质减少,结膜杯状细胞数量减少,角膜上皮及角膜缘干细胞形态学上无明显改变.结论去势后雄兔睾酮水平明显低下,泪腺上皮细胞萎缩,腺泡泡状黏液物质消失及结膜杯状细胞减少,导致泪液分泌的质和量改变及泪膜稳定性降低.     

泪道激光联合引流管植入对泪道阻塞患者的影响

目的：探究泪道激光联合引流管植入对泪道阻塞患者泪膜破裂时间及角膜荧光素钠染色的影响,以便提高泪道激光联合引流管植入术对患者的治疗效果。

方法：选取2013-01/2015-12在我院进行泪道激光联合泪道引流管植入术治疗的泪道阻塞患者200例200眼,分别在治疗前、术后1mo、拔管后进行眼表评估。检测内容包括患者泪膜破裂时间、泪液分泌量、角膜荧光素钠染色情况及干眼症状问卷等。

结果：患者治疗前、术后1mo、拔管后泪膜破裂时间,三者间比较差异无统计学意义(P>0.05)。患者术后1mo泪液分泌量增加,拔管后泪液分泌量减少,差异有统计学意义(P<0.05)。患者治疗前、术后1mo、拔管后角膜荧光素钠染色评分无明显变化,三者间比较差异无统计学意义(P>0.05)。进行干眼症状问卷显示行泪道激光联合引流管植入术后流泪症状明显改善。

结论：泪道激光联合引流管植入对患者泪膜破裂时间及角膜荧光素钠染色无明显影响,此种手术方式可以使患者术后流泪症状明显改善,在治疗泪道阻塞方面具有重大意义。     

Retinol secretion by the lacrimal gland

In order to determine the source of the retinol which has been identified in the tear fluid, the lacrimal gland ducts of rabbits and rats were cannulated and the collected lacrimal gland fluid was analyzed by high performance liquid chromatography. Retinol was identified in the lacrimal gland fluid of rabbits and rats, and it is concluded that the lacrimal gland is the source of retinol in the tears. Dose-response studies show that intravenously administered pilocarpine and intra-arterial acetylcholine stimulate secretion of retinol by the lacrimal gland. Intravenous administration of vasoactive intestinal peptide (VIP) also stimulates retinol secretion in a dose-response manner. These observations are similar to the effects of cholinergic drugs and VIP on protein secretion by the lacrimal gland.