Introdução: o estresse psicológico aumenta a permeabilidade intestinal em roedores e humanos, potencialmente levando a inflamação de baixo grau e aos sintomas em distúrbios gastrintestinais funcionais. No entanto, o efeito do estresse agudo combinado ao estresse da vida crônica, que mimetiza potencialmente melhor a situação humana, é desconhecido. Além disso, há poucos dados disponíveis sobre os efeitos do estresse em intestino delgado versus cólon. Métodos: ratos Wistar foram alocados em quatro protocolos de estresse: 1/ controles; 2/ estresse agudo (isolamento e movimentos limitados); 3/ Crowding stress:crônico e 4/ estresse agudo + estresse crônico. Amostras de jejuno e cólon foram colhidas para estudar a permeabilidade em câmaras deUssing, a expressão gênica de moléculas de junção firmes e a densidade de mastócitos. Níveis de corticosterona no plasma foram medidos. Principais resultados:corticosterona plasmática foi avaliada nas três condições de estresse, teve níveis mais altos na condição de estresse combinado. Permeabilidade do jejuno foi aumentada em todas as condições de estresse e correlacionada com os níveis de corticosterona. O aumento da expressão das claudinas 1, 5 e 8, daocludina e da ZO-1 foi detectado no estado de estresse agudo no jejuno. Em contraste, a permeabilidade do cólon foi aumentada no protocolo de estresse combinado, e a expressão de moléculas das junção firmes permaneceu inalterada. O aumento da densidade de mastócitos foi observado no cólon nos ratos submetidos aos estresses crônico e combinado. Conclusão e inferências:os estresses agudo, crônico e combinado influenciam diferentemente a permeabilidade intestinal, a expressão de moléculas de junção firmes e a atividade dos mastócitos, no jejuno e no cólon. Estes resultados fornecem uma visão mais aprofundada dos mecanismos de hiperpermeabilidade intestinal relacionadas ao estresse. / Background: Psychological stress increases intestinal permeability in rodents and humans, potentially leading to low-grade inflammation and symptoms in functional gastrointestinal disorders through disturbances in brain-gut axis. However, the effect of acute stress on the background of Crhonic life stress, potentially better approaching the human situation, is unknown. Moreover, only limited information is available on the effects in small intestine versus colon in animal model. Methods: Wistar rats were allocated to 4 stress protocols: 1/ sham; 2/ acute stress (isolation and limited movement); 3/ Crhonic crowding stress and 4/ acute + Crhonic stress (n = 8 per group). Jejunum and colon were harvested to study permeability in Ussing chambers, gene expression of tight junction molecules and mast cell density. Plasma corticosterone levels were measured. Key Results: Plasma corticosterone was elevated in all three stress conditions, with the highest levels in the combined stress condition. Permeability of the jejunum was increased in all stress conditions and correlated with corticosterone levels. Increased expression of claudin 1, 5 and 8, occludin and ZO-1 was detected in the acute stress condition in the jejunum. In contrast, colonic permeability was increased in the acute on Crhonic stress protocol only and the expression of tight junction molecules was unaltered. Increased mast cell density was observed in the Crhonic and acute on Crhonic stress condition in the colon only. Conclusion and Inferences: Acute, Crhonic and combined stress differentially affect intestinal permeability, expression of tight junction molecules and mast cells in the jejunum and the colon. These findings provide further insight in the mechanisms of stress-related intestinal hyperpermeability and barrier.

Introdução: o estresse psicológico aumenta a permeabilidade intestinal em roedores e humanos, potencialmente levando a inflamação de baixo grau e aos sintomas em distúrbios gastrintestinais funcionais. No entanto, o efeito do estresse agudo combinado ao estresse da vida crônica, que mimetiza potencialmente melhor a situação humana, é desconhecido. Além disso, há poucos dados disponíveis sobre os efeitos do estresse em intestino delgado versus cólon. Métodos: ratos Wistar foram alocados em quatro protocolos de estresse: 1/ controles; 2/ estresse agudo (isolamento e movimentos limitados); 3/ Crowding stress:crônico e 4/ estresse agudo + estresse crônico. Amostras de jejuno e cólon foram colhidas para estudar a permeabilidade em câmaras deUssing, a expressão gênica de moléculas de junção firmes e a densidade de mastócitos. Níveis de corticosterona no plasma foram medidos. Principais resultados:corticosterona plasmática foi avaliada nas três condições de estresse, teve níveis mais altos na condição de estresse combinado. Permeabilidade do jejuno foi aumentada em todas as condições de estresse e correlacionada com os níveis de corticosterona. O aumento da expressão das claudinas 1, 5 e 8, daocludina e da ZO-1 foi detectado no estado de estresse agudo no jejuno. Em contraste, a permeabilidade do cólon foi aumentada no protocolo de estresse combinado, e a expressão de moléculas das junção firmes permaneceu inalterada. O aumento da densidade de mastócitos foi observado no cólon nos ratos submetidos aos estresses crônico e combinado. Conclusão e inferências:os estresses agudo, crônico e combinado influenciam diferentemente a permeabilidade intestinal, a expressão de moléculas de junção firmes e a atividade dos mastócitos, no jejuno e no cólon. Estes resultados fornecem uma visão mais aprofundada dos mecanismos de hiperpermeabilidade intestinal relacionadas ao estresse. / Background: Psychological stress increases intestinal permeability in rodents and humans, potentially leading to low-grade inflammation and symptoms in functional gastrointestinal disorders through disturbances in brain-gut axis. However, the effect of acute stress on the background of Crhonic life stress, potentially better approaching the human situation, is unknown. Moreover, only limited information is available on the effects in small intestine versus colon in animal model. Methods: Wistar rats were allocated to 4 stress protocols: 1/ sham; 2/ acute stress (isolation and limited movement); 3/ Crhonic crowding stress and 4/ acute + Crhonic stress (n = 8 per group). Jejunum and colon were harvested to study permeability in Ussing chambers, gene expression of tight junction molecules and mast cell density. Plasma corticosterone levels were measured. Key Results: Plasma corticosterone was elevated in all three stress conditions, with the highest levels in the combined stress condition. Permeability of the jejunum was increased in all stress conditions and correlated with corticosterone levels. Increased expression of claudin 1, 5 and 8, occludin and ZO-1 was detected in the acute stress condition in the jejunum. In contrast, colonic permeability was increased in the acute on Crhonic stress protocol only and the expression of tight junction molecules was unaltered. Increased mast cell density was observed in the Crhonic and acute on Crhonic stress condition in the colon only. Conclusion and Inferences: Acute, Crhonic and combined stress differentially affect intestinal permeability, expression of tight junction molecules and mast cells in the jejunum and the colon. These findings provide further insight in the mechanisms of stress-related intestinal hyperpermeability and barrier.

Introdução: o estresse psicológico aumenta a permeabilidade intestinal em roedores e humanos, potencialmente levando a inflamação de baixo grau e aos sintomas em distúrbios gastrintestinais funcionais. No entanto, o efeito do estresse agudo combinado ao estresse da vida crônica, que mimetiza potencialmente melhor a situação humana, é desconhecido. Além disso, há poucos dados disponíveis sobre os efeitos do estresse em intestino delgado versus cólon. Métodos: ratos Wistar foram alocados em quatro protocolos de estresse: 1/ controles; 2/ estresse agudo (isolamento e movimentos limitados); 3/ Crowding stress:crônico e 4/ estresse agudo + estresse crônico. Amostras de jejuno e cólon foram colhidas para estudar a permeabilidade em câmaras deUssing, a expressão gênica de moléculas de junção firmes e a densidade de mastócitos. Níveis de corticosterona no plasma foram medidos. Principais resultados:corticosterona plasmática foi avaliada nas três condições de estresse, teve níveis mais altos na condição de estresse combinado. Permeabilidade do jejuno foi aumentada em todas as condições de estresse e correlacionada com os níveis de corticosterona. O aumento da expressão das claudinas 1, 5 e 8, daocludina e da ZO-1 foi detectado no estado de estresse agudo no jejuno. Em contraste, a permeabilidade do cólon foi aumentada no protocolo de estresse combinado, e a expressão de moléculas das junção firmes permaneceu inalterada. O aumento da densidade de mastócitos foi observado no cólon nos ratos submetidos aos estresses crônico e combinado. Conclusão e inferências:os estresses agudo, crônico e combinado influenciam diferentemente a permeabilidade intestinal, a expressão de moléculas de junção firmes e a atividade dos mastócitos, no jejuno e no cólon. Estes resultados fornecem uma visão mais aprofundada dos mecanismos de hiperpermeabilidade intestinal relacionadas ao estresse. / Background: Psychological stress increases intestinal permeability in rodents and humans, potentially leading to low-grade inflammation and symptoms in functional gastrointestinal disorders through disturbances in brain-gut axis. However, the effect of acute stress on the background of Crhonic life stress, potentially better approaching the human situation, is unknown. Moreover, only limited information is available on the effects in small intestine versus colon in animal model. Methods: Wistar rats were allocated to 4 stress protocols: 1/ sham; 2/ acute stress (isolation and limited movement); 3/ Crhonic crowding stress and 4/ acute + Crhonic stress (n = 8 per group). Jejunum and colon were harvested to study permeability in Ussing chambers, gene expression of tight junction molecules and mast cell density. Plasma corticosterone levels were measured. Key Results: Plasma corticosterone was elevated in all three stress conditions, with the highest levels in the combined stress condition. Permeability of the jejunum was increased in all stress conditions and correlated with corticosterone levels. Increased expression of claudin 1, 5 and 8, occludin and ZO-1 was detected in the acute stress condition in the jejunum. In contrast, colonic permeability was increased in the acute on Crhonic stress protocol only and the expression of tight junction molecules was unaltered. Increased mast cell density was observed in the Crhonic and acute on Crhonic stress condition in the colon only. Conclusion and Inferences: Acute, Crhonic and combined stress differentially affect intestinal permeability, expression of tight junction molecules and mast cells in the jejunum and the colon. These findings provide further insight in the mechanisms of stress-related intestinal hyperpermeability and barrier.

Abstract Acute pancreatitis is a common disease, Finland being among the countries with the highest incidence. The majority of patients have a mild, self-limiting disease. However, 20% of these patients develop severe necrotizing pancreatitis with a mortality rate of 7 to 25%. The mechanisms for developing the severe disease are not known, it is not possible to accurately forecast the severity of the disease and there is no curative treatment yet. This study was aimed at analyzing the early phase of acute experimental porcine oedematous and necrotizing pancreatitis. In Study I, the pancreatic microcirculatory changes were measured and the expression of tight junction proteins (claudins-2, -3, -4, -5 and -7) and the rate of apoptosis in the pancreas were all measured. In Study II, bacterial translocation to the blood in the portal vein blood or to the mesenteric lymph nodes was analyzed and the intestinal expression of tight junction proteins (claudins-2, -3, -4, -5 and -7) and the intestinal apoptosis/ proliferation rates were measured. The basic histology of the jejunum and colon were analyzed. Study III analyzed which cytokines are released from the pancreas to the portal venous blood. In Study IV, the ultrastructure of the epithelium of the jejunum and colon was analyzed and the expression of adherens junction proteins, E-cadherin and β-catenin, were measured from both jejunum and colon. The first study (I) showed that membranous immunoreactivity of claudin-2 in acinar cells appeared in the pancreas during acute oedematous and necrotizing pancreatitis. The expressions of claudins -3, - 4, - 5 and 7 were unaffected. The second study (II) showed that bacterial translocation from the gut was not present at the beginning of acute porcine pancreatitis. The expressions of claudins-2 and -5 do not become altered; however, there might be some decrease in claudin-3 expression in the colon and decrease in the expression of claudins-4 and -7 in the jejunum in necrotizing pancreatitis. Performing the laparotomy itself caused increased apoptosis in the colon and the jejunum. In the third study (III), the initial inflammatory process was diverse in oedematous and necrotizing pancreatitis. Increased monocyte count in combination with elevated PDGF and IL-6 are characteristic of necrotizing pancreatitis in our model. The fourth study (IV) indicated that necrotizing pancreatitis caused damage to the epithelial and endothelial cells of the colon in the early stages of the disease. The expression of E-cadherin immunoreactivity showed a decreasing trend in the colon in both oedematous and necrotizing pancreatitis. The results of this study suggest that claudin-2 increases in acinar cells during acute porcine pancreatitis. Bacterial translocation is not present during the early phase of acute porcine pancreatitis. Increased monocyte count and elevated PDGF and IL-6 are characteristic of early phase necrotizing porcine pancreatitis and necrotizing porcine pancreatitis causes damage to the epithelial and endothelial cells of the colon. / Tiivistelmä Akuutti haimatulehdus on yleinen sairaus, jonka ilmaantuvuus Suomessa on verrattain suuri. Suurimmalla osalla potilaista tauti on lievä ja itsestään paraneva. Kuitenkin 20 %:lle potilaista kehittyy vaikea haimatulehdus, johon liittyy 7–25 %:n kuolleisuus. On epäselvää, miksi toisinaan kehittyy vaikea tautimuoto. Taudin vaikeusastetta ei voida etukäteen tarkasti ennustaa, eikä tautiin ole parantavaa hoitoa. Väitöskirjatyön tarkoituksena oli tutkia lievän ja vaikean haimatulehduksen varhaisvaihetta kokeellisessa sikamallissa. Työssä I mitattiin haiman mikroverenkierron muutoksia, tutkittiin tiivisliitosproteiinien klaudiini-2:n, -3:n, -4:n, -5:n ja -7:n ilmenemistä sekä apoptoosin määrää haimassa. Toisessa työssä tutkittiin mahdollista bakteeritranslokaatiota porttilaskimovereen ja vatsaontelon imusolmukkeisiin, mitattiin suoliston tiivis liitos-proteiinien klaudiinien-2, -3, -4, -5 ja -7 ilmenemistä ja suoliston apoptoosin ja soluproliferaation määrää. Mahdollisia muutoksia ohut- ja paksusuolen perushistologiassa analysoitiin. Kolmannessa työssä mitattiin sytokiinipitoisuuksia porttilaskimoverestä. Neljännessä työssä analysoitiin ohut- ja paksusuolen mikrorakennetta elektronimikroskopian avulla ja mitattiin vyöliitosproteiinien E-cadherin ja β-catenin määrää. I työssä todettiin klaudiini-2:n ilmaantuvan haiman asinaarisolujen solukalvoille lievässä ja vaikeassa kokeellisessa haimatulehduksessa. Klaudiinien 3,- 4,- 5 ja 7 esiintyminen haimassa ei muuttunut. II työssä todettiin, että bakteeritranslokaatiota ei tapahtunut seuranta-aikana. Suolistossa klaudiinien-2 ja -5 ilmenemisessä ei tapahtunut muutoksia. Klaudiini-3:n ilmenemisessä paksusuolessa ja klaudiinien -4 ja -7 ilmenemisessä ohutsuolessa saattaa tapahtua vähenemistä vaikeassa haimatulehduksessa. Tutkimustoimenpide itsessään aiheutti ohut- ja paksusuolen apoptoosin lisääntymistä. III työn mukaan tulehdusvaste oli erilainen akuutissa lievässä ja vaikeassa kokeellisessa haimatulehduksessa. Monosyyttimäärän sekä PDGF:n ja IL-6:n pitoisuuksien lisääntyminen, olivat tyypillisiä vaikealle haimatulehdukselle tässä mallissa. IV työssä todettiin, että vaikea haimatulehdus vaurioittaa paksusuolen epiteeli- ja endoteelisoluja. E-cadherin: n määrässä todettiin jonkin verran vähentymistä sekä lievässä että vaikeassa haimatulehduksessa. Näiden tulosten mukaan klaudiini-2 lisääntyy sian haiman asinaarisoluissa akuutissa haimatulehduksessa. Sialla ei tapahdu bakteerien translokaatiota haimatulehduksen varhaisvaiheessa. Sian vaikeaan haimatulehdukseen liittyy monosyyttien, PDGF:n ja IL-6:n lisääntyminen. Kokeellisessa vaikeassa haimatulehduksessa paksusuolen epiteeli- ja endoteelisolut vaurioituvat jo varhaisvaiheessa.

No description available.

Abstract Squamous cell carcinoma (SCC) of the mobile tongue is the most common type of cancer of the oral cavity, accounting for 30-40% of oral cancers. It behaves aggressively and almost half of the affected patients still die of the disease despite great advances in its medical and surgical care. Ameloblastomas are the most common clinically significant type of odontogenic tumors, constituting approximately 1% of all cysts and tumors of the jaw. They are benign but locally invasive tumors with a strong tendency to recur after surgery. Ameloblastic carcinoma combines the histological features of ameloblastoma with cytologic atypia irrespective of the presence or absence of metastasis. The effectiveness of tight junction proteins (claudins 1, 4, 5, 7 and occludin) and cancer-associated fibroblasts (CAFs) as prognostic markers in OTSCC and as markers of malignancy in ameloblastomas was studied. Abundance of CAFs and Claudin 7 derangement was found to be associated with poor disease-specific survival in oral (mobile) tongue cancer. Appearance of CAFs within the epithelial islands of ameloblastoma was found to be a marker of malignancy in the tumor. The prognostic predictability of CAF density, Ki-67 (cell proliferation marker), maspin (tumor suppressor marker) and tumor DNA content (tumor ploidy using image cytometry) in tongue cancers was also tested. CAF density was the only marker strongly predictive of prognosis. In ameloblastomas, α-SMA (for CAFs), Ki-67, epithelial membrane antigen (EMA) and DNA content (using image and flow cytometry) were assessed as markers of ameloblastic carcinoma. Only α-SMA was able to predict ameloblastic carcinoma when found in the epithelial islands. In conclusion, staining for α-SMA and claudin 7 seems to be beneficial for prognostication in tongue cancer, while α-SMA staining may be beneficial in differentiating ameloblastoma from ameloblastic carcinoma.

Abstract Acute acalculous cholecystitis (AAC) is an insidious and increasingly recognized complication of critical illness, whose pathogenesis is poorly understood and clinical picture obscure. Diagnosis is difficult and there is no consensus on treatment. The medical records of all ICU patients who had undergone open cholecystectomy due to AAC during the years 2000–2001 and 2003–2004 were examined for clinical and organ failure data. The indication for open cholecystectomy was a suspicion of AAC based on clinical signs and symptoms of sepsis or deteriorating multiple organ dysfunction without other obvious foci and/or radiological (computed tomography or ultrasound) findings indicative of cholecystitis. A total of 73 patients had operatively treated AAC during the study periods, giving an incidence of 0.9% of all admissions (73/8184) and an incidence of 6.7% among the long-stayers (ICUstay >5 days). The hospital mortality of these patients was 43%. Infection was the most common admission diagnosis followed by cardiovascular surgery. The patients were severely ill, the mean (SD) APACHE II score being 25.5 (6.4) and the mean (SD) SOFA score 10.2 (3.5) on admission. In those patients who had AAC as the only intra-abdominal complication of multiple organ dysfunction, cholecystectomy was followed by a remarkable improvement of individual and total SOFA scores by the seventh postoperative day. The AAC gallbladders were histologically and immunohistologically compared to normal gallbladders and to gallbladders of patients with acute calculous cholecystitis (ACC). The ACC patients were admitted into hospital because of primary acute gallbladder disease, were treated on a normal ward and did not have severe sepsis or multiple organ dysfunction. The typical histopathological features of AAC (34 cases) in the gallbladder wall were bile infiltration, lymphatic dilatation and leucocyte margination of blood vessels, while epithelial degeneration and defects, widespread occurrence of inflammatory cells and extensive and deep muscle layer necrosis were typical features of ACC (28 cases). Tight junction proteins (claudin-1, -2, -3, -4, occludin, ZO-1 and E-cadherin) were uniformly expressed in normal gallbladder epithelium, with the exception of claudin-2, which was present in less than half of the cells. In AAC, the expression of cytoplasmic occludin and claudin-1 was decreased compared to control group. In ACC, the expression of claudin-2 was increased, but the expression of claudin-1, -3 and -4, occludin and ZO-1 was decreased compared to normal or AAC gallbladders. In conclusion, AAC is associated with severe illness, infection, long intensive care unit stay and deteriorating multiple organ dysfunction. Open cholecystectomy is one important contributing factor to reverse the course of multiple organ dysfunction in these patients. Histological and immunohistological studies suggest that AAC is a manifestation of systemic inflammatory disease, while ACC is a local inflammatory and often infectious disease.

Polymorphic light eruption (PLE) is the most common photodermatosis, affecting ~17% of the population. PLE is a delayed-type hypersensitivity response to an antigen induced by solar ultra-violet radiation (UVR). Its effects vary between patients, but the main symptom is a non-scarring, red papular rash in areas exposed to UVR. An effective therapy is low dose ultra-violet B (NBUVB) phototherapy. It is thought that NBUVB phototherapy desensitises the skin to further UVR exposure, but the mechanism by which this happens is unknown. Current immune based studies have been unable to clarify a mechanism as to how PLE arises. However, research in other skin diseases, such as psoriasis and atopic dermatitis, has shown that the barrier function of the skin is compromised by these disorders. Furthermore, research in lesional PLE skin showed an increase in barrier permeability of the skin. Recent research has specifically linked claudin proteins of tight junctions to the barrier dysfunction. Therefore, this study used quantitative immunofluorescent staining to measure tight junction (TJ) proteins and other barrier proteins of interest. Barrier function was also measured by transepidermal water loss (TEWL); a tracer dye penetration assay was used to measure TJ barrier function specifically. All measurements were made in non-lesional PLE skin, as compared to skin from healthy human volunteers. In photoprotected PLE skin the TJ protein claudin-1 was significantly reduced compared to healthy skin. The use of a tracer dye highlighted there was a reduction in TJ barrier function in PLE skin compared to healthy individuals. PLE and healthy skin were then exposed to ultra-violet B (UVB) and 24h later TJ proteins and TJ barrier function were measured. There was no change to claudin-1 after UVB exposure in PLE skin, but claudin-7 was reduced and claudin-12 increased. In contrast, in UVB-irradiated skin in normal controls after UVB exposure claudin-7 and claudin-12 were both increased, whilst claudin-1 was reduced. In PLE patients there was no further change to TJ barrier function, however, in normal controls, skin TJ barrier function was reduced post UVB. Both in healthy and PLE skin TEWL was unchanged before and after UVB exposure. Lastly TJ proteins were investigated after NBUVB in PLE patients. There was a further reduction in claudin-1 in PLE patients as well as a reduction in the TJ protein occludin, however the stratum corneum was significantly thickened. It could be suggested that this is a compensatory measure for the reduction seen in TJ barrier proteins, however further studies are needed to understand this. These data show significant differences in the TJ skin barrier in patients with PLE as compared to healthy human volunteers before and after UVB exposure. Furthermore, in PLE skin there is a significant change to the epidermis after NBUVB phototherapy. These data demonstrate that TJ protein expression and function is altered in PLE skin and may contribute to aetiology of the disorder, however the role of TJ barrier in aetiology is yet to be firmly established.

The mammalian skin provides a necessary barrier between the organism and the environment, defending against loss of water and solutes, preventing the invasion of pathogens as well as protecting against chemical and physical assault. Claudin (Cldn)-based Tight Junctions (TJs) are the main functional part of the skin barrier. In particular, Cldn6 through its cytoplasmic tail has been shown to be important for barrier function. In other to further investigate the role of the Cldn6 tail in TJ-function, we developed Cldn6 mouse mutants carrying varying truncations of the Cldn6 tail. Both of these mice present with epidermal differentiation perturbations and delayed barrier function that is repaired later in life. These studies support the importance of the tail portion of the Cldn molecules in epidermal differentiation and barrier function. In addition, both of these mouse models are useful for the study of barrier function in preterm infants and in aging, with the hope of developing novel therapeutics for the alleviation of barrier dysfunction. Cdx is a family of homeodomain (HD) transcription factors (TFs) essential for many key developmental processes. In particular, Cdx2 is important for the establishment and maintenance of posterior identity in the developing endoderm. In spite of this, only a few Cdx targets in the developing endoderm have been discovered. In addition, the interplay between Cdx and its targets within the endoderm is poorly understood. In this study, we show that the forkhead box transcription factor, Foxa2 is a Cdx2 target. We also show that Foxa2 and Cdx2 physically and genetically interact to regulate a subset of genes that are implicated in endodermal development. These studies help to further our understanding of endoderm biology with the goal of developing new strategies to diagnose and treat diseases associated with defective endoderm development.

Purpose. These experiments were designed to characterize tight junction formation by retinal pigment epithelial (RPE) cells in vitro and to compare the effects on this process of hormonally defined medium (HDM) and serum- containing medium. Methods. Formation of RPE tight junctions was analyzed in freshly isolated rat RPE cells maintained either in HDM or serum-containing medium. Junctions were evaluated functionally by measuring transepithelial electrical resistance (TER) and permeability and structurally by immunolocalization of the junction-associated actin microfilaments. Calcium dependency of the junction was determined by reducing media calcium concentration. Results. RPE cells cultured in serum-free HDM developed calcium-dependent tight junctions, which exhibited TER levels > 150 Ω · cm 2 and low paracellular permeability. Serum-containing media inhibited tight junction formation as indicated by significant reductions in TER and increases in permeability. Junction-associated actin microfilaments and cell density were unchanged. Conclusions. Tight junction formation by RPE cells is inhibited by serum. This activity may play an important role in responses of the RPE layer to injury, contributing to the pathologic progression of blood- retinal barrier dysfunction.