Permissive effect of glutamine on the differentiation of fetal mouse small intestine in organ culture

The EXTrauterine Environment for Neonatal Development Supports Normal Intestinal Maturation and Development

BACKGROUND AND AIMS

The Extra-Uterine Environment for Neonatal Development (EXTEND) aims to avoid the complications of prematurity, such as NEC. Our goal was to determine if bowel development occurs normally in EXTEND-supported lambs, with specific emphasis on markers of immaturity associated with NEC.

METHODS

We compared terminal ileum from 17 pre-term lambs supported on EXTEND for 2- 4 weeks to bowel from age-matched fetal lambs that developed in utero. We evaluated morphology, markers of epithelial integrity and maturation, enteric nervous system structure, and bowel motility.

RESULTS

EXTEND-supported lamb ileum had normal villus height, crypt depth, density of mucin-containing goblet cells, and enteric neuron density. Expression patterns for I-FABP, activated caspase-3 and EGFR were normal in bowel epithelium. Transmural resistance assessed in Ussing chambers was normal. Bowel motility was also normal as assessed by ex vivo organ bath and video imaging. However, Peyer's patch organization did not occur normally in EXTEND ileum, resulting in fewer circulating B cells in experimental animals.

CONCLUSION

EXTEND supports normal ileal epithelial and enteric nervous system maturation in pre-term lambs. The classic morphologic changes and cellular expression profiles associated with NEC are not seen. However, immune development within the EXTEND supported lamb bowel does not progress normally.

Effects of protein deprivation on growth and small intestine morphology are not improved by glutamine or glutamate in gastrostomy-fed rat pups

OBJECTIVES

Critically ill neonates often have their enteral intake severely limited shortly after birth. Whether glutamine (Gln) or glutamate (Glu) can preserve intestinal structure and function in the neonate undergoing limited enteral feeding is not clear. We hypothesize that Gln and Glu can similarly preserve intestinal structure in the developing small intestine of infant rats fed a low protein diet.

METHODS

Using a gastrostomy-fed "pup-in-a-cup" rat model, the effects of Gln and Glu on the developing rat small intestine were examined. Four groups of 6- to 7-day-old pups were fed rat milk substitute (RMS) via gastrostomy tube. One group was provided 100% and three were provided 25% of the protein normally received from their mothers. Two of the groups fed 25% protein received additional Gln or Glu for 6 days.

RESULTS

Pups receiving the 100% protein RMS were larger than pups receiving the 25% protein RMS with or without Gln/Glu supplementation (P < 0.001). Average villus height (P < 0.01) and area (P < 0.01) were greater in pups receiving 100% protein RMS than in pups given 25% protein RMS formula. There was no significant difference among the groups in mucosal maltase or alkaline phosphatase activities. Tight junction protein claudin-1 was significantly higher in the group fed 100% protein RMS diet, while occludin did not differ among the 4 groups. Neither Gln nor Glu increased claudin-1 or occludin in rats fed 25% protein.

CONCLUSIONS

These results suggest that neither Gln nor Glu supplementation can substitute effectively for whole protein in the developing rat small intestine for the outcomes that were evaluated.

Glutamine decreases lipopolysaccharide-induced intestinal inflammation in infant rats

Using a gastrostomy-fed (GF) rat infant "pup-in-a-cup" model, the effects of protein deprivation and supplemental glutamine (Gln) and glutamate (Glu) were examined to test the hypothesis that Gln decreases the proinflammatory response induced by LPS in the developing infant rat small intestine. Four groups of 6- to 7-day-old pups were fed a rat milk substitute (RMS), one providing 100% and three providing 25% of normal protein intake for another 6 days. Two of the 25% protein-fed groups received supplemental Gln or Glu. GF and LPS treatment blunted body growth and intestinal villus height and increased intestinal cytokine-induced neutrophil chemoattractant (CINC) mRNA in the protein-deprived, non-Gln-treated group compared with mother-fed pups (P < 0.05). Gln blunted intestinal CINC mRNA (P < 0.05), but Glu did not. Intestinal CINC peptide in the LPS-treated pups provided 100 and 25% protein was elevated approximately 13-fold compared with the mother-reared pups (P < 0.001). Gln and Glu decreased intestinal CINC peptide by 73 and 80%, respectively. GF, LPS-treated pups also had a higher level of plasma CINC peptide (P < 0.05). Gln but not Glu decreased plasma CINC peptide (P < 0.05). An approximate sixfold elevation of intestinal MPO activity in the GF, LPS-treated rats was decreased by Gln and Glu by 92% (P < 0.001) and 54% (P < 0.05), respectively. Intestinal and plasma TNF-alpha were increased in GF, LPS-treated pups (P < 0.01), and Gln and Glu both blunted this increase (P < 0.05) in the intestine but not in the plasma. The results indicate that Gln decreases the LPS-induced inflammatory response in infant rat intestine under different conditions of protein intake.

Glutamine supplementation and deprivation: effect on artificially reared rat small intestinal morphology

The mechanisms of how glutamine benefits critically ill patients have not been established. The purpose of this study was to determine the effects of dietary and endogenously produced glutamine on small intestinal morphology using light and transmission electron microscopy in artificially reared rat pups. It was hypothesized that deprivation of dietary glutamine leads to intestinal disease that is exacerbated by inhibition of glutamine synthetase by methionine sulfoximine (MS). Rat pups were placed into five different test groups: The first was a reference group that was reared by their mother. The other four groups were reared artificially and received a 10% Travasol amino acid solution at 5 g/kg per day, which does not contain glutamine, added to a mixture containing carbohydrates, lipids, and vitamins. This dose was chosen because it represents an approximation of the amount of glutamine these rats would be receiving in a normal rat diet (approximately 40 g/kg per day total protein, 10 to 15% of which is glutamine + glutamate). The glutamine was manipulated by adding glutamine (Q) or MS or both. The four groups were as follows: MS-Q-, MS-Q+, MS+Q-, and MS+Q+. Light microscopy revealed the greatest blunting of villus height in the ileum of rats from the MS+Q- group when compared with the MS-Q+ group (123 +/- 48.9 micro m versus 207 +/- 36 microm, p < 0.05). The other two groups exhibited intermediate villus heights, but all were shorter than the villi from the mother-reared animals. The number of villi per unit length of bowel was also lowest in the animals that were treated with MS and not provided with dietary glutamine. Transmission electron microscopy demonstrated breakdown of the epithelial junctions in the glutamine-deprived and glutamine synthetase-inhibited intestines. Glutamine-deprived animals also displayed sloughing of microvilli, decreased actin cores, and degeneration of the terminal web. In summary, these studies support the hypothesis that glutamine is involved with maintenance of intestinal epithelial integrity.

Potentiating the benefit of vascular-supplied glutamine during small bowel storage: importance of buffering agent

BACKGROUND

Glutamine (gln)-supplemented University of Wisconsin (UW) solution improves overall small bowel (SB) preservation. Sustained gln metabolism in a system devoid of hepatic detoxification will necessarily result in the accumulation of pH active end products leading to nonphysiologic pH shifts. We hypothesized that simultaneous addition of N,N-bis[2-hydroxyethyl]-2-aminoethane sulfonic acid (BES), a known buffering agent, would potentiate the beneficial effect of gln supplementation by addressing the fundamental metabolic principle of pH homeostasis.

METHODS

Sprague-Dawley SB rats were administered a vascular flush with one of four solutions: UW; UW+90 mM BES (UWB); UW+2% gln (UWG); or UW+2% gln+90 mM BES (UWBG). Indices of energetics, barrier function, gln catabolism, and histology (light and electron microscopy) were assessed over a 10-hr cold storage time course.

RESULTS

Superior gln utilization in the UWBG group was indicated by elevated levels of key catabolites (glutamate, aspartate, glycine, ammonia). The addition of BES and gln resulted in significantly higher levels of all energetic parameters (ATP, total adenylates) at 10 hr compared with UW, UWB, and/or UWG. Barrier function was markedly improved after 10 hr storage in the UWBG group; mannitol permeability was 169 nmol/cm2/hr versus 572 and 445 nmol/cm(2)/hr (for UW and UWG, respectively). Histologic injury at 10 hr was 5.5, 7.5, and 8 (Park's grade) for UWBG, UWG, and UW. Ultrastructural damage was markedly reduced with UWBG, as assessed by grade of mitochondria damage.

CONCLUSION

This study strongly supports that the beneficial effects of gln-enriched UW solution can be amplified when combined with an effective buffering agent such as BES.

In vitro evaluation of a customized solution for use in attenuating effects of ischemia and reperfusion in the equine small intestine

OBJECTIVE

To determine whether a customized solution could attenuate the effects of low-flow ischemia and reperfusion injury of the equine jejunum.

SAMPLE POPULATION

A segment of jejunum obtained from 21 healthy adult horses.

PROCEDURE

A segment of jejunum was maintained in an isolated extracorporeal circuit, and arterial flow was reduced to 20% of baseline for 40 minutes (ischemia) followed by 60 minutes of reperfusion. In 1 group, a customized solution was infused at a rate of 1 ml/min during low-flow ischemia and 3 ml/min during reperfusion. In a second group, the solution was infused at the same rate during low-flow ischemia, but it was infused at a rate of 7 ml/min during reperfusion. Control groups received lactated Ringer's solution administered at the same rates as for the customized solution. Various metabolic, hemodynamic, histologic, and permeability variables were recorded.

RESULTS

A lower flow rate during reperfusion (3 ml/min) had a beneficial effect, compared with lactated Ringer's solution or the higher flow rate (7 ml/min). Use of the solution at this rate resulted in less histomorphologic injury and reduced mucosal permeability to albumin.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of a customized solution at a lower flow rate during repurfusion appeared to have a protective effect on equine jejunum when administered IV during low-flow ischemia and reperfusion.

Glutamine supplementation in low-birth-weight infants: mechanisms of action

Very low-birth-weight infants have minimal endogenous nutritional reserves and are at high risk for stresses that induce further breakdown of these diminished reserves. They frequently receive very little glutamine because enteral feedings are often delayed and glutamine is not included in parenteral nutrition. Here we describe studies of glutamine supplementation in very low-birth-weight infants and discuss potential mechanisms for the beneficial effects.

Glutamine synthetase: a key enzyme for intestinal epithelial differentiation?

BACKGROUND

We have previously shown that glutamine synthetase protein and mRNA are concentrated in the crypt region of the rat small intestine and that the activity of this enzyme is highest around the time of weaning. This anatomical location and time of peak activity are sites and periods of active enterocyte differentiation. This led to our current hypothesis that glutamine synthetase is important in the differentiation of enterocytes.

METHODS

To test our hypothesis, we treated Caco-2 cells with physiologic (0.6 mM) glutamine concentrations in cell culture medium. The experimental group was treated with methionine sulfoximine, an irreversible glutamine synthetase inhibitor, and the control group with phosphate buffered saline. Three standard and well-defined markers of intestinal differentiation-sucrase-isomaltase activity, microvillus formation, and electrical impedance in transwell plates-were compared between the two groups.

RESULTS

The methionine-sulfoximine-inhibited group was found to have lower sucrase-isomaltase activity, a lower density of microvilli, and lower electrical impedance values over time compared with the control group.

CONCLUSION

The experimental group was found to be less differentiated by all three markers of differentiation. Therefore, glutamine synthetase is important for Caco-2 cell differentiation.

Glutamine protects function and improves preservation of small bowel segments

BACKGROUND

Improved organ preservation is essential for the success of small bowel transplantation. Small bowel is usually preserved in UW (University of Wisconsin) solution which does not contain glutamine (Gln), the principal fuel for the enterocyte. We hypothesized that Gln-supplemented UW would improve mucosal function and structure of cold preserved small intestine.

MATERIALS AND METHODS

Jejunum (40 cm) was harvested from Lewis rats and preserved for 18 hr at 4 degrees C in saline; UW solution only; UW with 1, 2, or 4% Gln; and UW containing 1, 2, or 4% isonitrogenous balanced nonessential amino acids (NEAA). 14C glucose transport, mucosal protein, mucosal maltase and alkaline phosphatase, jejunal villous height, and histologic damage were measured.

RESULTS

UW with 2% Gln significantly increased glucose transport and mucosal protein when compared to the 2% NEAA and UW-only groups. Two percent Gln significantly decreased histologic damage of jejunum following cold preservation. Increasing Gln to 4% did not significantly increase its efficacy when compared to the UW with 2% Gln group. There were no significant differences in the activities of mucosal maltase and alkaline phosphatase among the various treatment groups.

CONCLUSIONS

The addition of Gln, optimally provided at a concentration of 2%, to UW solution may protect the preserved small bowel segments from cold ischemic injury and improve mucosal function.

Characterization of glutamine synthetase transcript, protein, and enzyme activity in the human placenta

This study characterizes the molecular mechanisms necessary for glutamine synthesis in the human placenta. RNA hybridization and protein immunoblotting were used to verify the presence of glutamine synthetase (GS) transcripts and protein, respectively. Additionally, the presence of GS was determined by immunohistochemistry. RNA hybridization demonstrated the presence of 1.8- and 2.8-kB transcripts and protein immunoblotting yielded a single 49-kDa band, characteristics of GS transcripts and protein, respectively. The mean (+/- s.d.) specific activity of placental GS, expressed as mumol gamma-glutamyl hydroxamic acid/mg protein/h was 1.80 +/- 0.59, which is comparable to other organs which are net glutamine producers. Immunohistochemical analysis indicated the presence of GS within the cytotrophoblast and mesenchyme layers of placental villi, but not in the syncytiotrophoblast. Although these results suggest that the human placenta is capable of synthesizing glutamine, the fate of glutamine produced by this organ remains speculative.

Supplemental alanylglutamine, organ growth, and nitrogen metabolism in neonatal pigs fed by total parenteral nutrition

The objective of this study was to determine whether supplemental glutamine (alanylglutamine dipeptide) is effective in preventing small intestinal mucosal atrophy associated with total parenteral nutrition and whether it affects the growth of other organs in neonatal pigs. We compared organ growth, intestinal enzyme activity, and plasma nitrogen metabolites in 4-day-old pigs randomly selected to receive total parenteral nutrition supplemented with 0 g, 2.0 g, or 4.5 g of glutamine per deciliter for a total amino acid intake of either 11 or 25 g.kg-1.d-1 for 7 days. Glutamine supplementation increased (60% to 100%) plasma concentrations of glutamine, urea nitrogen, ammonia, and both jejunal villus height and surface area, but it did not significantly affect jejunal mucosal protein and DNA masses or the relative growth of liver, kidneys, and brain. No histologic evidence of tissue toxicity was found. Supplementing large amounts of glutamine (alanylglutamine dipeptide) did not completely prevent total parenteral nutrition-associated intestinal mucosal atrophy but did improve villus morphology without affecting vital organ growth or histology.

Treatment of acute diarrhea with oral rehydration solutions containing glutamine

OBJECTIVE

In a randomized, double-blind, controlled clinical trial we compared the efficacy of adding 90 mmol/L L-glutamine to the standard World Health Organization oral rehydration salts (WHO-ORS).

SUBJECTS

One hundred twenty male infants, > 1 month and < 1 year of age, with acute non-cholera diarrhea and dehydration were randomly assigned to one of the two treatment groups.

METHODS

Patients were kept in a metabolic unit where body weight, ORS, water and food intake, as well as stool, urine and vomitus output were recorded at 6-hour intervals. Laboratory evaluation, including blood gases and electrolytes, were monitored during hospitalization.

RESULTS

Diarrheal stool output, duration of diarrhea and volume of ORS required to achieve and maintain hydration was not significantly different between the treatment groups.

CONCLUSION

This study demonstrated that a glutamine-based ORS did not provide any additional therapeutic advantage over the standard WHO-ORS during treatment of dehydration in infants with acute non-cholera diarrhea.

Glutamine-supplemented parenteral nutrition improves gut mucosa integrity and function in endotoxemic rats

The effects of glutamine-supplemented parenteral nutrition on protein metabolism, small intestinal mucosal metabolism, morphology, and barrier function were studied in endotoxin-treated rats. Forty-six male Wistar rats were randomized to two groups of 23 animals each and received total parenteral nutrition solutions supplemented with either glutamine (GLN group) or glycine (GLY group) at 2% wt/vol. Endotoxemia was induced by continuous intravenous infusion of endotoxin at a dose of 2 mg/kg per day throughout the 4-day study period. The GLN group had a less-negative cumulative nitrogen balance (-14.0 +/- 132.8 mg of nitrogen in the GLN group and -86.8 +/- 161.7 mg of nitrogen in the GLY group, p < .05) and less cumulative excretion of urinary 3-methylhistidine (2910 +/- 593 nmol) than the GLY group (4447 +/- 933 nmol, p < .01). Jejunal mucosal glutaminase activity and the arterio-portal venous blood glutamine concentration differences were significantly higher in the GLN group compared with the GLY group (15.6 +/- 2.3 vs 11.1 +/- 1.9 mumol/g per minute, p < .05, and 181 +/- 52 vs 147 +/- 36 nmol/mL, p < .05, respectively). The morphology of the jejunal mucosa in the GLN group was significant for having greater mucosal weight (23.4 +/- 3.1 vs 17.6 +/- 2.5 mg/cm), villus height (445 +/- 75 vs 357 +/- 57 microns), crypt depth (197 +/- 34 vs 161 +/- 28 microns), and wall thickness (751 +/- 77 vs 648 +/- 102 microns) than the GLY group (p < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of parenteral nutrition supplemented with L-glutamine or glutamine dipeptides

Although glutamine is an important fuel used by the intestinal mucosa and other visceral organs, it is not present in any commercially available parenteral amino acid solution. To compare the effects of L-glutamine with glutamine dipeptides, we studied the effects of each in 8 dogs and 60 Wistar rats. In the dog study, three amino acid solutions were compared: standard commercial amino acid solution (control), alanine-glutamine dipeptide-enriched solution (glutamine 3.4%), and glycine-glutamine dipeptide-enriched solution (glutamine 3.6%). Arterial and venous samples were collected to compare the effects of the three solutions on skeletal muscle amino acid exchange. In the rat study, two studies were undertaken: group 1 rats underwent only central venous catheterization; group 2 rats underwent central venous catheterization and a 50% intestinal resection. Within each group, three different solutions were infused: standard amino acid solution (control), glutamine-enriched (1.5% glutamine) solution, or glutamine dipeptide-enriched (1% glutamine) solution. After 7 days of parenteral nutrition, samples of gut, blood, and muscle were collected for determination of mucosal thickness, villus area, serum amino acid profile, liver and renal function tests, and muscle composition. When glutamine or glutamine-dipeptide solutions were administered, the dogs showed increasing serum glutamine concentrations and enhanced glutamine uptake across the hind leg muscle. Similarly, both groups of rats demonstrated significant differences in serum glutamine levels, nitrogen balance, intestinal mucosa thickness, and villus area. We conclude that both glutamine and glutamine-dipeptide infusions increase serum glutamine concentrations and result in regional tissue effects. Both exerted similar metabolic effects with no apparent complications.

Effect of glutamine-supplemented total parenteral nutrition on the small bowel of septic rats

In order to study the effect of total parenteral nutrition (TPN) with or without glutamine supplementation in septic rats, septic Wistar albino rats were randomly assigned to receive 0.23 g of nitrogen and 113 kJ (100 g BW)(-1) per day in the form of amino acids with (group 2) or without (group 1) glutamine supplementation or 10% (w/v) glucose only (group 3). After 4 days of TPN treatments, rats receiving glutamine-supplemented TPN had a cumulative nitrogen balance of -24.4 +/- 3.3 mg N, which was significantly (P < 0.001) better compared to other TPN-treated groups. Septic rats of group 2 survived sepsis significantly (P < 0.001) better than those in groups 1 and 3. Glutamine-supplemented TPN treatment resulted in significant increases in jejunal weight (P < 0.001), DNA and protein contents (P < 0.001), villous height (P < 0.001) and crypt depth (P < 0.001) when compared with septic rats of group 1. Septic rats of group 2 extracted and metabolised glutamine by the small bowel at higher rates (P < 0.001) than that observed in septic rats of group 1. Increases in jejunal glutaminase (38.2%, P < 0.001) and decreases in glutamine synthetase (41.7%, P < 0.001) activities were observed in response to glutamine-supplemented TPN treatment. It is concluded that the administration of glutamine-supplemented TPN is beneficial to the small bowel of septic rats.

A review of the effects of glutamine-enriched diets on experimentally induced enterocolitis

Studies in animal models of enterocolitis have failed to confirm the purported metabolic and functional benefits of bowel rest induced by use of an elemental diet. Recent reports have demonstrated that glutamine-supplemented diets ameliorate or reverse many of the adverse effects of experimentally induced enterocolitis. Human studies are needed to confirm these findings.

Maintenance of small bowel mucosa with glutamine-enriched parenteral nutrition

Glutamine is an important fuel utilized by the intestinal mucosa that is not present in standard amino acid nutrition solutions. In order to determine the effects of glutamine on the intestine, glutamine enriched nutrition was administered intravenously to male Wistar rats. A standard amino acid solution was enriched with 1 and 2 g/100 ml of glutamine or glycine and used as part of a parenteral nutrition regime for 7 days. Intestinal samples were taken for measurements of jejunal weight, DNA, protein, mucosal thickness and villus height. Animals receiving 2 g glutamine/100 ml in the nutrition solution had increased intestinal weight, DNA, and villus height when compared to animals receiving 2 g/100 ml of glycine. No increase in the intestinal parameters was noted when 1 g/100 ml of glutamine was used. To investigate the dose-response effects of glutamine, further studies were performed using isonitrogenous and isocaloric solutions containing 0, 2, and 3 g of glutamine/100 ml. Animals receiving glutamine had a significant increase in mucosal weight, DNA, protein and villus height when compared to animals receiving no glutamine in the parenteral solutions. There was a dose-response relationship between the increase in jejunal DNA and the increased intake of glutamine (r = 0.93, p less than 0.01) but no correlation with the nitrogen content of the solutions (r = 0.18, p = 0.8). Total body nitrogen retention was greater in animals receiving 2 g/100 ml of glutamine (166 +/- 12 mg, days 6/7) when compared to those receiving 0 and 3 g of glutamine/100 ml (126 +/- 14 mg and 138 +/- 16 mg, respectively, p less than 0.05). These studies demonstrate that glutamine enriched nutrition protects against atrophy of the intestinal mucosa and when given at 2 g/100 ml improves nitrogen retention during intravenous feeding.

Effect of a glutamine-supplemented enteral diet on methotrexate-induced enterocolitis

Administration of an elemental diet to rats given methotrexate (MTX), 20 mg/kg intraperitoneally (ip), results in 100% mortality from severe enterocolitis. Previous studies indicate that glutamine (GLN), which is not present in elemental diets, is the preferred oxidative substrate for the gut and may facilitate intestinal recovery after injury. This study investigated the effects of a glutamine-supplemented elemental diet (GLN-ED) on nutritional status, intestinal morphometry, bacterial translocation and survival in this lethal model of intestinal injury. Three experiments were performed. In the first experiment, rats received an intragastric elemental diet supplemented with either 2% GLN or an equivalent amount of glycine (Control). After 4 days animals received either MTX, 20 mg/kg ip, or saline ip and were killed 3 days later. The GLN-ED resulted in significantly decreased weight loss, improved nitrogen retention, and increased mucosal weight, protein, and DNA content of the jejunum and colon. In the second experiment rats were assigned to diet as in the first experiment, but all animals received MTX. Control diet animals died within 120 hrs of MTX administration. The GLN-ED group had significantly longer survival time and decreased mortality. In the third experiment animals were assigned to diet and MTX as in the first experiment. Ninety-six hrs later aortic blood cultures revealed enteric bacteremia in animals administered MTX. GLN-ED resulted in a significant reduction in the incidence of bacteremia. These experiments showed that a GLN-ED significantly improved nutritional status, decreased intestinal injury, decreased bacterial translocation, and resulted in improved survival in a lethal model of enterocolitis.

Intestinal tissue and cell cultures

The culture of animal cells and tissues is a widely used technique in the field of cellular and molecular biology; one of the most interesting aspect being linked to the study of the mechanisms of cell differentiation. In the specific case of intestinal epithelial cells, various tissue culture technologies have proved to be important tools for the study of precise facets related to intestinal function, pathology and differentiation. Concerning this latter aspect, organ culture experiments have brought about interesting data on the hormonal or nutritional control of intestinal maturation. Nevertheless, the study of the precise mechanisms underlying epithelial proliferation and/or differentiation at the cellular level needs more adequate cell culture model systems. One of them has been described for two cell lines derived from human colonic adenocarcinomas, in which the cells can be induced to achieve enterocytic-like differentiation. Up to date, none of the continuous cell lines starting from normal undifferentiated cells have allowed generation of morphological or functional enterocytic polarity. In contrast, primary cell cultures which allow maintenance of a more physiological environment for the epithelial cells like contacts with their in vivo counterparts, mesenchymal cells or extracellular matrix molecules, have proved to be promising approaches.