Isolated bowel segment (Iowa Model 1): technique and histological studies

An isolated bowel segment (IBS) is a viable loop of bowel that is completely free of its mesenteric attachments. We created an IBS by staged procedures: (1) myoenteropexy between the undersurface of the abdominal wall muscle and a jejunal loop that is exteriorized at both ends as mucous fistulae; and (2) division of the IBS mesentery several weeks later. Viability of the IBS is preserved by vascular collaterals that develop at the myoenteropexy during the interval between these two procedures. In this study, histological observation of the IBS was performed to retrospectively determine the optimal interval required for adequate collateral circulation to develop. Twenty-eight rats were subdivided into seven groups of four rats each; each group underwent mesenteric division of the IBS at successive 1-week intervals after myoenteropexy (1 to 7 weeks). The bowel wall structures were histologically examined under light microscopy for each group after mesenteric division. Ischemic changes were observed in the groups in which the intervals were shorter than 6 weeks. With the time interval longer than 6 weeks, no ischemic changes were observed in the intramural ganglia and muscle layers and minimal changes were noted in the mucosa. This study concluded that an IBS can be safely created in the rat that preserve normal bowel structures when its mesentery is divided 7 weeks after myoenteropexy.     

Functional innervation of the isolated bowel segment

PURPOSE: The aim of this study is to investigate whether there might be an eventual change in the enteric nerve responses to electrical field stimulation (EFS) of the isolated bowel segment (IBS) created by omentoenteropexy. METHODS: In the experimental group, an IBS was created in 10 rats using the omentum as the host organ by dividing of its mesentery 4 weeks later. In the control group, a jejunal loop of identical site and length to the IBS was studied in another 10 rats as normal jejunal segment. Longitudinal muscle strips were prepared from the IBSs (n 20) and the normal jejunal segments (n = 20). The effects of atropine, tetrodotoxin, L-arginine, and L-nitroarginine methyl ester (L-NAME) on the responses to transmural EFS were examined in both the IBS and normal jejunal segment using the isometric tension recording technique. RESULTS: Transmural EFS of the IBS strips produced contractile responses. Atropine and tetrodotoxin abolished the EFS-induced contractions of the IBS. Moreover, EFS-induced contractions were increased by the application of L-NAME, and were decreased by the application of L-arginine in the IBS. Mean percent change of IBS's contraction was not found statistically different from mean percent change of normal jejunal segment's contraction on the effects of L-NAME and L-arginine. CONCLUSION: These results suggest that the IBS, created by omentoenteropexy, produced enteric nerve responses to EFS as seen as in normal jejunal segment.     

Isolated bowel segment (model 1): creation by myoenteropexy

A surgical technique to create an isolated bowel segment (IBS) that is completely free of its mesenteric attachment, and yet preserves the viability of IBS is described. The procedure consists of (1) initial myoenteropexy between the abdominal wall muscle and the IBS, and (2) secondary division of the mesentery of the IBS 6 weeks later when the vascular collaterals have formed at the myoenteropexy. The IBS created by this technique is useful for the studies of bowel physiology and possible elongation of the bowel in the management of the short-bowel syndrome.     

The isolated bowel segment (Iowa Model II): absorption studies for glucose and leucine.

A model of the isolated bowel segment (IBS, Iowa Model II) was successfully created in experimental animals using a new surgical technique we developed. The IBS is completely free of its mesenteric attachment, yet its viability is preserved. The technique consists of two staged procedures: (1) initial enteropexy between the anterior margin of the liver and the antimesenteric border of the IBS with its ends forming cutaneous stomas; and (2) division of the IBS mesentery 5 weeks later. The IBS is nourished by vascular collaterals that form at the hepatoenteropexy during the interval between these two procedures. Our previous studies demonstrated preserved viability and motility in the IBS. This study was undertaken to test absorption in the IBS. In 25 rats (experimental group), the IBS (Iowa Model II) was created using an 8-cm-long isolated segment of jejunum. In 15 rats (control group), an 8-cm-long segment of jejunum was arranged to form a Thiry-Vella loop. Five weeks later, the IBS mesentery was divided in the experimental group, and sham laparotomy was performed in the control group animals. Absorption of glucose and leucine was studied in 13 rats of the experimental group and 6 of the control group using a constant single perfusion technique at 3, 8, and 11 weeks after the initial operation. The results were compared between the two groups. There was a 25% to 35% reduction in absorption of glucose and leucine in both groups with the advance of time, but no significant difference was observed between the groups except in leucine absorption at 11 weeks after the initial operation. This study concludes that absorption of glucose and leucine is preserved in the IBS after its mesentery is divided, suggesting that the IBS can be used as a functioning bowel for bowel reconstruction.     

The isolated bowel segment (Iowa model II) created in functioning bowel

In experimental rats (n = 15), an isolated bowel segment (IBS) was created by (1) initial enteropexy between an 8-cm-long jejunal segment and the liver margin (hepatoenteropexy; Iowa model II) with its proximal and distal ends divided and immediately reanastomosed in an end-to-end fashion to reconstruct the bowel; and (2) secondary division of the IBS mesentery 5 weeks later. The IBS is then completely free of its mesentric and intramural nervous and vascular communications. The viability of the IBS is preserved by vascular collaterals developed at the hepatoenteropexy. Twelve rats proceeded to the second procedure, having tolerated regular rat chow with satisfactory weight gain. The Iowa model II created in functioning bowel was evaluated by contrast studies and myoelectrical activities. Contrast studies demonstrated peristalsis in the IBS. In the myoelectrical recordings, the frequency of slow wave was 32.5 +/- 1.0 in the IBS and 36.3 +/- 0.8 in the normal bowel (P less than .05). During fasting, the migrating motor complex (MMC) was observed to propagate aborally in the IBS in a coordinated fashion. The cyclic period of the MMC was 17.2 +/- 1.1 minutes in the IBS and 15.8 +/- 0.8 minutes in the normal bowel (P = .30). We conclude from this study that (1) the IBS (Iowa model II) retains motor function as demonstrated by successful feeding, as well as contrast studies and myoelectrical recordings that were essentially identical to those in the normal bowel; and (2) the IBS (Iowa model II) has significant research potential for studies of bowel physiology.     

The isolated bowel segment (Iowa model II): motility across the anastomosis with or without mesenteric division.

In previous reports, anastomosis has been shown to disrupt the myoelectric activity of the bowel. However, these studies have failed to delineate the role of the extrinsic nerves. Using an isolated bowel segment (IBS) and an amesenteric bowel segment (ABS), motility was evaluated by myoelectric recording across a bowel anastomosis. Ten rats were divided equally into the experimental group with the IBS and the control group with the ABS. In the IBS group, an 8-cm segment of jejunum was divided, reanastomosed, and coapted to the liver margin (Iowa model II). In the ABS group, an 8-cm segment of jejunum was coapted to the liver margin without disruption of bowel continuity (Iowa model II variant). Two weeks later, bipolar electrodes were implanted in the IBS and ABS, and normal jejunum in both groups. Mesenteric division (MD) was performed 4 weeks later to eliminate extrinsic innervation. Myoelectrical recordings were taken 2 weeks before and after MD. In the control group with IBS, incoordination in the propagation of the migrating motor complex (MMC) and reduction in the frequency of slow waves (FSW) were observed across the anastomosis and were unchanged by MD. In the control group with the ABS, the MMC and FSW were identical to that in the normal jejunum and were unaffected by MD. In both groups postprandial inhibition of the MMC was the same as in the normal jejunum and was unaffected by MD. This study confirms that incoordination in propagation of the MMC and reduction in FSW occur across a bowel anastomosis, and elimination of extrinsic innervation does not affect the autonomy of these changes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Restoration of mechanically lengthened jejunum into intestinal continuity in rats

PurposePrior studies demonstrated the feasibility of lengthening intestinal segments with mechanical force, but no previous studies have restored the lengthened segment back into intestinal continuity.MethodsA 1-cm segment of isolated rat jejunum was lengthened using a Nitinol spring. After lengthening, this segment was restored into intestinal continuity via a transection of the intact small intestine. Rats were euthanized 2 weeks later to retrieve the restored intestinal segment for histologic and enzymatic analyses.ResultsThe isolated jejunal segments were initially lengthened to 3.3 ± 0.9 cm. After the lengthened segments were restored into intestinal continuity for 2 weeks, the final length of the restored segment was 1.9 ± 0.7 cm. All rats continued to gain weight, and the intestine proximal to the restored jejunal segment remained normal 2 weeks later. The restored jejunal segment had an increase in crypt depth and no difference in villus height compared with normal jejunum. Sucrase activity in the restored segment was not different from that in normal jejunum.ConclusionMechanically lengthened jejunum can be restored into intestinal continuity and appears to have normal function. This further demonstrates the feasibility of mechanical enterogenesis as a potential therapy for short bowel syndrome.     

Changes in morphology and function in small intestinal mucosa after Roux-en-Y surgery in a rat model

BackgroundCurrently there is no an appropriate model to study intestinal mucosal atrophy in vivo that preserves the nutritional status of the organism.Materials and methodsWe created a defunctionalized segment of jejunum via a dead-end Roux-en-Y anastomosis in rats. We compared tissue morphometric parameters in the intestinal mucosa of the defunctionalized bowel with that of the mucosa proximal and distal to the anastomosis. We further measured extracellular signal-regulated kinase (ERK) activation within the mucosa as well as sucrase-isomaltase and dipeptidyl peptidase-4 levels as markers of intestinal mucosal differentiation by Western blotting of mucosal scrapings.ResultsThree days after anastomosis, the defunctionalized bowel exhibited decreased diameter and thickness of both the mucosa and the fibromuscular layer compared with adjacent bowel in continuity for luminal nutrient flow or with bowel from control animals. Sucrase-isomaltase and dipeptidyl peptidase-4 levels also were decreased. Furthermore, mucosal ERK activation, assessed as the ratio of phosphorylated to total ERK, also was reduced. Animal weights did not differ between bypassed and control animals.ConclusionsDeprivation of nutrient flow in a segment of bowel by defunctionalizing Roux-en-anastomosis produces mucosal atrophy as indicated by altered histology, differentiation marker expression, and ERK signaling, in animals that are otherwise able to maintain enteral nutrition.     

Effect of luminal contents on postresectional longitudinal and mucosal growth in the ileum of suckling rats

Effects of luminal contents on adaptive growth of the ileum in the neonatal small bowel of rats were examined after resection of the proximal small bowel and after removal of the ileum from the enteric stream. Four weeks after resection of the proximal 60% of the small bowel in 10-day-old rats, the distal 40% of the small bowel elongated by 281%. This distal segment also elongated by 265% after intestinal transection (p greater than 0.05), but by only 191% when it was bypassed (bypass versus resection or transection, p less than 0.001). After resection the distal villi were 81% taller, but after transection they were only 19% taller (p less than 0.001 versus resection); after bypass they did not grow (p less than 0.001 versus resection or transection). The distal crypts were 404% deeper after resection, 331% deeper after transection (p less than 0.05), and 291% deeper after bypass (p less than 0.001 versus resection, p greater than 0.05 versus transection). The DNA content was 517% greater after resection, 364% greater after transection (p less than 0.001), and 73% greater after bypass (p less than 0.001 versus transection or resection). Maximal elongation of the small bowel occurs in the presence of luminal nutrition. Increasing luminal nutrition is associated with increasing mucosal hyperplasia.     

Autotransplantation of splenic tissue in an isolated segment of small intestine.

The ability of splenic tissue to regenerate when implanted in an isolated segment of small intestine with intact circulation was studied in six pigs. After total splenectomy, 10 per cent of the weight of the spleen was implanted in a 10-15-cm long isolated segment of small intestine with an intact vascular supply. Bowel continuity was established by end-to-end anastomosis. Before implantation, the mucosal layer was completely removed from the isolated segment of the small intestine. The animals were killed 6 months later and the isolated segment of small bowel containing the splenic tissue identified. Most of the implanted splenic tissue was recovered in the isolated segment of small intestine; the weight ranged from 43 to 120 (mean 80) per cent of that of the implanted tissue. An isolated segment of small intestine with an intact circulation produces a higher index of regeneration than other previously reported sites.     

Transport of small bowel contents after interposition of an antiperistaltic jejunal segment in the rat

The effect of antiperistaltic jejunal segments on small bowel transport was studied with an experimental technique permitting quantitative studies of propulsive as well as non-propulsive intestinal motility. 79 rats were provided with a permanent duodenal catheter and at the same laparotomy an anti- or isoperistaltic bowel segment based on 1, 2 or 3 mesenteric end-arcades was inserted at the midpoint of the jejunum. The evaluation of intestinal motility 1 week later indicated that an antiperistaltic segment of the rat jejunum has to be made at least 6 cm long, i.e. based on 3 mesenteric end-arcades in order to retard the propulsion of luminal contents. Not even this relatively long antiperistaltic segment was found to influence propulsion through bowel segments proximal to it, and distally the propagation velocity was accelerated. This implied that the total small bowel transit was unaffected. The non-propulsive intestinal motility, i.e. the mixing of small bowel contents was increased by the long reversed loop. The possible beneficial effect of antiperistaltic jejunal segments in short bowel syndrome and post-vagotomy diarrhoea in man may thus be attributed not to a retardation of the propulsive intestinal motility but to an increase in the non-propulsive intestinal motility.     

Maintenance of gut mass in bypassed bowel of orally vs parenterally nourished rats.

Twenty-six male rats (experimental) underwent bypass of more than 70% of their small bowel, and nine rats underwent small bowel transection and reanastomosis. Rats were fed standard laboratory chow for the initial 18 postoperative days and then were switched to an elemental nutrient solution consisting of amino acids, glucose, vitamins, and minerals, either orally or parenterally (TPN) for the next 10 to 13 days. At the end of the experiment, in continuity and bypassed segments from both parenterally and orally fed animals were compared to each other and to similar segments of bowel from control rats.Our data indicate that oral intake and/or digestion and absorption of nutrients by the in continuity alimentary tract provides a stimulus to hyperplasia in the in continuity small bowel and a systemic stimulus that permits maintenance of gut mass in large bypassed segments of small bowel. Such a systemic stimulus is absent when the rat is provided TPN. Bypassed rats, remaining on oral nutrition throughout the 28- to 31-day postoperative period, had bypassed segments that maintained bowel weight, mucosal protein and RNA and DNA content similar to comparable segments of control bowel, while bypassed bowel from the TPN group underwent atrophy. The systemic stimulus does not appear to be gastrin since serum and antral gastrin levels were similar in bypassed and control rats and serum levels were consistent with previously determined fasting gastrin levels.     

Roux-en-Y on-line intussusception to avoid ascending cholangitis in biliary atresia

A child with biliary atresia experienced severe ascending cholangitis when the exteriorized loop of a portahepatoenterostomy was internalized. Backwash of bowel contents was reduced and further cholangitis was prevented by creation of an on-line intussuscepted Roux-en-Y jejunal loop to provide a nonrefluxing conduit from the porta hepatis. The loop could then be internalized and antibiotic therapy could be discontinued. The success of this procedure in this case encourages us to use this method in similar cases of biliary atresia. It may be of use in all Roux-en-Y loops, such as those used to drain the transplanted liver of patients with biliary atresia and those created to drain the distal part of the transplanted pancreas.     

Short-Term Adaptation of Rat Intestine to Ileostomy: Implication for Pediatric Practice

Background: Surgical neonates with complex intestinal conditions, such as enterocolitis, midgut volvulus with bowel loss and multiple atresias, often require temporary stomas. Little is known on the postsurgical response of the altered gut segments, although adaptation is an important consideration in neonatal postoperative care, particularly after stoma closure. Materials and Methods: Rats underwent bowel resection at a point 15 cm proximal to the ileocecal valve, and a split ileostomy was performed. On the 6th postoperative day the mucosal thickness was calculated with Soft Imaging System Analysis Pro, the rate of proliferation was measured following Ki67 immunohistochemistry and the apoptotic index was determined on sections stained with ApopTag Plus. The intestinal motor activity was recorded on isolated gut segments. Neuronal nitric oxide synthase (nNOS) expression and distribution was examined with NADPH-diaphorase histochemistry and Western blot analysis. Results: An increased wet weight of the mucosa and a pronounced mucosal thickening were observed in the proximal functional bowel segment. Enterocyte proliferation rate was increased significantly, while the apoptotic index remained unchanged in the epithelial layer. The dilation of the gut lumen resulted in a morphological change in the nitrergic myenteric network with an overexpression of nNOS. As a consequence of the surgical procedure, the functional proximal gut segment showed strong and frequent contraction waves, with an enhanced responsiveness to cholinergic stimuli. Conclusions: The dilated functional bowel segment was characterized by hyperplasic changes in the mucosa and stronger mechanical activity with overproduction of nNOS. Although early restoration of intestinal continuity is recommended, our observations on adaptive changes may partly explain intestinal motility disorders after early stoma closure, suggesting the need for a careful approach to a redo-laparotomy.     

Analysis of mucosal regeneration and the expression profile of heat shock protein 70 in the isolated small bowel segment

To investigate the mechanisms underlying the regeneration of the denervated small bowel segment from ischemic injury, we examined the heat shock protein 70 (HSP70) expression profile and compared it to the histopathological and proliferative changes seen in an isolated bowel segment using a rabbit model. Two weeks following the small bowel segment transplantation to the subcutaneous space, the vascular pedicle was sectioned. Next, the rabbits were divided into three groups based on the collection time of the isolated bowel segment (3, 7, and 14 days for Groups A, B, and C, respectively). The unsectioned small bowel segment was utilized as a control for each group. Histological studies showed that no experimental group recovered from mucosal injury. The bromodeoxyuridine labeling index showed that the more severe mucosal injury group had the highest bromodeoxyuridine incorporation. In the experimental groups, HSP70 immunoreactivity was intensely seen in the regenerating epithelial cells and inflammatory cells. In the control mucosa, HSP70 immunoreactivity was weakly seen in the subepithelial stromal tissue, crypt cells and not in epithelial cells. The present study shows that the isolated bowel segment requires longer periods for regeneration from ischemic injury and HSP70 may play an important role during the regeneration process.     

Bombesin stimulates mucosal growth in jejunal and ileal Thiry-Vella fistulas.

OBJECTIVE: The authors determined whether the trophic effects of bombesin (BBS) on the small bowel mucosa are mediated by either nonluminal factors or endogenous luminal secretion. SUMMARY BACKGROUND DATA: The gut hormone bombesin stimulates growth of small bowel mucosa. The mechanisms responsible for this trophic effect are not known. METHODS: Rats underwent construction of a Thiry-Vella fistula (TVF) of either the jejunum or ileum. On postoperative day 10, the two groups were subdivided to receive either saline (control) or bombesin (10 micrograms/kg, subcutaneously, three times a day). After 14 days, rats were killed and the TVF was removed. The mucosa was scraped and weighed, and DNA and protein content was determined. RESULTS: Bombesin significantly increased mucosal weight and DNA and protein content of both the jejunal and ileal TVF compared with the control rats. CONCLUSIONS: Bombesin-mediated stimulation of small bowel mucosal growth is mediated by factors that are independent of luminal contents and pancreaticobiliary secretion. Bombesin may prove to be an important enterotrophic factor for gut mucosal proliferation.     

Mucosal hyperplasia of the ileum after subtotal colectomy in rats

To evaluate factors contributing to ileal mucosal hyperplasia following extensive colectomy, the following three models were designed in rats: Subtotal colectomy and end-to-end ileoproctostomy; retransection of the terminal ileum with end-to-side proximal-ileoproctostomy (colectomy group), End-to-side ileoproctostomy without colectomy (bypass group), and End-to-side ileocecostomy (control group). In all groups the terminal ileum was left out of the intestinal stream as defunctionalized segments. Specimens were obtained from the terminal ileum during operation and from both the functioning and the defunctionalized segments at various intervals postoperatively. The mucosal hyperplasia was evaluated by measuring eight variables including numbers of villi around the bowel circumference, villous height, crypt depth, total mucosal thickness, epithelial cell counts per villus, DNA contents per unit length of the bowel and radioactivity of incorporated 3H-thymidine as well as DNA specific activity. The results showed that marked mucosal hyperplasia of the functioning segments was noted in all groups, and that mucosal hyperplasia was also observed in defunctionalized segments of colectomized rats, but not in those of both bypass rats and control. These findings were statistically confirmed by analysis of variance. It was concluded that although intraluminal factors played an important role in mucosal hyperplasia of the ileum following extensive colectomy, humoral factors might be also involved in intestinal adaptation.     

Laparoendoscopy (laparoscopy assisted enteroscopy) and partial resection of small bowel

We investigated laparoendoscopy (laparoscopy-assisted small bowel enteroscopy) as an aid in managing the challenging diagnostic problem of occult small bowel hemorrhage. We performed laparoendoscopy on five nonconditioned dogs and five conditioned dogs. In each case, we laparoscopically inspected the small bowel from the ligament of Treitz to the ileocecal valve. Small bowel was exteriorized, an enterotomy was performed, and an Olympus CL-100L colonoscope was inserted and advanced proximally to the stomach and distally to the ileocecal valve. A segment of small bowel was arbitrarily designated as the site of bleeding, was exteriorized, and was resected. Bowel continuity was restored with a stapled anastomosis. The animals were euthanized and autopsied immediately after the procedure (n=5) or a week following the procedure (n=5), at which time the anastomoses were tested for leakage and patency. The entire small bowel was visualized in all animals. There were no anastomotic leaks. Laparoendoscopy may be of value in identifying and treating hemorrhagic origin of small bowel. Clinical trials are warranted.     

Replacement of colonic mucosa by free and pedicled grafts of ileal mucosa in the dog

Mucosal proctectomy and ileo-anal anastomosis is a relatively new operation in the treatment of patients with ulcerative colitis. The diseased rectal mucosa is removed and gastrointestinal continuity is restored by replacing it with full thickness ileum. The main disadvantage of the operation is frequency of bowel action and in an attempt to resolve this problem the feasibility of replacing colonic mucosa with grafts of ileal mucosa alone was explored. Both free and pedicled autografts of ileal mucosa were grafted into isolated loops of canine colon, denuded of mucosa and submucosa. All the free grafts failed to survive (eight dogs) and histological examinations confirmed graft rejection. All the pedicled grafts (seven dogs) were successful and retained the histological characteristics of normal ileum. Net movement and bidirectional transport of water and electrolytes were measured in six dogs with pedicled grafts and in four control dogs with intact full thickness ileal loops. Whereas the control loops absorbed water and electrolytes, the grafted loops secreted water, sodium, and potassium and absorbed significantly less chloride. The mucosa of the colon can thus be replaced successfully by grafts of ileal mucosa provided that an intact blood supply is preserved. The capacity and sensation of the neorectum after mucosal proctectomy may be improved if mucosal grafts were used instead of full thickness ileum. Before this procedure could be used in man, however, practical problems would have to be overcome and the secretion of water and electrolytes corrected.     

Recombinant human interleukin-11 prevents mucosal atrophy and bowel shortening in the defunctionalized intestine

BACKGROUND: Mucosal atrophy and bowel shortening are the hallmark of proximal intestinal diversion for extensive necrotizing enterocolitis (NEC) or Thiry-Vella fistulas (TVF), in which the ends of a defunctionalized loop of intestine are exteriorized as stomas. Recombinant human interleukin-11 (rhIL-11) is a pleiotropic cytokine that promotes epithelial regeneration and enhances adaptation after bowel resection. The authors hypothesized that rhIL-11 may prevent mucosal atrophy and bowel shortening in rats with TVF METHODS: After creation of ileal TVF, Sprague-Dawley rats were selected randomly to receive either rhIL-11 or equal volume of 0.1% bovine serum albumin (BSA) subcutaneously daily. On day 14, the TVF were excised and examined morphologically. Enterocyte apoptosis was measured using the TUNEL assay. Mucosal DNA and protein content were measured. RESULTS: Administration of rhIL-11 resulted in a significantly greater weight gain and less shortening of TVF than BSA treatment. TVF from the rhIL-11-treated group showed evidence of hyperplasia and hypertrophy and increased crypt to villus ratio. The BSA group had substantial mucosal atrophy. There was a qualitative decrease in the incidence of apoptosis in the rhIL-11 group. CONCLUSIONS: Recombinant human IL-11 prevents mucosal atrophy and shortening of defunctionalized intestinal loops. It may help reduce the incidence of short gut syndrome in infants with extensive NEC.