Digestive diseases affect more than 60 million Americans each year and account for more than $100 million in direct and indirect medical costs. Most digestive diseases are very complex and have subtle symptoms. In many cases, the cause(s) remain(s) unknown. Resection of the small intestine may be required, which can lead to a state of malnutrition and malabsorption if the functional gut mass is reduced below the minimum amount required for digestion and absorption to satisfy the nutrient and fluid requirements. This condition is commonly referred to as short bowel syndrome [88]. The normal physiologic process of intestinal adaptation after extensive resection usually allows for recovery of sufficient intestinal function within weeks to months, and during this time, patients can be sustained on total parenteral nutrition. However, prolonged parenteral nutrition can lead to complications such as hepatic dysfunction, progressive nephric insufficiency, and bone demineralization [89]. Surgical procedures such as small intestine tapering and lengthening have been undertaken to lengthen the bowel or increase intestinal transit time, but none have found widespread clinical application [90, 91]. Small intestinal transplantation is a promising surgical alternative, but the usual concerns regarding immunosuppression, rejection and limited donor supply remain [92].

Tissue engineering has been proposed as an alternative to allogeneic transplantation. In initial studies, enterocytes were isolated from neonatal Lewis rats, seeded on nonwoven PGA sheets, and formed into tubular structures. These constructs were implanted into the omentum or mesentery of syngeneic adult rats. Stratified epithelium was observed after 2 weeks, but the newly formed tissue had the histological appearance of embryonic intestine rather than adult intestine [93, 94]. It was hypothesized that this approach was limited due to the absence of epithelial-mesenchymal cell-cell interactions that are indispensable for survival, morphogenesis, proliferation, and differentiation. To allow for these interactions, the concept of epithelium organoid units was developed. These epithelium organoid units consist of a villus structure with overlying epithelium and a core of mesenchymal stromal cells. Mixed populations of enterocytes and stromal cells were harvested from the small intestine of neonatal Lewis rats, seeded on nonwoven PGA sheets, and transplanted into syn-geneic adult rats. The epithelium organoid units maintained their epithelial-mesenchymal interactions and resulted in the formation of large cystic structures [95]. The inner lumen was lined with a neomucosa consisting of columnar epithelium containing goblet and Paneth cells, indicative of organ morphogenesis, cytodifferentiation, and phenotype maturation [96]. Subsequent studies showed that small bowel resection provides significant regenerative stimuli for morphogenesis and differentiation of tissue-engineered small intestine. Portacaval shunts were also stimulatory, but to a lesser extent [97, 98]. Implantation of the organoid unit/polymer constructs in highly vascularized beds such as the omentum or mesentery emerged as a reliable approach to form cystic structures with a small intestine-like morphology. The next step was to assess whether anastomosis between the tissue-engineered and native small intestine had an effect on cyst growth. Three weeks after implantation in the omentum, the tissue-engineered small intestine was anastomosed to the native jejunum in a side-to-side fashion. There was no evidence of stenosis or obstruction at the anastomosis site. Following anastomosis, the cysts were lined with a neomucosa that was continuous with the native small intestine across the anastomotic site. A positive effect of the anastomosis on the cyst size and the development of the mucosa in the tissue-engineered intestine were noted. Furthermore, crypt-villus structures were observed [99, 100]. Subsequent investigations to demonstrate the feasibility of end-to-end anastomoses showed a moderately high patency rate and a positive effect on the size of the neointestine and the development of the neomucosa [101]. Further studies were conducted to assess the effect of anastomosis alone or in combination with small bowel resection on neoin-testine organization [102]. A long-term investigation showed that anastomosis between tissue-engineered and native small intestine had a low complication rate after the operation and resulted in a high patency rate for up to 36 weeks. During this period, the neointes-tine increased in size and was lined with a well-developed mucosa [103].

The concept of epithelium organoid units was also applied to other organs of the gastrointestinal tract. A tissue-engineered colon was assessed as an alternative to an ileal pouch after a colectomy in a rat model. An end ileostomy alone was compared to an end ileostomy combined with a side-to-side ileum-tissue-en-gineered colon anastomosis. The tissue-engineered colon resulted in higher transit times, with lower stool moisture content and higher total serum bile acids [104]. The effect of cell source, i.e., adult or neonatal tissue, was also assessed. The architecture of the tissue-engineered colon resembles that of native colon (Fig. 16.4). Furthermore, it was found that the in vitro function was consistent with that of mature colono-cytes [105].

Fig. 16.4 a Gross morphology of tissue-engineered colon at 4 weeks after implantation in a rat model. Intestinal organoid units, mesenchymal cell cores surrounded by a polarized epithe-lia, were isolated from full-thickness sigmoid colon dissection, seeded on a polymer scaffold and implanted into the omentum of syngeneic hosts, resulting in cyst formation. The cysts were subsequently anastomosed to either the small or large intestine in a side-to-side fashion. b Immunohistochemical staining for actin of native (a) and tissue-engineered (b) colon. Both stain positively in the muscularis propria Original magnification x 10. (Reprinted with permission from [105])

The concept of a tissue-engineered stomach has also been investigated as an alternative to currently used reconstruction techniques after a total gastrectomy. Tissue-engineered stomachs were created from stomach organoid units isolated from neonatal and adult donor rats and implanted in syngeneic adult rats. The resulting cysts resembled native stomachs histologi-cally [106]. Tissue-engineered stomachs were successfully used as replacement stomachs in a rat model by resecting the native stomach and anastomosing the tissue-engineered stomach between the native esophagus and jejunum. An upper gastrointestinal study revealed no evidence of bowel stenosis or obstruction at both anastomosis sites. Histologically, the tissue-engineered stomachs had well-developed, vascularized tissue with a neomucosa continuously lining the lumen and stratified smooth muscle layers [107].

Tissue engineering of the gastrointestinal tract has been shown to be a versatile model for studying the gastric physiology. Using this approach, important insights into tissue development and potential therapy can be gained. A recent study has characterized the mi-crovasculature and angiogenic growth factor profile of tissue-engineered intestine. While tissue-engineered intestine has the histological appearance of native tissue, the mechanism driving angiogenesis differs in tissue-engineered intestine and in normal small intestine. Delivery of angiogenic factors like vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) is proposed as a remedy, and this may bring tissue-engineered intestine closer to clinical applications [108].

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