Experimental models of small intestinal transplantation in rats: orthotopic versus heterotopic model.

Two kinds of surgical models of small intestinal transplantation (SITx) in rats, namely heterotopic (HIT) and orthotopic transplantion (OIT), have been reviewed. In OIT, the small intestine of the recipient is removed and the transplanted intestine replaces it in continuity. On the other hand, in the HIT model, the small intestinal grafts are rendered dysfunctional without alimentary tract continuity. Histological evidence showed that acute rejection appeared earlier in HIT as compared to OIT. Hyperplasia and hypertrophy of the muscularis externa produced in the chronic rejection process were more pronounced in HIT allografts. The HIT grafts showed severe mucosal atrophy due to the lack of intraluminal trophic factors, because oral feedings can stimulate tropic hormones for mucosal growth, and provide nutrients for enterocytes. Intestinal permeability was consistently higher after HIT than after OIT. The HIT grafts demonstrated less contractility and less response to chemical stimulation than did OIT grafts. The OIT models are advantageous in studies of intraluminal nutrients, and intestinal secretions in these models might modulate the intestinal immune status and possibly delay rejection. The superior intestinal barrier function and the delayed onset of rejection in OIT rats suggest that nutrients and other factors in the succus entericus are important for the maintenance of intestinal graft function.