HUMAN INTESTINAL DEVELOPMENT IN A SEVERE-COMBINED IMMUNODEFICIENT XENOGRAFT MODEL

The present work describes a severe-combined immunodeficient murine xe nograft model used to investigate human gastrointestinal ontogenesis. Specifically, the study has tested whether carefully selected regions of human fetal gut are able to undergo region-specific morphogenesis a nd epithelial cytodifferentiation when transplanted subcutaneously int o immunodeficient mice. In addition, double-label in situ hybridisatio n techniques, utilising specific human and mouse DNA probes, have been adopted to characterise host and donor cell types and to investigate the potential developmental roles for non-epithelial cells in the regu lation of epithelial differentiation pathways in vivo. Human fetal sma ll and large bowel developed to form a characteristic mucosa 10 weeks after transplantation, which displayed clear region-specific structura l and functional gradients. The initial phase of xenograft epitheliali sation closely resembled the stratified type of epithelium which is pr esent during early fetal gastrointestinal development. Idiosyncratic e pithelial differentiation pathways were recorded during xenograft rege neration, with an absence of Paneth cells and an abundance of enteroen docrine cells when compared with developed xenograft and paediatric in testine. Such differences may, therefore, be important in ensuring rap id and region-specific development in the absence of conventional lumi nal stimuli and hormonal changes that occur normally during pregnancy. In situ hybridisation demonstrated an exclusively human origin for th e intestinal xenograft epithelium and muscularis mucosa and externa. A lthough the submucosa and lamina propria were comprised of a chimeric mixture, murine cells were rarely seen to contact with the epithelium, which interacted primarily with human myofibroblasts and human intrae pithelial lymphocytes. It is proposed that a 'selection' process opera tes to maintain species-specific cellular interactions, and this mecha nism may subsequently play an important role in regulating epithelial cell differentiation, orchestrated in part by juxtaposed non-epithelia l cell types.