Chimeric-transgenic mice represent a powerful tool for studying how the proliferation and differentiation programs of intestinal epithelial cell lineages are regulated.

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Abstract
An in vivo system has been developed for examining the effects of wild-type or mutant proteins on cell fate determination in the mouse intestinal epithelium or on the proliferation and differentiation programs of its component epithelial lineages. This system takes advantage of the fact that at the conclusion of gut morphogenesis, each intestinal crypt is composed of a monoclonal population of cells descended from a single active multipotent stem cell, each villus is supplied by several monoclonal crypts, and the four principal cell types of the intestinal epithelium differentiate during a rapid, geographically well-organized migration along the crypt-to-villus axis. Embryonic stem (ES) cells (129/Sv origin) are initially transfected with recombinant DNAs consisting of a reporter of interest linked to transcriptional regulatory elements that control the cell lineage-specific, differentiation-dependent, and axial patterns of expression of fatty acid binding protein genes in the gut. Stably transfected ES cells are subsequently introduced into host C57BL/6 blastocysts to generate chimeric-transgenic mice. At the borders of ES cell-derived and host blastocyst-derived epithelium, intestinal villi are found that are supplied by both ES cell- and host blastocyst-derived crypts. These villi can be rapidly identified in fixed whole-mount preparations of intestine using the alpha-L-fucose-specific Ulex europaeus agglutinin type I (UEA-I) lectin. They appear striped because UEA-I recognizes a cell-surface carbohydrate polymorphism between the inbred strains used to generate the chimeric animals. The strength of this system derives from the fact that two gut epithelial populations can be compared and contrasted that occupy virtually identical positions along the crypt-to-villus and duodenal-to-colonic axes within the same animal and differ only by the presence or absence of a single gene product. The band of blastocyst-derived epithelium in these striped, polyclonal villi can be used as an internal control to assess the biological effect of the transfected gene product produced in the adjacent stripe of ES-derived cells. The system can be used for either gain-of-function or loss-of-function experiments.

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