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Fig. 1 The zebrafish intestine and its pattern of cell renewal. (A) Micrograph of the posterior intestine of a living 2-week-old fish. Intestinal villi are clearly visible through the transparent body wall. The epithelial cells are coloured brown by uptake of pigment in the diet. (B) Sketches showing development of the gross anatomy of the zebrafish gut. A, anterior segment; M, middle segment; P, posterior segment. (C-F) Toluidine Blue-stained plastic sections of intestinal bulb (C,E) and posterior intestine (D,F) at 3 days and 1 month. The intestine matures rapidly from a simple tube to a more elaborate structure with villi. The intestinal bulb initially has a flattened shape (white outline in C). (G-J) Electron micrographs identify goblet cells (G), brush-border cells (H) and enteroendocrine cells (I,J). (J) Enlargement of the area boxed in the top-left corner in I, showing characteristic secretory granules. (K-M) Proliferating cells are progressively restricted to the intervillus pockets as shown by PCNA labelling (green) in the intestinal bulb. (N) Pulse-chase experiment showing the migration of BrdU-labelled cells (green) along the villi of the intestinal bulb 0, 24, 72 and 96 hours after the pulse. (O) TUNEL labelling (green) showing programmed cell death in cells at the tops of the intestinal villi. (P) At 1 month of development, proliferation is more plentiful in the posterior part of the gut, as shown by PCNA labelling (green). (K-P) Confocal optical sections; TOPRO-3 nuclear stain is in red.