(a and b) Four-week-old grafts of colonic epithelium derived from isolated crypts. Invaginations similar to those observed during normal crypt development can be seen, with differentiated goblet cells and secreting mucins at the apical pole, and proliferating cells (labeled with tritiated thymidine) at the basal pole. (c) At later times some of these grafts develop a structure more typical of adult intestinal crypts. Adapted from ref. 19.