Fig. 5. Electron microscopy revealed abnormal ultrastructural morphology in the colonic epithelium of Klf4^–/– mice. (A) Colonic epithelium from wild-type mice on postnatal day 1 showed the normal goblet cell morphology (arrows). The nucleus (black arrowhead) was densely packed and located at the base of the cell, while the apical portion of the cell was comprised largely of membrane-bound secretory vacuoles containing mucigen. Normal colonocytes (*) were interspersed. (B,C) Colonic epithelium from Klf4^–/– mice on postnatal day 1. (B) Several abnormally shaped or immature cells of the goblet lineage (arrows) were seen emptying their contents into the lumen. Adjacent to these cells, several cells had densely packed nuclei (black arrowheads) suggestive of a goblet cell lineage but lacked the typical secretory vacuoles and normal goblet cell shape. Colonocytes (*), recognized by their paler cytoplasm and less condensed nuclei, appeared normal. (C) The colonic epithelium of the Klf4^–/– mouse also demonstrated small clusters of secretory vacuoles (arrows) scattered throughout the epithelium. Scale bars: 2 µm.