Figure 1: Time line of intestinal development in human and mice. Definitive endodermal cells are specified during gastrulation (mouse: embryonic day 6 E6, human: week 3 W3) and initiate the formation of the primitive gut tube, fully formed at E9.5 in mice and W4 in humans. At later stages, the tube is patterned along the anterior-posterior axis. Cytodifferentiation and villus formation take place from E14.5 in mouse and from W9 in human. In mice, crypt formation starts around day 15 after birth (postnatal day 15, P15), whereas in humans, mature crypt-villus architecture is already defined during fetal period from W11.