The development of the bovine ileal mucosa was studied with particular reference to maturation during the fetal and neonatal period. In this region, by 4–5 months of fetal development, vacuolation of the epithelial cells had occurred on the villi, and the goblet and absorptive cells in the crypts were present. By 6–9 months, the villi were longer and more numerous than in the previous stages. At the same time, the vacuolated cells could be seen predominantly on the upper half of each villus. The absorptive cells and goblet cells were more distinct in the crypt and lower half of each villus. Moreover, the goblet cells showed differences in mucin, while in the submucosa the lymphoid follicles were seen to have enlarged to become a prominent feature of the Peyer’s patches at this stage. At birth, in suckled animals, the ileal cells on the lower area of each villus and in the crypt appeared more like mature cells. In contrast, there were numerous inclusion bodies in epithelial cells on the upper half of each villus. They appeared in the apical portion of the cytoplasm as vacuoles with stainable or dense contents. By 1 week, however, epithelial cells no longer contained inclusion bodies, and absorptive and goblet cell populations had begun to emerge from the crypts.These histological results suggest that the bovine ileal mucosa has two distinct turning points during its development in the fetus and the neonate. Initially all the mucosal structures are present in fetuses at 6–7 months of gestation, and then the vacuolated cells covering the ileal villi are replaced by mature, nonpinocytosing epithelium which emerges from the crypts on or before the 7th day after birth (ileal closure).

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