Morphogenesis of the small intestine during fetal development.

During the last five days ('last trimester') of the 22-day gestation period of the rat the mucosa of the small intestine changes from undifferentiated stratified epithelium without villi to a mucosa with villi covered with simple columnar epithelium. During this process many secondary lumina form in the primitive stratified epithelium; these lumina enlarge and eventually fuse with the main intestinal lumen as degenerating superficial epithelial cells are exfoliated and as upward growth of mesenchyme towards the main lumen takes place. Proliferation of intestinal epithelial cells occurs along the entire length of the newly formed villi until one or two days before birth, when proliferating epithelial cells become confined to incompletely formed crypts at the base of the villi. In contrast, differentiation of the small intestinal mucosa in human fetuses begins much earlier in gestation. Villi form at 9 to 10 weeks and crypts are well developed by 12 weeks (first trimester). By 17 weeks, all epithelial cells types seen in intestinal crypts of adults are present. Absorptive cells on villi have a prominent apical tubular system, large meconium-filled lysosomes and abundant glycogen between 10 and 22 weeks' gestation. Whereas there is uptake of the macromolecular marker, ferritin, into the apical tubular system after eight or more minutes of in vitro exposure, there is no evidence of transport of ferritin across the absorptive cells after up to 40 minutes of exposure between 11 and 12 weeks' gestation.