Specialized cell types in the human fetal small intestine

In the present study we describe the time of appearance and morphological differentiation of specialized epithelial cells in human fetal small intestine (SB). Proximal and distal SB from 36 nonviable fetuses was studied by light and electron microscopy. During the 9‐ to 10‐week period, villi lined by simple columnar epithelium replaced the stratified epithelial lining which was two to six cell layers thick. During this transition, distinctive junctional complexes and a single secondary lumen were identified in the deeper layers of stratified epithelium, and there was evidence of cellular degeneration of some superficial cells. Oligomucous and mature goblet cells were present in both the stratified and simple columnar epithelium. Crypt formation began proximally at 10 to 11 weeks and, within a week, crypts lined by undifferentiated crypt cells (UCC) could also be identified in the distal SB. These cells resembled adult UCC's except for the presence of large aggregates of glycogen, and the absence of large adult‐type secretory granules (SG) until 16 weeks. At all ages SG's were smaller and less numerous than in adults. Paneth cells appeared with crypt development at 11 to 12 weeks. Unlike adult Paneth cells their SG's were structurally heterogeneous and frequently had cores with halos of differing density. Caveolated or tuft cells with dense bundles of microfila‐ments extending from microvilli into apical cytoplasm, apical granules, occasional caveolae, and a microvillus membrane denser than that of adjacent cells were identified by 16 weeks. Putative microfold (“M”) cells were seen in the distal SB of a 17‐week fetus. These cells had an unusual apical border with irregular projections, many small membrane bound vesicles in the cytoplasm, and were in direct contact with underlying lymphoid cells. The glandular cells of Brunner's glands at 14 to 15 weeks resembled those of normal adult.