An electron microscopic study of intraepithelial lymphocytes in human fetal small intestine.

Human small intestine was obtained from fetuses at 8 to 20 weeks of gestation. The tissues were examined by electron microscopy. Lymphocytes were present as isolated cells in the mucosal mesenchyme as early as the 8th week of intrauterine life and were found, in increasing numbers, in the epithelium of specimens obtained from 11- to 20-week-old fetuses. All intraepithelial lymphocytes were located in the lateral spaces between contiguous columnar cells. Some displayed cytoplasmic pseudopods and were thought to migrate within the extracellular spaces of the epithelial layer. Surface invaginations and cytoplasmic vesicles in fetal lymphocytes suggested that these cells were capable of pinocytotic activity. The possibile origin and function of lymphocytes in fetal small intestine are discussed.