Pinocytosis by epithelium associated with lymphoid follicles in the bursa of fabricius, appendix, and Peyer's patches. An electron microscopic study

Abstract

The fine structure and micropinocytotic capabilities of epithelial cells closely associated with lymphoid follicles in the chicken bursa of Fabricius, rabbit appendix, and mouse Peyer's patch were compared. Epithelial cells capable of transporting ferritin and India ink tracers from the lumen were demonstrated in all three locations. Epithelial cells not associated with lymphoid follicles in the bursa and appendix did not express pinocytotic activity. Lymphoid cells were identified in bursal epithelium of chick embryos as early as the twelfth day of incubation. These lymphoid cells were smaller than typical bursal lymphocytes, had dense cytoplasm, numerous cytoplasmic projections, and prominent nucleoli. The small lymphoid cells were first demonstrable at a time in incubation during which lymphoid stem cells have been shown to migrate into the bursal epithelium. Lymphoid cells were seen earlier than the specialized follicle‐associated epithelium. It is concluded that specialized pinocytotic follicle‐associated epithelium does not induce initial migration of stem cells into areas along the intestinal tract, but that this transepithelial pinocytotic flow of intestinal contents after birth may provide a significant stimulus for attraction, proliferation and egression of lymphocytes.