It was suggested originally that human peripheral blood lymphocytes could be separated into two broad categories on the basis of findings of scanning electron microscopy, depending on their cell-surface morphology (25
). One population had a fairly smooth surface, whereas the other was described as hairy, being covered by numerous microvilli (Fig. 15.3
). The former was considered to correspond to thymus-derived (or T) lymphocyte, whereas the latter was thought to represent bone marrow–derived (or B) lymphocyte. Results of subsequent studies demonstrated that the surface features of lymphocytes depend on the methods used for preparation of the cells as well as the functional state of the cells (27
). Thus, lymphocytes stimulated by various mitogens have villi independent of their origin. In addition, both T and B lymphocytes have microvilli while in circulation and especially when they pass through the venules of LNs characterized by high endothelial cells (29
). On the other
hand, all lymphocytes have a smooth surface when they reach their respective home microenvironment (30
). Therefore, the smooth cell surface likely is associated with resting lymphocytes, whereas the appearance of microvilli is triggered by environmental stimuli that interact with cell-surface receptors. These newly formed microvilli may help lymphocytes to interact with target substrates.