HA25 shows the central artery in cross-section. Fine capillaries, which cannot be seen in this image, from the central artery course through the white pulp to terminate in the sinuses of the marginal zone (mz). The white pulp immediately surrounding the central artery is called the PeriArteriolar Lymphatic Sheath (PALS). This is the zone of T cells. Outside of this region is a zone where lymphocytes are less densely packed - the Peripheral White Pulp (PWP). This is the zone of B cells. The boundary between the peripheral white pulp and the marginal zone is sometimes quite distinct (HA25, arrow).