FIGURE 14.29 • Schematic diagram and photomicrograph of splenic structure. a. The substance of the spleen is divided into white pulp and red pulp. White pulp consists of a cylindrical mass of lymphocytes arranged around a central artery that constitutes the periarterial lymphatic sheath (PALS). Splenic nodules occur along the length of the PALS. When observed in cross section through part of the sheath that contains a nodule, the central artery appears eccentrically located with respect to the lymphatic mass. The red pulp consists of splenic sinuses surrounded by splenic cords (cords of Billroth). A capsule surrounds the spleen; trabeculae project from it into the substance of the spleen. Both capsule and trabeculae give the appearance of dense connective tissue infiltrated by numerous myofibroblasts. Blood vessels traverse the capsule and trabeculae before and after passage within the substance of the spleen. Lymphatic vessels originate in the white pulp near the trabeculae. b. This low-magnification photomicrograph of the spleen reveals the same components shown in the previous drawing. Note the capsule with several trabeculae projecting into the substance of the spleen. In the center, there is a trabecula containing a trabecular vein through which blood leaves the organ. The red pulp constitutes the greater bulk of the splenic tissue. The white pulp contains lymphatic tissue that follows and ensheathes the central artery. Expansion of the white pulp creates the splenic nodules. 45.
and trabeculae contains myofibroblasts. These contractile cells also produce extracellular connective tissue fibers. In many mammals, the spleen holds large volumes of red blood cells in reserve. In these species, contraction in the capsule and trabeculae helps discharge stored red blood cells into the systemic circulation. The human spleen normally retains relatively little blood, but it has the capacity for contraction by means of the contractile cells in the capsule and trabeculae.
The hilum, located on the medial surface of the spleen, is the site for the passage of the splenic artery and vein, nerves, and lymphatic vessels. The lymphatic vessels originate in the white pulp near the trabeculae and constitute a route for lymphocytes leaving the spleen.
Most of the spleen consists of splenic pulp. Splenic pulp, in turn, is divided into two functionally and morphologically different regions: white pulp and red pulp, based on the color of fresh sections. White pulp appears as circular or elongated whitish gray areas surrounded by red pulp.
White pulp consists of a thick accumulation of lymphocytes surrounding an artery.
The white pulp consists of lymphatic tissue, mostly lymphocytes. In H&E–stained sections, white pulp appears  basophilic because of the dense heterochromatin in the nuclei of the numerous lymphocytes (Plate 39, page 483). Branches of the splenic artery course through the capsule and trabeculae of the spleen and then enter the white pulp. Within the white pulp, the branch of the splenic artery is called the central artery. Lymphocytes that aggregate around the central artery constitute the periarterial lymphatic sheath (PALS). The PALS has a roughly cylindrical configuration that conforms to the course of the central artery. In cross sections, the PALS appears circular and may resemble a lymphatic nodule. The presence of the central artery, however, distinguishes the PALS from typical lymphatic nodules found in other sites. Nodules appear as localized expansions of the PALS and displace the central artery so that it occupies an eccentric rather than a central position.