The nodules are the territory of B lymphocytes; other lymphocytes of the PALS are chiefly T lymphocytes that surround the nodules. Thus, the PALS may be considered a thymus-dependent zone similar to the deep cortex of a lymph node. The nodules usually contain germinal centers, which, as in other lymphatic tissues, develop as B cells proliferate after their activation. In humans, germinal centers develop within 24 hours after antigen exposure and may become extremely large and visible with the naked eye. These enlarged nodules are called splenic nodules or Malpighian corpuscles (not to be confused with the renal corpuscles that have the same name).
Red pulp contains large numbers of red blood cells that it filters and degrades.
Red pulp has a red appearance in the fresh state as well as in histologic sections because it contains large numbers of red blood cells (Plate 40, page 485). Essentially, red pulp consists of splenic sinuses separated by splenic cords (cords of Billroth). Splenic cords consist of the now-familiar loose meshwork of reticular cells and reticular fibers that contain large numbers of erythrocytes, macrophages, lymphocytes, dendritic cells, plasma cells, and granulocytes. Splenic macrophages phagocytose damaged red blood cells. The iron from destroyed red blood cells is used in the formation of new red blood cells; splenic macrophages begin the process of hemoglobin breakdown and iron reclamation. Megakaryocytes are also present in certain species, such as rodents and the cat, but not in humans except during fetal life.
The splenic or venous sinuses are special sinusoidal vessels lined by rod-shaped endothelial cells.
The endothelial cells that line the splenic sinuses are extremely long. Their longitudinal axis runs parallel to the direction of the vessel (Fig. 14.30). There are few contact points between adjacent cells, thus producing prominent intercellular spaces. These spaces allow blood cells to pass readily into and out of the sinuses. Processes of macrophages extend between the endothelial cells and into the lumen of the sinuses to monitor the passing blood for foreign antigens.
The sinuses do not possess a continuous basal lamina. Strands of basal lamina loop around the outside of the sinus much like the hoops that loop around the staves of a barrel. These strands are at right angles to the long axes of the endothelial cells. This material stains with silver-containing reagents or with the PAS reaction (Plate 40, page 485). Neither smooth muscle nor pericytes are present in the wall of splenic sinuses. Reticular cell processes may extend to the basal side of the endothelial cells and are probably associated with the reticular fibers that appear to merge with the perisinusoidal loops of basal lamina. Blood fills both the sinuses and cords of the red pulp, often obscuring the underlying structures and making it difficult to distinguish between the cords and the sinuses in histologic sections.
Circulation within red pulp allows macrophages to screen antigens in the blood.