splenic red pulp

Definition: The red pulp is the area of spleen in between white pulp and consists of open sinuses and cellular cords. The red pulp can be expanded markedly by venous congestion, extramedullary hematopoiesis, or by involvement by leukemias.

Histology

The red pulp is composed of a three dimensional meshwork of splenic cords and splenic sinuses.

 splenic cords

• The splenic cords are composed of reticular fibers, reticular cells, and associated macrophages (Saito et al., 1988).
• The reticular cells are considered to be myofibroblasts and may play a role in splenic contraction (Saito et al., 1988).
• The reticular fibers are composed of collagenous and elastic fibers, microfibrils, reticular cell basal laminae, and unmyelinated adrenergic nerve fibers (Saito et al., 1988). With electron microscopy, it is apparent that the reticular fibers are actually ensheathed by the reticular cells and their processes (Saito et al., 1988).
• The cellular splenic cords provide a tissue framework maintaining the network of sinuses.
• The splenic cords contain passing red blood cells, lymphocytes, monocytes, granulocytes, in addition to resident reticular fibroblasts, plasma cells, and macrophages.

 splenic sinuses

• Splenic sinuses are open vascular spaces lined by a discontinuous layer of endothelial cells and supported by a fenestrated basal lamina and reticular fibers.
• The sinus is lined by a discontinuous population of endothelial cells with slit-like spaces.
• Larger gaps in endothelial coverage are often present that serve as the conduit for blood cells to pass from surrounding splenic cords into the sinuses.
• The sinus contains all the blood elements.

The red pulp macrophages are actively phagocytic and remove old and damaged erythrocytes and blood-borne particulate matter.

Extra medullary hematopoiesis is common in rodent red pulp, especially in fetal and neonatal animals. Any combination of erythroid, myeloid, and megakaryocytic cells may be evident.

Venous sinuses can be found throughout the red pulp, including, as mentioned previously, directly adjacent to the marginal zone. They are lined by loose network of endothelial cells which sit on a basement membrane that is sandwiched between the endothelial cells and reticular fibers of the red pulp.

The penicillar arteries and arteriolar capillaries are also located in the red pulp, though they are more difficult to identify light microscopically.

Various pigments may be present in the spleen. Hemosiderin deposits in the cytoplasm of macrophages in the red pulp, and sometimes in the white pulp as well, are a typical finding.

In fact, iron pigments (i.e., hemosiderin and ferritin) are the most common pigments in the macrophages of the red pulp (Losco, 1992). Iron from the hemoglobin of phagocytized erythrocytes is converted to hemosiderin for storage in the spleen.

Ceroid and lipofuscin derived from oxidation of lipids is also typically found in the spleen, though they are less abundant than hemosiderin (Ward et al., 1999).

See also

 Spleen

• splenic white pulp