Microscopic Structure of the Spleen

Stroma and Trabecular Vessels

The stroma of the spleen performs both a supportive and a motor function. The organ is covered externally by a serous membrane (mesothelium), beneath which lies a tough capsule composed of dense fibrous connective tissue. Branching partitions, known as trabeculae, extend inward from the capsule.

A key feature of the capsule and trabeculae is the presence of numerous smooth myocytes. Upon contraction, these cells cause the spleen to compress, expelling stored blood into the general circulation.

Blood vessels travel within the connective tissue partitions:

• Trabecular arteries feature a classic structure with a well-developed circular layer of myocytes in the middle tunic (tunica media).
• Trabecular veins lack a muscular layer. They consist solely of an endothelium and an adventitial layer that is tightly fused with the collagen matrix of the trabecula itself. Because of this structural arrangement, venous vessels do not collapse when emptied and remain constantly patent, ensuring an immediate outflow of blood when the organ contracts.

White Pulp and Lymphatic Follicles

The basis of the white pulp is formed by lymphatic follicles (nodules). Their primary histological marker is the presence of a central artery. Contrary to its name, this vessel lies eccentrically, meaning it is offset toward the edge of the nodule.

The follicle exhibits a distinct zonal organization and consists of four cellular regions:

1. Periarterial lymphatic sheath (PALS). Surrounds the artery. This is a T-dependent area (the functional analogue of the lymph node paracortex). T lymphocytes and antigen-presenting interdigitating cells predominate here.
2. Germinal center (reactive zone). Located to the side of the vessel. This is the site of proliferation and selection of B lymphocytes. Structurally, it is divided into dark, light basal, and light apical zones.
3. Mantle zone. Surrounds the germinal center and the periarterial sheath. Contains macrophages, proplasmacytes, and memory B cells. Plasma cells do not linger here but migrate into the red pulp.
4. Marginal zone. A transitional area bordering the red pulp, surrounded by a network of venous sinuses and capillaries. This is a unique splenic structure with no analogue in lymph nodes. Both T and B lymphocytes are found here, and the zone serves as a sorting site for incoming blood cells.

Red Pulp: Cords and Sinuses

The red pulp fills all the space between the trabecular network and the lymphoid nodules. In histological sections, it appears lighter than the white pulp (due to a lower density of cell nuclei), but in the living organ, it imparts a deep red color due to the abundance of erythrocytes.

Two structural and functional elements are distinguished within it:

• Splenic cords (cords of Billroth). These are regions of reticular tissue whose meshes contain blood cells, macrophages, and plasma cells. Plasma cells actively synthesize antibodies, fulfilling the same role as medullary cords in lymph nodes.
• Venous sinuses. These are wide sinusoidal capillaries (ranging from 12 to 40 µm in diameter). Their basement membrane is discontinuous, and gaps called fenestrae exist between elongated endothelial cells. Externally, the wall lacks pericytes. Sphincter-like structures at the entry and exit of the sinuses allow them to dilate and store significant volumes of blood when blood flow is restricted.

Functions: Erythrocyte Clearance and Iron Metabolism

In addition to acting as an immune filter, the spleen functions as an "erythrocyte graveyard." Macrophages within the splenic cords are responsible for screening out old cells.

Macrophages recognize aged and damaged erythrocytes based on two criteria:

1. Biochemical: a decrease in sialic acid levels on the erythrocyte membrane, leading to a drop in surface charge.
2. Mechanical: aging erythrocytes lose cytoskeletal flexibility (become rigid), causing them to get trapped in the reticular meshwork of the cords and preventing them from squeezing back through the fenestrae into the venous sinus.

After engulfing an erythrocyte, the macrophage hydrolyzes the protein portion of hemoglobin into amino acids. The heme group loses its iron ion and is converted into bilirubin (a bile pigment). The liberated iron binds to the protein transferrin and is transported via the bloodstream to the red bone marrow or delivered directly to erythroblasts for the synthesis of new hemoglobin.