Microscopic structure of the spleen

Stroma and trabecular vessels

The stroma of the spleen performs not only a supporting function, but also a motor function. Externally, the organ is covered by a serous membrane (mesothelium), beneath which lies a tough capsule composed of dense fibrous connective tissue. Branching partitions—trabeculae—extend inward from the capsule.

A key feature of the capsule and trabeculae is the presence of numerous smooth myocytes within them. When these contract, the spleen compresses, forcing pooled blood into the general circulation.

Blood vessels run inside the connective tissue partitions:

• Trabecular arteries have a classic structure with a well-developed circular layer of myocytes in the tunica media (tunica media).
• Trabecular veins lack a muscular layer. They consist solely of endothelium and an adventitial layer that is tightly fused with the collagen matrix of the trabecula itself. As a result, the venous vessels do not collapse when emptied and always remain gaping, ensuring immediate blood outflow upon organ contraction.

White pulp and lymphoid follicles

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

The follicle has a distinct zonal organization and consists of four cellular areas:

1. Periarterial zone. Surrounds the artery. This is a T-dependent area (the functional equivalent of the lymph node paracortex). T lymphocytes and antigen-presenting interdigitating cells predominate here.
2. Germinal center (reactive zone). Lies away from 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 at the border with the red pulp, surrounded by a network of venous sinuses and capillaries. This is a unique structure of the spleen having no analogs in lymph nodes. Both T and B lymphocytes meet 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 preparations, it appears lighter than the white pulp (due to a lower density of cell nuclei), but in the living organ, it gives it a rich red color due to the abundance of erythrocytes.

It comprises two structural and functional elements:

• Splenic cords (Bilroth's cords). These are areas of reticular tissue in the meshes of which blood cells, macrophages, and plasma cells are located. Plasma cells actively synthesize antibodies, performing the same role as in the medullary cords of lymph nodes.
• Venous sinuses. These are wide sinusoidal capillaries (ranging from 12 to 40 µm in diameter). Their basement membrane is discontinuous, and there are gaps—fenestrae—between the elongated endothelial cells. Externally, the wall lacks pericytes. At the entry and exit of the sinuses, there are sphincter-like structures: by restricting blood flow, they allow the sinuses to stretch and store significant volumes of blood.

Functions: erythrocyte utilization and iron metabolism

In addition to acting as an immune filter, the spleen functions as an "erythrocyte graveyard." Macrophages of the splenic cords are responsible for their utilization.

Macrophages recognize aged and damaged erythrocytes by two criteria:

1. Biochemical: the level of sialic acids on the erythrocyte membrane decreases, leading to a drop in surface charge.
2. Mechanical: the aging erythrocyte loses its cytoskeletal flexibility (becomes rigid), causing it to get trapped in the reticular tissue meshwork and preventing it from squeezing back through the fenestrae into the venous sinus.

Having engulfed the erythrocyte, the macrophage hydrolyzes the protein part of hemoglobin into amino acids. The heme loses its iron ion and is converted into bilirubin (bile pigment). The released iron binds to the protein transferrin and is transported via the bloodstream to the red bone marrow, or transferred directly to erythroblasts for the synthesis of new hemoglobin portions.

Class and exam questions

What else to study on this topic

• Embryogenesis of the spleen: local stromal sources and migrating cell pools
• Comparative characteristics of T- and B-dependent zones of the spleen and lymph node
• Cellular composition of the germinal center: centroblasts, centrocytes, and immunoblasts
• Mechanisms of iron reutilization and the role of the carrier protein transferrin
• Structural features of the sphincter apparatus of red pulp venous sinuses

Other topics in the section "Hematopoietic and lymphoid organs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.