Spleen

Supportive and contractile apparatus

Histologically, the framework of the spleen is formed by dense fibrous connective tissue. It is represented by an outer capsule and septa extending inward from it into the organ — trabeculae. Inside the parenchyma, the trabeculae anastomose (connect) repeatedly, forming a dense three-dimensional supporting network.

A unique feature of the organ's stroma is the presence of smooth myocytes in the connective tissue. These muscle elements turn the capsule and trabeculae into a kind of musculoskeletal apparatus of the spleen: upon their contraction, the organ compresses, ensuring the emergency expulsion of pooled blood into the systemic circulation.

White and red pulp

The spleen parenchyma is based on reticular tissue and is functionally divided into two major zones.

White pulp is an organized lymphoid tissue that forms around arterial vessels. Unlike lymph nodes, which filter lymph, this zone is responsible for the immune response against foreign antigens circulating directly in the blood. It includes:

• Periarterial sheaths: dense accumulations of T lymphocytes that surround the pulp arteries like a cellular "cuff".
• Lymphoid nodules (follicles): spherical structures around the central arteries, whose cell composition is represented predominantly by B lymphocytes (with a small number of T cells).

Red pulp occupies all the free space between the lymphoid structures of the white pulp and the connective tissue trabeculae. It consists of two key structures:

1. Splenic cords: accumulations of reticular tissue in the meshes of which blood elements, plasma cells, and macrophages are located. The main task of local macrophages is the capture and destruction of worn-out erythrocytes and platelets.
2. Venous sinuses: wide post-capillary vessels whose main function is the accumulation (pooling) of certain volumes of blood.

Specifics of blood supply

The vascular system of the organ is strictly hierarchical. Blood flow begins from the splenic artery and goes through the following stages:

1. Trabecular arteries run inside the connective tissue septa.
2. Emerging from the trabecula, the vessel enters the white pulp and becomes the pulp artery, around which the T-zone forms. Sinking into the lymphatic follicle, the vessel is renamed the central artery (artery of the nodule). Feeding capillaries diverge radially from it.
3. Leaving the nodule, the trunk enters the red pulp, where it breaks down into small branches — penicillar arterioles.
4. Next are the ellipsoid (sheathed) arterioles. Their wall is thickened due to accumulations of macrophages and lymphocytes that capture antigens. Their endothelial cells contain contractile filaments to regulate the blood filling of the organ.

From the short capillaries terminating the arterial pathway, the microcirculation divides into two beds:

• Closed circulation: capillaries empty directly into venous sinuses.
• Open circulation: blood from the capillaries pours directly into the stroma (splenic cords). There, old cells are phagocytosed, and viable erythrocytes squeeze through the wall of the sinuses back into the bloodstream.

Venous outflow begins from the sinuses (the phenomenon of "reverse movement" of blood into the cords upon overflow is possible in them), collects into short pulp veins, and then into non-muscular trabecular veins. The latter merge into the splenic vein.

Functions of the spleen

All physiological tasks of the organ are closely related to its histological structure:

• Storage: implemented in the venous sinuses of the red pulp, where erythrocytes and platelets accumulate until they are needed.
• Elimination of blood cells: localized in the cords of the red pulp, where macrophages destroy old and damaged formed elements.
• Immune defense: provided by T and B lymphocytes of the white pulp, which react to blood antigens, while plasma cells migrate to the red pulp.
• Participation in myelopoiesis: in the embryonic period, the organ performs universal hemopoiesis (production of all types of cells), and in an adult, it secretes humoral factors that inhibit the process of erythropoiesis in the red bone marrow.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of the structure of ellipsoid arterioles and macrophage sheaths
• Structure of reticular tissue as the basis of the stroma of hematopoietic organs
• Differences in the cellular composition of periarterial sheaths and lymphoid nodules
• Mechanism of migration of healthy erythrocytes from the red pulp through the wall of venous sinuses
• Humoral regulation of erythropoiesis by spleen factors

Other topics in the section "Hematopoietic and lymphoid organs"

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