Supporting and Contractile Apparatus
Histologically, the framework of the spleen is formed by dense irregular connective tissue. It is represented by an outer capsule and inward-projecting partitions called trabeculae. Within the parenchyma, trabeculae repeatedly anastomose to form a dense three-dimensional supporting network.
A unique feature of the organ's stroma is the presence of smooth myocytes within the connective tissue. These muscular elements turn the capsule and trabeculae into a functional muscular skeletal apparatus: upon contraction, the organ compresses, ensuring the emergency release of pooled blood into the systemic circulation.
White and Red Pulp
The splenic parenchyma is supported by reticular tissue and functionally divided into two major zones.
White pulp is organized lymphoid tissue formed around arterial vessels. Unlike lymph nodes, which filter lymph, this zone mounts immune responses to foreign antigens circulating directly in the blood. It includes:
- Periarterial lymphatic sheaths (PALS): Dense accumulations of T lymphocytes surrounding pulp arteries like a cellular sleeve.
- Lymphoid follicles (malpighian corpuscles): Spherical structures around central arteries, composed predominantly of B lymphocytes (with a small number of T cells).
Red pulp occupies all the space between the lymphoid structures of the white pulp and the connective tissue trabeculae. It consists of two key structures:
- Splenic cords (cords of Billroth): Aggregations of reticular tissue whose meshes contain blood cells, plasma cells, and macrophages. The primary task of local macrophages is the phagocytosis and destruction of aged erythrocytes and platelets.
- Venous sinuses: Wide postcapillary vessels whose main function is the pooling (storage) of specific blood volumes.
Vascular Supply and Circulation
The vascular system of the organ is strictly hierarchical. Blood flow begins at the splenic artery and follows these stages:
- Trabecular arteries travel within the connective tissue partitions.
- Upon exiting the trabecula, the vessel enters the white pulp as a pulp artery, surrounded by the T-cell zone. Entering a lymphatic follicle, it becomes the central artery (arterule of the follicle), from which radiating capillaries branch off.
- Leaving the follicle, the vessel enters the red pulp, branching into penicillar arterioles.
- These lead to sheathed arterioles (ellipsoid capillaries). Their walls are thickened by accumulations of macrophages and lymphocytes that trap antigens. Their endothelial cells contain contractile filaments to regulate organ blood content.
From the short terminal capillaries, microcirculation splits into two pathways:
- Closed circulation: Capillaries empty directly into venous sinuses.
- Open circulation: Blood from capillaries pours directly into the stroma (splenic cords). There, aged cells are phagocytosed, while viable erythrocytes squeeze through the sinus walls back into the circulation.
Venous drainage begins at the sinuses (where reverse flow of blood back into the cords can occur during congestion), collects into short pulp veins, and then into non-muscular trabecular veins. The latter merge to form the splenic vein.
Functions of the Spleen
All physiological tasks of the organ are closely tied to its histological structure:
- Blood storage (reservoir): Carried out in the venous sinuses of the red pulp, where erythrocytes and platelets accumulate until needed.
- Elimination of blood cells: Localized in the red pulp cords, where macrophages destroy old and damaged formed elements.
- Immune defense: Provided by T and B lymphocytes of the white pulp responding to blood-borne antigens, with plasma cells migrating into the red pulp.
- Participation in myelopoiesis: During the embryonic period, the organ performs universal hematopoiesis (production of all cell types); in adults, it secretes humoral factors that inhibit erythropoiesis in the red bone marrow.