Spleen

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The Spleen - Anatomy & Histology

Overview

The spleen is the largest secondary lymphoid organ in the body, weighing approximately 150 g in adults. It lies in the left upper quadrant (left hypochondrium) of the abdomen, against the diaphragm at the level of ribs IX-X. It develops from the dorsal mesentery of the stomach and retains peritoneal connections to nearby viscera.

Spleen position and ligaments relative to stomach, left kidney, and colon

Gross Anatomy

Surfaces

The spleen has two surfaces:

Diaphragmatic surface - smooth and convex, facing upward and laterally against the diaphragm
Visceral surface - concave medial face bearing impressions from adjacent organs (stomach, left kidney, tail of pancreas, splenic flexure of colon)

Peritoneal Ligaments

The spleen is covered by visceral peritoneum except at the hilum, and is held in place by two peritoneal folds (both parts of the greater omentum):

Ligament	Connects To	Contents
Gastrosplenic ligament	Greater curvature of stomach	Short gastric & left gastro-omental vessels
Splenorenal ligament	Left kidney	Splenic artery, splenic vein, tail of pancreas

Vasculature

Arterial supply: Splenic artery (largest branch of the celiac trunk) - it runs a tortuous course along the superior border of the pancreas before entering the hilum
Venous drainage: Splenic vein drains into the portal vein (after joining the superior mesenteric vein behind the neck of the pancreas)

Arterial supply to the spleen from the celiac trunk

Internal Architecture / Histology

The splenic parenchyma ("pulp") is divided into two functionally distinct compartments:

Red Pulp (~75% of splenic volume)

Composed of blood-filled vascular sinusoids lined by macrophages and littoral cells
Sinusoids are filled with large numbers of erythrocytes
Drain into venules - splenic vein - portal circulation
The red pulp macrophages are the key filter: they phagocytose senescent red cells, damaged cells, opsonized microbes, and immune complexes
This is where splenic "slow circulation" occurs: red cells must squeeze through gaps between sinusoidal lining cells - rigid/damaged cells cannot pass and are destroyed

White Pulp (lymphoid tissue)

Organized around central arteries (branches of the splenic artery distinct from those supplying sinusoids):

Zone	Cell Population	Function
Periarteriolar lymphoid sheath (PALS)	Mainly T cells	T cell zone; chemokines CCL19/CCL21 attract naive T cells via CCR7
B cell follicles	B cells	Primary follicles (naive B cells) and secondary follicles with germinal centers after antigen stimulation
Marginal zone	Marginal zone B cells + specialized macrophages	Boundary between red and white pulp; samples blood-borne antigens; contains functionally distinct MZ B cells with limited antigen repertoire

The white pulp architecture is analogous to a lymph node - T and B zones are segregated by chemokine gradients produced by fibroblastic reticular cells (FRCs). CXCL13/CXCR5 drives B cells into follicles; CCL19/CCL21/CCR7 drives T cells into the PALS.

Marginal Sinus

A vascular channel that forms the border between white and red pulp. Antigens from the blood are delivered into the marginal sinus by circulating dendritic cells, or sampled directly by marginal zone macrophages.

Blood Flow Through the Spleen

Blood enters via the splenic artery → divides into trabecular arteries (surrounded by fibrous trabeculae) → central arteries → two routes:

"Open" / slow circulation - arterioles open into the red pulp cords; blood percolates through the reticular meshwork, squeezes through sinusoidal walls back into venous sinusoids (allows macrophage screening of red cells)
"Closed" / fast circulation - arterioles connect directly into sinusoids, bypassing the cords

Both routes ultimately drain into trabecular veins → splenic vein.

Key Histological Cell Types

Cell	Location	Role
Red pulp macrophages	Sinusoidal lining	Phagocytose senescent RBCs, opsonized bacteria
Marginal zone macrophages	Marginal zone	Sample blood antigens, present to B cells
Follicular dendritic cells	B cell follicles	Trap antigen for B cell activation
Fibroblastic reticular cells	White pulp stroma	Form conduit network; produce CCL19/CCL21/CXCL13
Marginal zone B cells	Marginal zone	T-independent antibody responses to polysaccharides
Plasma cells	Red pulp	Terminal antibody secretion

Clinical Note: Asplenia

Individuals without a functional spleen (congenital asplenia, splenectomy, or functional asplenia e.g. sickle cell) are at high risk for overwhelming post-splenectomy infection (OPSI), particularly from encapsulated bacteria - Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae. This is because:

The spleen is a major site of opsonin (IgM) production
Splenic macrophages clear encapsulated bacteria that have been opsonized - this function is lost
The marginal zone B cells, which respond to polysaccharide antigens in a T-independent manner, are absent

Sources: Gray's Anatomy for Students; Cellular and Molecular Immunology (Abbas & Lichtman)

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