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Histology of spleen

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I now have comprehensive content from Junqueira's, Robbins, and Cellular Immunology textbooks along with multiple excellent histology images. I have enough to write a thorough, well-illustrated answer.

Histology of the Spleen

The spleen is the largest secondary lymphoid organ and the only one dedicated to filtering blood (not lymph). It weighs ~150 g in adults and sits in the left upper quadrant. Its parenchyma - the splenic pulp - is divided into red pulp and white pulp, embedded in a reticular framework.

1. Capsule and Trabeculae

The spleen is covered by a dense connective tissue capsule (smooth muscle + collagen fibers, stained red in Picro-Sirius-hematoxylin). From the capsule, trabeculae extend inward carrying branches of the splenic artery, vein, lymphatics, and nerves. The trabeculae form the internal scaffolding of the organ.
Low-power micrograph - Capsule (C), Trabeculae (T), White Pulp (W) and Red Pulp (R):
Spleen low power - capsule, trabeculae, white and red pulp
Capsule (C), trabeculae (T), white pulp (W, dashed circles), and red pulp (R). x20; Picro-Sirius-hematoxylin. - Junqueira's Basic Histology 17e, Fig. 14-21

2. White Pulp (~20% of splenic mass)

The white pulp appears as pale gray-white nodules on fresh cut surface and as darker blue-purple islands on H&E (densely packed lymphocytes). It has two main components:

a. Periarteriolar Lymphoid Sheath (PALS)

  • Surrounds the central arteriole (a small muscular arteriole that has left the trabecular connective tissue)
  • Composed predominantly of T lymphocytes (T cell zone), with scattered macrophages, dendritic cells, and plasma cells
  • This is analogous to the paracortex of lymph nodes

b. Lymphoid Nodules (Follicles)

  • B cells are located in follicles within and adjacent to the PALS
  • On antigenic stimulation, B cells proliferate and form germinal centers - pale central areas with activated blasts, identical to those in lymph nodes
  • When a nodule enlarges, the arteriole is pushed to an eccentric position but is still called the central arteriole
  • Nodules include primary follicles (quiescent, small dark B cells) and secondary follicles (with pale germinal centers)
White pulp (W) with central arteriole (arrowhead) in PALS, surrounded by red pulp (R):
White pulp of spleen with central arteriole
White pulp (W) - PALS with central arteriole (arrowhead), surrounded by red pulp (R). - Junqueira's Basic Histology 17e, Fig. 14-23a

c. Marginal Zone

  • A ring of specialized B cells and macrophages encircling each lymphoid nodule at the outer edge of the white pulp
  • Receives blood from marginal zone sinuses (branches of the central arteriole)
  • Marginal zone B cells have a limited repertoire and respond quickly to blood-borne antigens (esp. bacterial polysaccharides)
  • In humans there is an inner and outer marginal zone plus a perifollicular zone
  • Antigens are delivered here by circulating dendritic cells or sampled by macrophages

3. Red Pulp (~80% of splenic mass)

The red pulp is filled with blood and consists of two components:

a. Splenic Cords (Cords of Billroth)

  • Irregular masses of reticular tissue (reticular cells + reticular fibers) between the sinusoids
  • Contain T and B lymphocytes, macrophages, other leukocytes, and red blood cells
  • Macrophages in the cords form a "labyrinth" connected by long dendritic processes
  • Key function: macrophages phagocytose aging/damaged RBCs that squeeze through the sinusoidal wall; they also "pit" inclusions such as Heinz bodies and Howell-Jolly bodies from RBCs

b. Splenic Sinusoids

  • Wide, irregular, blood-filled channels lined by specialized stave cells (endothelial cells)
  • Stave cells are elongated, oriented parallel to blood flow, with large nuclei bulging into the lumen
  • The basement membrane beneath them is highly discontinuous - blood cells must squeeze through gaps between stave cells to re-enter the sinusoid from the cords
  • This acts as a mechanical filter: healthy, deformable RBCs pass through; rigid or abnormal RBCs are trapped and phagocytosed
H&E micrograph of red pulp - sinusoids (S) and cords (C):
Red pulp sinusoids and cords H&E
(a) Red pulp with sinusoids (S) and cords (C), both containing blood cells. x140. (b) Higher magnification - stave cell nuclei (arrows) bulging into sinusoidal lumens (S); splenic cords (C) lateral to sinusoids. x200. H&E. - Junqueira's Basic Histology 17e, Fig. 14-25

4. Blood Circulation in the Spleen

The splenic microvasculature is uniquely organized to facilitate immune surveillance. Blood flow:
Splenic artery → Trabecular artery → Central arteriole (in PALS) → Penicillar arterioles (in red pulp) → Sheathed capillaries → Two routes:
RoutePathSpeedFunction
Closed (fast) circulationCapillaries → directly into sinusoids → trabecular veinRapidMost blood takes this route
Open (slow) circulationCapillaries dumped into splenic cords → blood cells squeeze through sinusoidal wallsSlowBlood cells examined by macrophages; effete cells trapped
Only a small fraction takes the open route at any moment, but over a day the entire blood volume is screened by this system.
Schematic of splenic blood flow (PALS, white pulp, red pulp circulation):
Splenic blood flow diagram - PALS, white pulp, germinal center, penicillar arterioles, open and closed circulation
PALS (T cells), white pulp B cell nodule with germinal center, penicillar arterioles with macrophage sheaths (S), open and closed circulation routes. - Junqueira's Basic Histology 17e, Fig. 14-22

5. Normal Splenic Architecture Diagram (Robbins)

Normal splenic architecture - Robbins
Normal splenic architecture showing trabecular artery/vein, central artery, lymphoid follicle with germinal center, penicilliary arterioles, venous sinusoids, and splenic cords. - Robbins Cotran & Kumar, Pathologic Basis of Disease, Fig. 13.41

6. Framework and Supporting Stroma

  • The entire parenchyma rests on a reticular fiber framework (Type III collagen, demonstrated by silver stain)
  • Reticular fibers wrap around the sinusoids and fill the cords
  • Reticular cells are the stromal cells that produce these fibers and form channels guiding lymphocyte traffic
  • A small amount of smooth muscle in the capsule and trabeculae allows minor contraction to release stored blood

7. Functions of Each Histological Compartment

CompartmentCell PopulationMain Functions
White pulp - PALST lymphocytes, DCsT-cell activation; antigen presentation
White pulp - FolliclesB lymphocytesAntibody production; germinal center reactions
Marginal zoneMarginal zone B cells, macrophages, DCsFirst-line response to blood-borne antigens, esp. encapsulated bacteria
Red pulp - CordsMacrophages, lymphocytes, plasma cells, all blood cellsPhagocytosis of old/damaged RBCs; platelet sequestration; extramedullary hematopoiesis (fetal)
Red pulp - SinusoidsStave cells (endothelium)Blood filtration; discrimination of healthy vs. rigid RBCs

Key Histological Landmarks to Identify on a Slide

  1. Capsule - thick pink dense CT layer at periphery
  2. Trabeculae - pink CT bands projecting inward
  3. White pulp - darker blue-purple islands (dense lymphocytes) containing a central arteriole
  4. Germinal center - pale zone within a secondary follicle
  5. PALS - T cell sleeve around central arteriole
  6. Red pulp - lighter pink area with alternating pale sinusoids and darker cords
  7. Stave cells - elongated endothelial nuclei protruding into sinusoidal lumen
Sources:
  • Junqueira's Basic Histology, 17e - Figs. 14-21 to 14-26 (primary source)
  • Robbins, Cotran & Kumar - Pathologic Basis of Disease - Spleen section, Fig. 13.41
  • Cellular and Molecular Immunology (Abbas) - Spleen microarchitecture
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