Human spleen microanatomy: why mice do not suffice

Abstract

The microanatomical structure of the spleen has been primarily described in mice and rats. This leads to terminological problems with respect to humans and their species-specific splenic microstructure. In mice, rats and humans the spleen consists of the white pulp embedded in the red pulp. In the white pulp, T and B lymphocytes form accumulations, the periarteriolar lymphatic sheaths and the follicles, located around intermediate-sized arterial vessels, the central arteries. The red pulp is a reticular connective tissue containing all types of blood cells. The spleen of mice and rats exhibits an additional well-delineated B-cell compartment, the marginal zone, between white and red pulp. This area is, however, absent in human spleen. Human splenic secondary follicles comprise three zones: a germinal centre, a mantle zone and a superficial zone. In humans, arterioles and sheathed capillaries in the red pulp are surrounded by lymphocytes, especially by B cells. Human sheathed capillaries are related to the splenic ellipsoids of most other vertebrates. Such vessels are lacking in rats or mice, which form an evolutionary exception. Capillary sheaths are composed of endothelial cells, pericytes, special stromal sheath cells, macrophages and B lymphocytes. Human spleens most probably host a totally open circulation system, as connections from capillaries to sinuses were not found in the red pulp. Three stromal cell types of different phenotype and location occur in the human white pulp. Splenic white and red pulp structure is reviewed in rats, mice and humans to encourage further investigations on lymphocyte recirculation through the spleen.

Keywords: human spleen; rat and mouse spleen; sheathed capillaries; splenic follicles; splenic stromal cells.

Figure 1

(a) Drawing of a small part of the white pulp in a cross-section of an adult rat spleen as visualized by standard haemalum-eosin staining. The follicles are attached to the PALS at relatively large intervals. The MZ primarily covers the T-cell zone. GC, germinal centre; MZ, marginal zone; PALS, periarteriolar lymphocyte sheath; RP, red pulp. (b) Visualization of the marginal zone and certain follicular B lymphocytes in a cross-section of an adult male LEW rat spleen by monoclonal antibody HIS 57. Cryosection, avidin–biotin complex technique with diaminobenzidine chromogen (ABC-DAB) and haemalum nuclear stain. The MZ is interrupted by several MZ bridging channels. Scale bar =100 μm. (c) Drawing of part of the white pulp in a cross-section of an adult mouse spleen as visualized by standard haemalum-eosin staining. The follicles tend to fuse around the PALS (hatched region). Most central arteries are cut transversely. MZ bridging channels are not depicted. (d) Visualization of the marginal zone and follicles by detection of IgM in a cross-section of an adult male C57BL/6 mouse spleen. Paraffin section, ABC-DAB technique with haemalum nuclear stain. Arrows indicate marginal sinus. Scale bar = 100 μm. (e) Drawing of a part of the human splenic white pulp as visualized by standard haemalum-eosin staining. MANZ mantle zone, SZ, superficial zone; PFZ, perifollicular zone. (f) Sequential visualization of IgD (ABC-DAB technique, brown) and CD20 (alkaline phosphatase polymer technique with Fast blue chromogen, blue) in a human splenic PALS (left) and follicle (right). In the most superficial zone of the follicle IgD⁻CD20⁺ B cells prevail. In other experiments most of these cells are CD27⁺. A marginal sinus is absent. Photograph taken from reference 55. Scale bar =100 μm.

Figure 2

(a) Schematic drawing of a periarteriolar lymphocyte sheath (PALS) (left), a secondary follicle (centre) and red pulp arterioles (right) in the normal adult human spleen. Only different stromal cells and B-cell types, but not T cells, are depicted. PALS, follicles, vessels and cells are not drawn to scale. The superficial stromal cells and the fibroblastic reticulum cells (FRCs) are sheet-like, when sectioned tangentially. The expression of smooth muscle α-actin (SMA) in FRCs and non-FRC/non-follicular dendritic cell (FDC) stromal cells is individually variable. Follicles may also be traversed by side branches of central arteries. (b) Visualization of CD271 (brown) and SMA (blue) in a paraffin section of an adult human spleen by subtractive double-staining. FRCs in a PALS (left) and FDCs in a follicle (right) are strongly CD271⁺. Capillary sheaths in the surrounding red pulp are also strongly stained. Scale bar = 100 μm. (c) Visualization of CD271 (brown) and MAdCAM-1 (blue) in a cryosection of an adult human spleen by subtractive double-staining. A follicle has CD271⁺ FDCs and is surrounded by MAdCAM-1⁺ non-FRC/non-FDC stromal cells and CD271⁺ capillary sheaths. Scale bar = 100 μm.

Figure 3

(a) Schematic drawing of a branching arteriole and a sheathed capillary in the adult human splenic red pulp. The vessels are not drawn to scale, the course of the capillary has been shortened and the form of the open capillary end is hypothetical. Whether pericytes also occur in arterioles is not clear. T cells are not depicted. (b) Visualization of a capillary sheath by subtractive double staining for CD20 (brown) and CD271 (blue) in a paraffin section. The CD271⁺ stromal sheath cells are surrounded by CD20⁺ B cells. Scale bar = 50 μm. (c) Visualization of a capillary sheath by subtractive double-staining for CD163 (brown) and CD68 (blue). The majority of the capillary sheath-associated macrophages are CD68⁺CD163⁻. Scale bar = 50 μm.