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2006 34: 455Toxicol Pathol
Mark F. Cesta
Normal Structure, Function, and Histology of the Spleen
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Toxicologic Pathology, 34:455–465, 2006
Copyright C ?by the Society of Toxicologic Pathology
ISSN: 0192-6233 print / 1533-1601 online
Normal Structure, Function, and Histology of the Spleen
MARK F. CESTA
Integrated Laboratory Systems, Inc., Durham, North Carolina 27713, USA
The spleen is the largest secondary immune organ in the body and is responsible for initiating immune reactions to blood-borne antigens and for
filtering the blood of foreign material and old or damaged red blood cells. These functions are carried out by the 2 main compartments of the spleen,
the white pulp (including the marginal zone) and the red pulp, which are vastly different in their architecture, vascular organization, and cellular
composition. The morphology of these compartments is described and, to a lesser extent, their functions are discussed. The variation between species
and effects of aging and genetics on splenic morphology are also discussed.
Immunopathology; lab animal; marginal zone; morphology; red pulp; spleen; white pulp.
The spleen is a dark red to blue-black organ located in the
left cranial abdomen. It is adjacent to the greater curvature of
size of the spleen are variable, depending on the species and
the degree of distension; nonetheless, spleen weights can be
important in its evaluation. The ratio of splenic weight to
body weight remains fairly constant regardless of age and, in
rats, is typically around 0.2% (Losco, 1992).
The functions of the spleen are centered on the systemic
circulation. As such, it lacks afferent lymphatic vessels. It
is comprised of 2 functionally and morphologically distinct
compartments, the red pulp and the white pulp (Figures 1
and 2). The red pulp is a blood filter that removes foreign
material and damaged and effete erythrocytes. It is also a
storage site for iron, erythrocytes, and platelets. In rodents, it
is a site of hematopoiesis, particularly in fetal and neonatal
animals. The spleen is also the largest secondary lymphoid
per et al., 2002; Nolte et al., 2002; Balogh et al., 2004). This
function is charged to the white pulp which surrounds the
central arterioles. The white pulp is composed of three sub-
compartments: the periarteriolar lymphoid sheath (PALS),
the follicles, and the marginal zone.
The spleen is surrounded by a capsule composed of dense
fibrous tissue, elastic fibers, and smooth muscle. The outer-
most layer of the splenic capsule is composed of mesothelial
cells, which may not be evident on histologic section. Irreg-
ularly spaced trabeculae of smooth muscle and fibroelastic
tissue emanate from the capsule into the splenic parenchyma
Address correspondence to: Mark F. Cesta, Integrated Laboratory Sys-
tems, Inc., 601 Keystone Park Drive, Suite 100, Durham, NC 27713, USA;
This work was supported by NIEHS contracts N01ES35513 and
of Integrated Laboratory Systems Inc., Experimental Pathology Laborato-
ries Inc., and the National Institute of Environmental Health Sciences with
from Dr Jerrold Ward.
(Figure 10). These trabeculae also contain blood and lymph
vessels and nerves. The lymph vessels are efferent vessels
through which lymphocytes migrate to the splenic lymph
but is an important and sometimes controversial concept.
splenic artery divides into trabecular arteries located within
the trabeculae entering the splenic parenchyma. Small arte-
rioles branch from the trabecular arteries and enter the red
pulp where they become central arterioles which are sur-
rounded by lymphoid tissue. Smaller arterioles branch from
the central arterioles and feed the white pulp capillary beds
(Satodate et al., 1986; Valli et al., 2002). Some of these ter-
minate in the marginal sinus at the junction of the white pulp
and the marginal zone, others terminate within the marginal
zone, and a few extend beyond the white pulp to terminate
in the red pulp (Dijkstra and Veerman, 1990; Schmidt et al.,
percolates through the marginal zone in the direction of the
red pulp. Once through the marginal zone, the blood either
flows directly into adjacent venous sinuses whose open ends
are continuous with the marginal zone, the so-called “fast
pathway,” or enters the reticular meshwork of the red pulp,
the “slow pathway” (Schmidt et al., 1993). As much as 90%
of the total splenic blood flow travels through the adjacent
venous sinuses, bypassing the reticular meshwork of the red
the white pulp wanes and they become the penicillar arteries
surrounded by red pulp. These give rise to the arterial capil-
laries, which terminate in the reticular meshwork of the red
pulp in rodents (open circulation; Mebius and Kraal, 2005).
Blood from the red pulp collects in the venous sinuses which
enter the trabeculae and merge into the trabecular veins (Fig-
the splenic vein which drains into the hepatic portal system.
of splenic cords and venous sinuses. The splenic cords are
composed of reticular fibers, reticular cells, and associated
FIGURE 1.—Cross-section of the spleen. C57Bl/6 mouse. Relative to the rat (see Figure 2), the mouse spleen typically has a slightly blue tint due to the increased
amounts of extramedullary hematopoiesis in the red pulp. In general, the marginal zones are much smaller and more variable than those of the rat and the white pulp
is more prominent. H&E stain. 2—Cross-section of the spleen. Wistar rat, female. Compare to Figure 1.
macrophages (Saito et al., 1988). The reticular cells are con-
sidered to be myofibroblasts and may play a role in splenic
contraction (Saito et al., 1988). With electron microscopy,
it is apparent that the reticular fibers are actually ensheathed
FIGURE 3.—B6C3F1 mouse, female, 20 weeks old (left) and F344/N rat, male, 12 weeks old. (right). Relative to the mouse spleen, the rat spleen has a larger and
more uniform marginal zone (MZ) and a more pronounced marginal sinus region (MS). The follicle (F) in the rat spleen is better demarcated from the PALS (P).
Hematopoietic tissue is more prevalent in the red pulp (RP) of the mouse. A = central artery, T = trabeculus, H = hilus.
by the reticular cells and their processes (Saito et al., 1988).
The reticular fibers are composed of collagenous and elastic
nated adrenergic nerve fibers (Saito et al., 1988). For more
Vol. 34, No. 5, 2006
STRUCTURE OF THE SPLEEN457
FIGURE 4.—White Pulp. C57Bl/6 mouse. The germinal centers (G) are prominent, highlighting the number and location of the normally inconspicuous follicles
(F) in the mouse spleen. A = central artery, P = PALS, MZ = marginal zone, RP = red pulp. 5.—White Pulp. C57Bl/6 mouse. Higher magnification view of the
white pulp from Figure 4. G = germinal center, F = follicle, A = central artery, P = PALS, MS = marginal sinus, MZ = marginal zone, RP = red pulp.
FIGURE 6.—F344/N rat, male, 12 weeks old. The marginal sinus (MS) of the rat and its sinus lining cells are readily apparent. The marginal zone metallophilic
macrophages can occasionally be seen on the PALS side of the marginal sinus. The outer ring of the marginal sinus is composed of marginal zone macrophages,
dendritic cells, B-cells, and reticular cells and blends with the red pulp (RP). Erythrocytes percolate through the marginal zone (MZ). OP = outer PALS.
information on the ultrastructure of the red pulp, see Saito
et al. (1988) or Schmidt et al. (1993). Within the spaces be-
ulocytes, and circulating mononuclear cells. Also associated
with the splenic cords, are lymphocytes and hematopoietic
cells as well as plasma cells and plasmablasts that migrate
from the follicles and the outer PALS after antigen specific
differentiation (Matsuno et al., 1989; Mebius and Kraal,
2005). The red pulp macrophages are actively phagocytic
and remove old and damaged erythrocytes and blood-borne
in rodent red pulp, especially in fetal and neonatal animals.
Any combination of erythroid, myeloid, and megakaryocytic
cells may be evident (Figure 10).
Venous sinuses can be found throughout the red pulp, in-
cluding, as mentioned previously, directly adjacent to the
marginal zone (Figure 8). They are lined by loose network of
endothelial cells which sit on a basement membrane that is
sandwiched between the endothelial cells and reticular fibers
arteriolar capillaries are also located in the red pulp, though
they are more difficult to identify light microscopically.
Various pigments may be present in the spleen.
Hemosiderin deposits in the cytoplasm of macrophages in
the red pulp, and sometimes in the white pulp as well, are
a typical finding (Figure 9). In fact, iron pigments (i.e.,
hemosiderin and ferritin) are the most common pigments in
the macrophages of the red pulp (Losco, 1992). Iron from
the hemoglobin of phagocytized erythrocytes is converted to
control data from the National Toxicology Program (NTP),
hemosiderin pigmentation is more prevalent in females than
in males (Ward et al., 1999). Ceroid and lipofuscin derived
from oxidation of lipids is also typically found in the spleen,
though they are less abundant than hemosiderin (Ward et al.,
spleen, particularly in black mice, usually in the trabeculae
or focally in the red pulp (Ward et al., 1999).
The white pulp is subdivided into the PALS, the follicles,
and the marginal zone (Figures 3, 4, and 5). It is composed
of lymphocytes, macrophages, dendritic cells, plasma cells,
arterioles, and capillaries in a reticular framework similar to
that found in the red pulp (Saito et al., 1988). As the cen-
tral arterioles enter the red pulp, they are surrounded by the
PALS which are composed of lymphocytes and concentric
layers of reticular fibers and flattened reticular cells (Dijkstra
Vol. 34, No. 5, 2006
STRUCTURE OF THE SPLEEN459
FIGURE 7.—B6C3F1 mouse, female, 20 weeks old. Two examples of the marginal zone (MZ) of the mouse showing the variation in thickness. The marginal
zone is thicker in 3b than in 3a. Note that the marginal sinus (MS) is sometimes obscured in the mouse as opposed to the rat where it is more readily apparent (see
Figure 6). F = follicle, RP = red pulp.
and Veerman, 1990; Satodate et al., 1986). The PALS are
divided into the inner PALS and the outer PALS (Matsuno
et al., 1989; Nicander et al., 1993; Van Rees et al., 1996).
more intensely than the outer PALS due to its cellular com-
position of predominantly small lymphocytes (Dijkstra and
Veerman, 1990; Matsuno et al., 1989). The difference, how-
as interdigitating dendritic cells, and migrating B-cells (Van
medium lymphocytes (both B- and T-cells), macrophages,
1989; Van Rees et al., 1996). It is an important site of lym-
phocyte traffic where the formation of plasma cells occurs
(Dijkstra and Veerman, 1990; Matsuno et al., 1989).
The follicles are continuous with the PALS and are typi-
cally found at bifurcation sites of the central arterioles (Ward
et al., 1999). They are composed primarily of B-cells with
fewer follicular dendritic cells and CD4+ T-cells but typi-
cally do not contain CD8+ T-cells (Van Rees et al., 1996).
The follicles have larger lymphocytes at the follicular center
small to medium lymphocytes (Ward et al., 1999). Follicles
may contain germinal centers, which form upon antigenic
stimulation, that stain less intensely due to the presence of
at the interface of the red pulp with the PALS and follicles
rather than part of the white pulp, it is designed to screen
the systemic circulation for antigens and pathogens and
plays an important role in antigen processing (Kuper et al.,
2002; Mebius and Kraal, 2005). A band of macrophages, the
marginal zone metallophilic macrophages, and the marginal
sinus (Dijkstra and Veerman, 1990; Mebius and Kraal, 2005;
Satodate et al., 1986), separate the marginal zone from
the PALS and follicles. The marginal zone metallophilic
macrophages are a unique subset of macrophages at the
inner margin of the marginal zone adjacent to the PALS
and follicles (Dijkstra and Veerman, 1990; Matsuno et al.,
1989; Mebius and Kraal, 2005). They can be visualized by
silver staining and with the monoclonal antibody MOMA-1
FIGURE 8.—F344/N rat, male, 12 weeks old. Several venous sinuses are readily apparent in the red pulp, particularly the 3 largest ones (VS) at the margin of the
marginal zone (MZ). Aggregates of erythropoietic cells (EP) are scattered throughout the red pulp. MS = marginal sinus, F = follicle. 9.—F344/N rat, male, 12
weeks old. A venous sinus (VS) can be seen penetrating a trabeculus (T) to merge with a trabecular vein (TV). Granulocytes, erythropoietic cells, lymphocytes, and
hemosiderin-laden macrophages are present amid the splenic cords.
Vol. 34, No. 5, 2006
STRUCTURE OF THE SPLEEN 461
FIGURE 10.—B6C3F1 mouse, female, 20 weeks old. Hematopoietic cells are prevalent in the red pulp of the mouse. Granulopoiesis can be seen in the center of
the photo immediately adjacent to the capsule. Erythroid cells and megakaryocytes are also present. A trabeculus arises from the capsule and enters the red pulp.
11.—NIH Nude rat, 9 weeks old. The PALS are sparsely populated by T-cells, particularly the inner PALS (IP). The marginal zone (MZ) and red pulp (RP) are
relatively normal. MS = marginal sinus, OP = outer PALS, A = central artery.
FIGURE 12.—BALB/cA nude mouse, 9 weeks old. As with the rat, the PALS (P) are depleted of T-cells. The B-cell follicle (F), marginal zone (MZ), and red pulp
(RP) are relatively normal. MS = marginal sinus, A = central artery. 13.—CB-17 SCID mouse, 9 weeks old. The white pulp region is small and both B- and T-cell
regions are affected. The PALS (P) and the follicle (F) are markedly depleted of lymphocytes. The marginal zone is nearly nonexistent (the asterisk indicates where
the marginal zone should be). As with the nude animals, the red pulp (RP) appears relatively normal. A = central artery, MS = marginal sinus.
Vol. 34, No. 5, 2006
STRUCTURE OF THE SPLEEN463
(Mebius and Kraal, 2005). Adjacent and peripheral to the
marginal zone metallophilic macrophages is the marginal
sinus. It is continuous with vessels that feed the capillary
sinus-lining endothelial cells (Mebius and Kraal, 2005).
Peripheral to the marginal sinus, is the thick outer ring of the
marginal zone, composed of reticular fibroblasts, marginal
zone macrophages, dendritic cells, and medium sized
marginal zone B-cells (Dijkstra and Veerman, 1990; Mebius
The marginal zone macrophages are another population of
ERTR-9 (Van Rees et al., 1996). While all the potential
functions of the marginal zone metallophilic macrophages
are not known, the marginal zone macrophages are im-
portant in clearance of microorganisms and viruses. They
express a number of pattern recognition receptors such as
toll-like receptors (TLRs) and the macrophage receptor with
collagenous structure (MARCO), which are important in the
uptake of various bacteria (Mebius and Kraal, 2005). The
marginal zone B-cells are a unique subset of noncirculating
B-cells that have an IgM+/IgD- phenotype as opposed to
follicular B-cells which are IgM+/IgD+ (Van Rees et al.,
Factors Affecting Splenic Morphology
Species: There are a number of species differences in the
gross and histologic appearance of the spleen. In dogs, for
example, the spleen is somewhat dumbbell shaped, while in
mice and rats, it’s more uniform along the longitudinal axis.
The spleen in dogs is able to expand to store large numbers
of erythrocytes, but it is also capable of rapid contraction.
large and dark red to blue-black to smaller and lighter red.
The capsule and trabeculae of dogs contains more smooth
muscle than that of mice and rats, so the spleens of rodents
do not contract as rapidly and tend to vary less in their gross
2001), while in the rat, there are as many as eight branches
(Satodate et al., 1986).
Vascular arrangements are perhaps the greatest source of
species variation in splenic architecture. Species variation in
the structure and morphology of the venous sinuses forms
the basis for the classification of spleens into two groups, si-
Sinusal spleens are found in rats and dogs and nonsinusal
spleens are found in mice (Schmidt et al., 1985a). The ve-
nous sinuses of sinusal spleens are larger, more abundant,
make numerous anastamoses, and have a characteristic wall
structure relative to the venous sinuses of nonsinusal spleens
(for an in depth description of these differences, see Snook
(1950) and for more detail on the wall structure of each ves-
sel type, see Blue and Weiss (1981) (Schmidt et al., 1985a).
The venous sinuses of nonsinusal spleens are so different, in
fact, that some investigators use the term pulp venules rather
sinuses of the rat spleen are far more conspicuous than those
of the mouse spleen.
There are also species differences in the arterial vascula-
ture. Schmidt et al. have reported that, in dogs, the arterial
capillaries both terminate in the reticular meshwork (open
circulation) and empty directly into the venous sinuses with
no interruption of the endothelial lining (closed circulation)
(Schmidt et al., 1982, 1983, 1993). In dogs, but not rats,
the arterial capillaries are surrounded by dense, circumfer-
ential clusters of macrophages known as ellipsoids or peri-
arterial macrophage sheaths (PAMS) (Blue and Weiss, 1981;
Satodate et al., 1986). In dogs, there are very few capillar-
ies within the PALS, as opposed to rats and mice where the
Extra medullary hematopoiesis is more prevalent in
spleens of mice than rats. In dogs, hematopoietic tissue is
present in the spleen in pathologic conditions such as neo-
plasia and anemia, but may be present in the absence of un-
derlying disease (HoganEsch and Hahn, 2001). When the
hematopoietic tissue is predominantly myeloid, the term
Though there is a lot of individual variation, mice tend
to have a greater proportion of white pulp than rats, but the
of rats (Figures 1, 2, and 7) (Ward et al., 1999). In rats, the
marginal zone comprises up to 28% of the splenic volume
and is the largest B-cell region in the spleen (Dijkstra and
Veerman, 1990; Schmidt et al., 1993). Approximately one-
third of the B-cells in the rat spleen have the marginal zone
B-cell phenotype, whereas in the mouse, only 15% of the
splenic B-cells have this phenotype (Van Rees et al., 1996).
consistently discernible in rats, electron microscopic studies
show that the marginal sinus is up to 6 times larger in mice
(Schmidt et al., 1993).
In the fetus, the spleen begins as a collection of
primitive reticular cells in the dorsal mesogastrium. The first
cells to appear are hematopoietic, which are evident by ges-
tation day 17 in the rat (Losco, 1992). In the mouse, splenic
tissue can first be identified, light microscopically, at gesta-
tion day 12.5 and the first hematopoietic cells can be seen
at gestation day 15.5 (Seymour et al., 2006). In the dog,
lymphocytes first appear in the spleen at gestation day 52,
while the rodent spleen contains little or no white pulp at
birth (HoganEsch and Hahn, 2001; Van Rees et al., 1996).
The first lymphocytes to appear are T-cells that accumulate
in the PALS regions (Losco, 1992; Van Rees et al., 1996).
In rats, this begins by 2 days of age, by day 5, dendritic cell
precursors appear, after which B-cell follicles begin to de-
velop, and immunologic function begins at 14 days of age
when cell to cell contact of antigen presenting dendritic cells
becomes apparent (Losco, 1992). The spleen reaches peak
development at puberty, in rats, followed by gradual involu-
tion (Losco, 1992). In dogs, the spleen increases in weight
Numerous references discuss the effects of aging on
lymphocyte function and changes in the distribution of
lymphocyte subsets. Lymphocyte numbers, however, may
also decrease with age. One study showed a greater than
80% decrease in lymphocyte numbers in the white pulp
of Fisher rats between 4 and 30 months of age (Cheung
and Nadakavukaren, 1983). This change corresponded, light
and electron microscopically, to a decrease in lymphocyte
density in the white pulp (Cheung and Nadakavukaren,
1983). There was also an increase in the number of retic-
ular cells and macrophages in the same regions (Cheung and
1992). The spleens of older dogs and rodents typically have
fewer germinal centers (HoganEsch and Hahn, 2001; Losco,
Extra medullary hematopoiesis tends to be decreased in
adult animals, but can increase in any animal when there is
increased demand for these cells as in cases of anemia, in-
flammation, decreased production by the bone marrow, or in
present in the spleen tends to increase with age in both ro-
dents and dogs (HoganEsch and Hahn, 2001; Losco, 1992;
Van Rees et al., 1996) and, in mice, is more prevalent in
females than males (Ward et al., 1999).
Genetic mutations in rats and mice, either
spontaneous or engineered, resulting in immunodeficiency
munodeficient strains, nude rats and SCID (severe combined
immunodeficiency disease) mice are perhaps the best known
and most commonly used in scientific studies. Nude rats are
congenitally athymic and so are deficient in T-lymphocytes.
The spleens of nude rats (and mice) are smaller than those
of their wild-type counterparts. They have sparsely popu-
lated PALS regions and, since T-cell activity is required for
the formation of germinal centers, lack secondary follicles
(Figures 11 and 12) (Bell et al., 1987; Hanes, 2005). SCID
mice are homozygous for the Prkdcscidmutation, a muta-
tion in the gene encoding the catalytic subunit of DNA-
dependent protein kinase (DNA-PKcs) (Perryman, 2004;
Seymour et al., 2006). This results in a defect in V(D)J
recombination of T-cell receptors and B-cell immunoglob-
ulin receptors and a lack mature B- and T-cells (Perryman,
2004). The spleens of SCID mice are smaller than those of
wild-type mice and all three regions of the white pulp con-
tain few lymphocytes but do contain macrophages (Custer
et al., 1985). The follicles are variable in size and contain oc-
casional plasma cells, however, follicular dendritic cells are
these cells (Custer et al., 1985; Seymour et al., 2006). The
marginal zone is markedly decreased in size and is poorly
demarcated from the PALS and follicles (Figure 13). In both
the SCID and nude mutants, the reticular framework of the
sparsely populated white pulp is intact (Custer et al., 1985;
Balogh, P., Horvath, G., and Szakal, A. K. (2004). Immunoarchitecture of dis-
tinct reticular fibroblastic domains in the white pulp of mouse spleen.
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spleen including vascular arrangements, periarterial macrophage sheaths
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