The mouse spleen is a small, crescent-shaped organ tucked against the left side of the abdomen that serves as the body’s primary blood filter and a major hub for immune activity. Despite weighing only around 80 to 100 milligrams in a healthy adult mouse, it packs an extraordinary density of immune cells and specialized tissue compartments that make it indispensable for fighting blood-borne infections, recycling old red blood cells, and even producing new blood cells in emergencies. Because the mouse is the most widely used mammalian model in immunology, its spleen has been mapped in remarkable detail, and the findings underpin much of what we know about splenic biology more broadly.
Gross Anatomy and Blood Supply
The mouse spleen sits in the upper left quadrant of the abdomen, near the stomach. It has a smooth, deep red surface and an elongated shape. Blood enters through the splenic artery at a depression called the hilum, and the arterial tree immediately becomes surrounded by immune tissue (the white pulp) as soon as it enters. From there, relatively few arterioles extend directly into the surrounding red pulp. Instead, the arterial branches divide into a profusion of capillaries that fan out through the immune compartments before blood eventually drains into the red pulp’s open meshwork and is collected by venules for return to the circulation.
Detailed vascular casting studies using scanning electron microscopy have shown that these capillaries form bifurcating systems rather than interconnected networks, and they terminate in the marginal sinus and marginal zone, a transitional region between white and red pulp. Blood fills the entire marginal zone before appreciable flow reaches the surrounding red pulp, which means the marginal zone acts as a kind of antechamber where immune cells get first access to anything circulating in the blood.1PubMed. Microcirculation in mouse spleen (nonsinusal) studied by means of corrosion casts When blood does enter the red pulp, capillary endings merge continuously with the reticular meshwork, an open lattice of connective tissue fibers and cells. No evidence of a sealed “closed” circulation has been found in the mouse spleen; blood percolates freely through this meshwork before being collected by pulp venules. Those venules differ from the venous sinuses seen in some other species and converge into large anastomosing veins that ramify beneath the capsule before draining into the splenic vein.
Microscopic Architecture
Under the microscope, the mouse spleen resolves into two interwoven compartments. The white pulp consists of dense accumulations of lymphocytes organized around central arteries. T cells form sleeves called periarteriolar lymphatic sheaths (PALS) directly around these vessels, while B cells cluster into adjacent follicles.2PubMed Central. Human spleen microanatomy: why mice do not suffice The red pulp fills the space between these lymphoid nodules and is a loose reticular connective tissue permeated with blood cells of every type. It is the site where aged or damaged red blood cells are culled from the circulation.
Between white and red pulp sits the marginal zone, a well-delineated compartment that is especially prominent in mice and rats. The marginal zone contains its own specialized B-cell population along with several types of macrophages, and it is bordered on its inner edge by a discontinuous marginal sinus.3PubMed. White pulp compartments in the spleen of rats and mice. A light and electron microscopic study of lymphoid and non-lymphoid celltypes in T- and B-areas In vascular casts, the marginal sinus appears as flattened, anastomosing spaces that are up to six times larger in the mouse than their counterparts in the rat, giving the mouse marginal zone a distinctive architecture.1PubMed. Microcirculation in mouse spleen (nonsinusal) studied by means of corrosion casts
The Marginal Zone as an Immunological Gatehouse
The marginal zone deserves special attention because it is, functionally, the first line of defense against anything arriving in the bloodstream. Marginal zone B cells sit at the interface between the circulation and the lymphoid tissue and respond rapidly to blood-borne antigens. They blur the usual boundary between innate and adaptive immunity by mounting fast antibody responses without waiting for the slower T-cell-dependent activation process.4PubMed Central. Marginal zone B cells: virtues of innate-like antibody-producing lymphocytes
Alongside these B cells, specialized macrophage populations patrol the marginal zone. Marginal zone macrophages and marginal metallophilic macrophages are essential for physically trapping particulate material, including bacteria, from the blood. When researchers selectively depleted these macrophages in mice, the ability to capture microspheres and live Listeria monocytogenes organisms from the circulation was lost, and early control of infection was impaired.5The Journal of Immunology. Macrophages of the Splenic Marginal Zone Are Essential for Trapping of Blood-Borne Particulate Antigen but Dispensable for Induction of Specific T Cell Responses Interestingly, the same experiments showed that even without these macrophages, antigen presentation still occurred and protective T-cell immunity could still develop. So the macrophages serve as rapid-capture sentinels rather than as the sole means of alerting the adaptive immune system.
A separate population of SIGN-R1-positive macrophages in the marginal zone contributes to a different part of the immune response: they help germinal center B cells mature inside the follicles. Deleting these macrophages through pharmacologic and genetic approaches revealed that they regulate humoral immunity, linking the marginal zone’s gatekeeper function to the deeper antibody-producing machinery of the white pulp.6PubMed Central. Marginal zone SIGN-R1(+) macrophages are essential for the maturation of germinal center B cells in the spleen
Immune Surveillance and Pathogen Clearance
The spleen’s physical layout is not just anatomically interesting; it is functionally optimized for immune defense. By forcing blood through a meshwork that slows its flow and brings it into contact with resident immune cells, the spleen facilitates interactions between antigen-presenting cells and the lymphocytes that recognize specific threats.7PubMed Central. Structure and function of the immune system in the spleen These encounters are inherently low-probability events because any one lymphocyte recognizes only a narrow slice of possible antigens, so the spleen’s architecture effectively concentrates and corrals the relevant players.
The red pulp contributes its own layer of defense. Red pulp macrophages are powerful phagocytes that clear bacteria the liver cannot handle. A striking demonstration comes from work on Streptococcus pneumoniae, a major human pathogen. Certain pneumococcal serotypes resist clearance by the liver, and in mice without a spleen, these liver-resistant bacteria grew unchecked in the blood. Red pulp macrophages were identified as the primary phagocytes responsible for clearing them, working in concert with natural antibodies against the bacterial cell wall and the complement system.8PubMed Central. Splenic red pulp macrophages eliminate the liver-resistant Streptococcus pneumoniae from the blood circulation of mice This backup role explains why loss of the spleen dramatically increases susceptibility to encapsulated bacteria.
Germinal Centers and Antibody Production
Within the white pulp follicles, the spleen generates germinal centers, the microstructures where B cells undergo rapid proliferation, mutation of their antibody genes, and selection for the highest-affinity antibodies. Germinal centers are classically driven by T-cell help, but the mouse spleen can also form them in response to polysaccharide antigens without significant T-cell involvement. In one model, mice immunized with a polysaccharide antigen developed large germinal centers in every follicle, and the B cells within them displayed the full hallmarks of germinal center activity, including expression of the transcription factor Bcl-6 and downregulation of surface IgD, despite containing very few T cells.9PubMed Central. Germinal Centers without T Cells This finding is relevant because many bacterial pathogens display polysaccharide capsules as their dominant surface antigen, and the spleen’s ability to mount antibody responses to these antigens without full T-cell cooperation may be one reason spleen loss increases vulnerability to encapsulated organisms.
Red Blood Cell Clearance and Iron Recycling
The spleen’s red pulp doubles as the body’s quality-control station for red blood cells. Red pulp macrophages engulf senescent or damaged erythrocytes, break down hemoglobin, and release iron back into the circulation for reuse in new red blood cell production.10PubMed Central. Reshaping Erythrophagocytosis and Iron Recycling by Reticuloendothelial Macrophages In a mouse, which has a red blood cell lifespan considerably shorter than the roughly 120 days in humans, this turnover is brisk. The reticular meshwork of the red pulp mechanically tests red blood cells as they squeeze through narrow spaces; cells that are too stiff or misshapen get trapped and consumed. Iron homeostasis depends heavily on this recycling loop, and disruptions to splenic macrophage function can contribute to anemia or iron overload.
Extramedullary Hematopoiesis and a Capsular Stem Cell Niche
Unlike the human spleen, the mouse spleen readily takes on blood cell production when the bone marrow’s capacity is exceeded. This “extramedullary” hematopoiesis is a carryover from the spleen’s role during fetal and perinatal development, when it serves as a major blood-forming organ. In adult mice, conditions such as severe anemia, infection, or myelofibrosis can reactivate splenic hematopoiesis, causing the organ to enlarge substantially.11Frontiers in Hematology. Exploring extramedullary hematopoiesis: unraveling the hematopoietic microenvironments Large mammals, including humans, have bigger bone marrow reserves and do not shift production to the spleen as readily, making this a significant species difference for anyone interpreting mouse data.
Recent work has uncovered that the mouse spleen maintains a dedicated niche for hematopoietic stem cells within a narrow capsular zone roughly 200 micrometers wide. This niche is populated by myofibroblasts, a type of contractile connective-tissue cell, and keeps resident stem cells in a quiescent, dormant state. Unlike the bone marrow niche, where stem cells tend to cluster near blood vessels, splenic stem cells were not spatially associated with vascular structures.12PubMed Central. A capsular myofibroblastic niche maintains hematopoietic stem cells in the spleen The existence of this niche helps explain how the mouse spleen can ramp up blood production so quickly: a reserve of stem cells is already in place, held dormant until signals arrive telling them to proliferate.
Infections, Splenomegaly, and Tissue Disruption
Splenomegaly, or enlargement of the spleen, is one of the most common pathological findings in mouse infection models. The organ swells because immune cells proliferate, inflammatory signals recruit additional cells from the blood, and in some cases the pathogen itself replicates within splenic tissue. The degree of enlargement and the pattern of tissue damage depend on the pathogen and the mouse strain.
In a Salmonella enterica model, C57BL/6 mice carrying a specific resistance gene variant developed splenic bacterial counts a thousand-fold higher than a comparison strain (Sv129S6 mice), along with greater splenomegaly and a strongly pro-inflammatory cytokine profile dominated by interferon-γ and tumor necrosis factor-α. Despite both strains carrying the same resistance gene, the C57BL/6 mice mounted a more aggressive, Th1-skewed inflammatory response.13PubMed Central. Salmonella enterica causes more severe inflammatory disease in C57/BL6 Nramp1G169 mice than Sv129S6 mice This highlights how background genetics influence splenic pathology, something researchers must account for when choosing mouse strains.
Viral infections produce their own patterns. In an AG129 mouse model of dengue virus 3 infection, high viral loads accumulated in the spleen along with histological changes to splenic tissue.14PubMed Central. A lethal murine infection model for dengue virus 3 in AG129 mice deficient in type I and II interferon receptors leads to systemic disease Parasitic infections are particularly dramatic. In mice infected with Plasmodium berghei, the malaria parasite disrupts splenic architecture so severely that the normal separation between red and white pulp breaks down, impairing the organ’s filtering and immune functions.15PubMed. In vivo study on splenomegaly inhibition by genistein in Plasmodium berghei-infected mice Treatment with the isoflavone genistein inhibited splenomegaly and restored the disrupted architecture, suggesting that pharmacological intervention can preserve splenic structure even during active infection.
The Spleen in Cancer Models
Mouse models of blood cancers often center on the spleen because it is where malignant cells first expand. In the widely used Eμ-TCL1 transgenic model of chronic lymphocytic leukemia (CLL), the spleen was identified as the initial site of disease growth. Stromal cells isolated from the spleens of these mice induced CLL cell division in culture through both direct cell-cell contact and soluble factors, and these supportive stromal cells were present in significantly larger numbers in the spleen than in other lymphoid organs. Remarkably, removing the spleen delayed CLL development in transgenic mice and completely prevented it in adoptive-transfer recipients.16PubMed. Role of the splenic microenvironment in chronic lymphocytic leukemia development in Eµ-TCL1 transgenic mice
A separate transgenic model targeting the microRNA miR-29 showed a different angle on splenic leukemia. In these mice, an expanded population of CD5-positive B cells appeared in the spleen by two months of age, with about 85% of transgenic animals affected. By two years of age, the spleens were significantly enlarged and the CD5-positive fraction had grown to nearly all B cells, with roughly one in five mice developing frank leukemia.17PubMed Central. Chronic lymphocytic leukemia modeled in mouse by targeted miR-29 expression Together, these models paint the spleen not just as a passive site of disease but as an active microenvironment that nurtures malignant growth.
What Happens When You Remove the Spleen
Splenectomy in mice does not cause the kind of dramatic behavioral or systemic changes you might expect from losing such an immunologically active organ. One study found no significant differences in the circulating proportions of CD4 and CD8 T cells, B cells, natural killer cells, or macrophages between splenectomized and sham-operated mice, and the animals showed no measurable changes in behavior.18PubMed Central. Splenectomy does not affect mouse behaviors Other lymphoid tissues compensate to some degree for routine immune functions.
However, the immune consequences of splenectomy become apparent in specific contexts. In a colon cancer model, removing the spleen had opposing effects on primary tumors versus metastases. Splenectomized mice showed increased density of cytotoxic T cells and fewer immunosuppressive macrophages in their primary tumors, suggesting enhanced anti-tumor immunity at that site. But they had markedly fewer natural killer cells in the lungs, and the proportion of natural killer cells was inversely correlated with the number of lung metastases. The spleen, it turns out, simultaneously supplies immunosuppressive cells that dampen the local anti-tumor response and natural killer cells that patrol distant organs for metastatic seeds.19Scientific Reports. Splenectomy has opposite effects on the growth of primary compared with metastatic tumors in a murine colon cancer model Losing it tips the balance in both directions at once.
Aging and the Deterioration of Splenic Architecture
The mouse spleen does not age gracefully. In young mice, the T-cell and B-cell zones are crisply demarcated, but by 12 months of age the boundary between them becomes increasingly blurred, with mixed T-cell and B-cell areas growing significantly larger compared to younger animals.20PubMed Central. Disorganization of the splenic microanatomy in ageing mice This disorganization is not just cosmetic. The marginal zone undergoes structural breakdown: its macrophages scatter from their normal continuous boundary, and their ability to capture particles from the blood declines. Marginal zone B cells lose their capacity to shuttle between the marginal zone and follicles, and their antibody responses to certain antigens become impaired.21PubMed Central. Influence of ageing on the microarchitecture of the spleen and lymph nodes
These changes offer a structural explanation for the well-known decline in vaccine efficacy and increased infection susceptibility in aged mice. When the architecture that brings immune cells into contact with antigens breaks down, the whole system becomes less efficient, even if the individual cell types are still present in reasonable numbers.
How the Spleen Develops
The mouse spleen originates from a primordium in the dorsal mesogastrium during embryonic development, and its formation depends on a small set of transcription factors. Work in knockout mice identified two key regulators, Tlx1 and Pbx1, that govern the early patterning and proliferation of the splenic anlage.22PubMed. Development and function of the mammalian spleen Loss of Tlx1 leads to failed spleen development, and detailed analysis of Tlx1-deficient embryos revealed that the transcription factor normally keeps retinoic acid signaling in check. Without Tlx1, genes involved in retinoic acid synthesis and signaling become upregulated while a gene encoding a retinoic acid-degrading enzyme is reduced, creating an imbalance that derails organogenesis.23JCI Insight. Transcription factor TLX1 controls retinoic acid signaling to ensure spleen development
After the organ forms, organizing its internal compartments requires signals through the NF-κB pathway. Gene-knockout studies have shown that an alternative NF-κB signaling route, activated by membrane-bound lymphotoxin and working through specific protein complexes, drives the early steps of secondary lymphoid organ development, including the spleen. A separate, classical NF-κB pathway contributes later, particularly to the proper organization of B-cell follicles.24PubMed. Regulation of secondary lymphoid organ development by the nuclear factor-kappaB signal transduction pathway Mice lacking components of either pathway can end up with spleens that are present but architecturally disorganized, which has downstream consequences for immune competence.
Neural Connections to the Spleen
The spleen does not operate in isolation from the nervous system. It receives sympathetic nerve fibers that influence immune cell behavior, forming what researchers call the brain-spleen axis. This connection allows the central nervous system to modulate splenic immune responses, balancing pro-inflammatory and anti-inflammatory outputs in response to systemic signals.25PubMed Central. Neural control of the spleen as an effector of immune responses to inflammation: mechanisms and treatments The practical significance of this axis has attracted growing interest, particularly regarding whether neural stimulation (including vagus nerve stimulation, which activates the splenic nerve indirectly) can be used therapeutically to dampen excessive inflammation. Mouse models have been central to testing this idea, though translating findings to human treatments remains a work in progress.
Advances in Live Imaging of the Mouse Spleen
One long-standing challenge for splenic research has been watching immune processes unfold in real time inside a living animal. The spleen’s deep abdominal location and constant motion from breathing make it technically difficult to image. Two recent advances have pushed the boundaries considerably. A two-photon microscopy protocol using a biocompatible abdominal imaging window now allows researchers to follow germinal center dynamics in the same mouse spleen for over two weeks, tracking structures up to 180 micrometers below the capsule without disrupting normal physiology.26Cell Press. Serial intravital imaging of the murine spleen via an abdominal window reveals dynamic germinal center remodeling A complementary approach uses three-photon microscopy, which employs longer-wavelength infrared light that scatters less in tissue and therefore penetrates deeper. Combined with a custom mounting platform that isolates the spleen from respiratory motion, this method enables visualization of leukocyte migration within the white pulp at depths previously inaccessible.27Cell Reports Methods. Intravital 3-photon imaging of the mouse spleen These tools are beginning to reveal how immune cells behave in their native splenic environment over time scales that matter for processes like germinal center formation and pathogen clearance.
Why the Mouse Spleen Is Not a Perfect Stand-In for the Human One
Despite the mouse spleen’s dominance as an experimental model, there are real structural differences that limit direct translation. The human spleen lacks the well-defined marginal zone found in mice, and its venous drainage relies on true venous sinuses rather than the pulp venules seen in the mouse.2PubMed Central. Human spleen microanatomy: why mice do not suffice These distinctions matter because the marginal zone is a critical site for rapid immune responses in the mouse, and findings about marginal zone B-cell behavior or macrophage trapping may not map cleanly onto human splenic immunology. The readiness of the mouse spleen to take on extramedullary hematopoiesis is another divergence; researchers seeing enlarged spleens in their mouse models need to distinguish between immune-driven splenomegaly and hematopoietic expansion, a distinction that rarely applies in human clinical settings. None of this makes the mouse spleen a poor model, but it does mean that results involving marginal zone function, venous architecture, or blood cell production need careful interpretation before being applied to human disease.