nodes. C57BL/6J (a–c), B6.Cg‐Foxn1nu (d–f), B6.129S7‐Rag1tm1Mom (g–i), and NOD.Cg‐Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice (j–l). a, d, g, j – cervical lymph node; b, e, h, k – jejunal lymph node with higher magnification of subcapsular sinus and cortex (c, f, i, l). Arrows point to lymph nodes in NSG mice.Figure 7.4 Amyloid. (a) Amyloid in mesenteric lymph node of 615‐day old C57BL/6J mouse. (b) Macrophages with intracellular accumulation of aluminum adjuvant in medullary cords. Iliac lymph node of male CD‐1 mouse after injection of an aluminum adjuvant.Figure 7.5 Mucosal lymphoid tissues. (a) Peyer's patch in C57BL/6 J mouse. (b) Colonic lymphoid patch in C57BL/6J mouse. (c) Cross section of the nose of a CD‐1 mouse. Arrows point to the nasopharynx‐associated lymphoid tissue (NALT) and * identifies the lacrimal duct with the lacrimal duct‐associated lymphoid tissue (LDALT). Higher magnification of the squamous epithelium overlying the LDALT (d) and ciliated epithelium overlying the NALT (e).
Peyer's patches: Peyer's patches are located in the anti‐mesenteric wall of the small intestine. The number of Peyer's patches varies between 6 and 10 depending on the mouse strain. Each Peyer's patch is composed of several lymphoid follicles [8–12] which extend from the submucosa into the lamina propria. The follicles have a prominent germinal center as a result of immune stimulation by antigens from the intestinal lumen, and are separated by small interfollicular areas which are analogous to the paracortex of lymph nodes and contain high endothelial venules. The formation of Peyer's patches begins before birth and continues in the first weeks after the pups are born. Several mutations are associated with a lack or reduced number and size of Peyer's patches (Table 7.2). In SHARPIN‐deficient mice (Sharpincpdm and Sharpincpdm‐Dem), Peyer's patches are initially formed and populated by B and T cells. However, lymphoid follicles are not formed, and the lymphoid tissues undergo regression at about two to three weeks of age resulting in the absence of Peyer's patches in adult mice [54]. The majority of lymphocytes in Peyer's patches are B cells. Foxn1 mutant mice that lack T cells have well‐developed Peyer's patches with sparsely populated interfollicular areas. Peyer's patches are absent from mice with null mutations in Rag1, Rag2, Prkdc and Il2rg genes, although anlagen of VCAM1‐positive stromal cells could be detected in neonatal C.B17/Icr‐SCID Jc mice [53]
Solitary intestinal lymphoid tissues (SILTs): These are individual follicles located in the lamina propria of the small and large intestine. They are formed after birth, but are considered secondary lymphoid tissues with a similar function to Peyer's patches. There are about 100–200 SILTs in the mouse intestine, the number depending on the mouse strain. Cryptopatches are small collections of leukocytes, about 80 μm in diameter, in the intestinal lamina propria that do not extend to the epithelium. Their cellular composition includes dendritic cells and innate lymphoid cells. Cryptopatches may represent immature forms of SILTs.
Large intestinal lymphoid tissue: There are two to five colonic patches in the colon of C57BL/6J mice. Each is composed of two large lymphoid follicles in the submucosa extending into the lamina propria of the colon separated by an interfollicular T cell area. M cells are present in the overlying mucosal epithelium. In addition, there are SILTs scattered throughout the lamina propria of the colon with increasing density toward the distal colon. Lymphoid patches similar to colonic patches are also present in the cecum and rectum.
Nasopharynx‐associated lymphoid tissue: The NALT consists of two bilateral rows of five to six lymphoid follicles in the ventral meatus of the nasal cavity. The overlying epithelium consists of ciliated cells interspersed with M cells. The NALT is absent or hypoplastic in several mutant mouse strains (Table 7.2)
Lacrimal duct‐associated lymphoid tissue: The LDALT consists of isolated lymphoid nodules in the propria mucosae of the lacrimal duct. They consist mostly of a B cell follicle with fewer T cells and dendritic cells. The overlying squamous epithelium contains M cells. Absence of the LDALT in selected mutant mouse strains has identified genes involved in the development of these lymphoid tissues (Table 7.2).
Spleen
The spleen is a slightly curved elongated organ on the left side of the stomach in the abdominal cavity. It is comprised of red pulp and white pulp surrounded by a thin fibroelastic capsule. The development of the spleen starts with the formation of the splanchnic mesodermal plate on ED 12. Defects in the formation and further development of this splenic anlage occur in mice with the dominant hemimelia mutation and mice with genetic deletion of the transcription factors NK3 homeobox 2 (Nkx3‐2), transcription factor 21 (Tcf21), Wilms tumor 1 homolog (Wt1), NK2 homeobox 5 (Nkx2‐5), and T cell leukemia homeobox 1 (Tlx1). These mice are asplenic, but also have other organ defects and usually die perinatally, with exception of TLX1‐deficient mice which have a selective absence of the spleen. The formation of the white pulp is dependent on the secretion of chemokines that attract B cells into follicles (CXCL13) and T cells to the periarteriolar lymphocyte sheath (CCL19 and CCL21). The secretion of these chemokines relies on induction by lymphotoxin (a heterotrimer composed of LTA and LTB) and tumor necrosis factor (TNF) which activate the NF‐κB signaling pathway when they engage with their receptors. The formation of the white pulp is disrupted when any of these factors are absent (Table 7.2). The separation of white pulp and red pulp is not complete until about two to three weeks postpartum.
The structure of the spleen is best understood in the context of the blood circulation. The splenic artery enters the spleen through the hilus and forms trabecular branches that extend into the parenchyma. Central arterioles branch off from the trabecular arteries and are surrounded by the lymphoid tissue that constitutes the white pulp. The arterioles terminate open‐ended in the marginal sinus and the cords of the red pulp. The cords consist of fibroblasts and reticular fibers without an endothelial lining and are filled with red pulp macrophages. Blood percolates through the cords and collects in venous sinuses which merge into the splenic vein. The spleen lacks afferent lymphatics, but it does have efferent lymphatics. Red pulp macrophages phagocytize damaged or aged red blood cells and recycle iron. Plasma cells induced by immune responses in the white pulp localize in the red pulp. The red pulp of the adult mouse also contains hemopoietic stem cells and precursor cells representing all lineages.
The white pulp is comprised of the periarteriolar lymphocyte sheath populated mostly by T cells and dendritic cells, flanked by B cell follicles. The marginal zone is the outermost layer of the white pulp which is thin and fairly indistinct in H&E‐stained sections. It is composed of an inner layer of metallophilic marginal zone macrophages, a marginal sinus lined by endothelial cells and an outer layer that contains marginal zone macrophages mixed with marginal zone B cells. The marginal zone is not fully formed until about three to four weeks of age.
Examination of the spleen: Routine examination is usually performed on two H&E‐stained cross sections of the spleen. Immunohistochemistry is performed to identify specific regions of the white pulp such as the marginal zone, and specific cell populations.
Lymphoid hypoplasia: Similar to the lymph nodes, mutations that affect the production of lymphocytes result in greatly decreased numbers of lymphocytes in the white pulp. Null mutations of Foxn1 cause a marked decrease of lymphocytes in the periarteriolar lymphocyte sheath (Figure 7.6). Mutations of the Rag1, Rag2, and Prkdc genes result in a lack of B and T cells, and the white pulp is markedly reduced in size and largely devoid of lymphocytes (Figure 7.6). Il2rg mutations affect the production of lymphocytes as well as other immune cells. NOD.Cg‐Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice have very small rudimentary white pulp (Figure 7.6).
Aging‐associated