Читать книгу Pathology of Genetically Engineered and Other Mutant Mice - Группа авторов - Страница 120

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 changes: The white pulp of older mice (>12 months of age) becomes increasingly disorganized with less clear separation of B and T cell areas [55]. B cell follicles are reduced in size and there is a reduction in the number and function of follicular dendritic cells. The marginal zone and metallophilic marginal zone macrophages no longer form a continuous cell layer [55, 56].

 Amyloidosis: Amyloid in the spleen is usually AA‐amyloid associated with chronic inflammation. Experimental models indicate that AA amyloid first accumulates in the marginal zone and is associated with loss of marginal zone macrophages [57]. As the amyloidosis increases in severity, deposits expand into the red pulp.

 Pigment: The appearance of hemosiderin pigment is normal in the red pulp of the spleen. Red pulp macrophages are engaged in erythrophagocytosis and frequently contain hemosiderin. The amount of hemosiderin is greater in female mice, but varies by mouse strain. Hemosiderin can be distinguished from other pigments by Prussian blue staining (Figure 7.7). Lipofuscin‐ceroid pigment may be observed in red pulp macrophages, but is less common than hemosiderin. In mice with black fur, the dorsal one‐third of the spleen is often dark red to black as a result of accumulation of melanin. Light microscopically, dendritic melanocytes and melanin‐laden macrophages are present in the red pulp, and occasionally in the white pulp (Figure 7.7).Figure 7.6 Spleen. Spleen of C57BL/6J (a), B6.Cg‐Foxn1nu (b), B6.129S7‐Rag1tm1Mom (c), and NOD.Cg‐Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice (d).

 Increased extramedullary hematopoiesis (EMH): While modest EMH is normal in the red pulp of the spleen of adult mice, increased EMH can lead to marked expansion of the red pulp and is a common cause of splenomegaly. The myeloid lineage predominates when there is inflammation in other organs, while hemorrhage and red blood cell destruction may lead to an increase of erythrocyte precursors and sometimes megakaryocytes.

 Red pulp atrophy and hypoplasia: A reduced size of the red pulp may be seen in mice with blocks in cell division as a result of genetic mutations or chemical treatment.