Migration of macrophages from the marginal zone to germinal centers in the spleen of mice

Histological observations of the mouse spleen were carried out at different times after intravenous carbon injection. Large carbon-laden macrophages appeared in great numbers in the marginal zone soon after injection. They came together favorably around the germinal centers. Possible migration of these cells toward the germinal centers diffusely from the periphery of the white pulp or through the periarterial lymphoid sheath was suggested. These macrophages entered the germinal centers on a large scale and clustered for a long period--at least 180 days. Since the same type of macrophages were observed persistently in the marginal zone, it was thought that some of them might arise from the blood stream. Possible migration of these cells from the marginal zone toward the germinal centers was also persistently observed. A second type of much smaller carbon-laden macrophages was seen in the white pulp. However, they never showed any favorable localization in the germinal centers as did large carbon-laden macrophages.     

The rat FcR for monomeric IgG is preferentially expressed on red pulp macrophages in the spleen.

Rat monoclonal IgG2b immunoglobulins labelled with 125I or biotin were localized in the red pulp areas but were not found in the marginal zones of the white pulp of the spleen 8 hr after their i.v. injection. In cell suspensions made from the spleen, 90% of red pulp macrophages (M phi) bound 125I-IgG2b monomeric proteins in vitro, whereas similar estimates for marginal zone M phi were 12-30%. Red pulp and marginal zone M phi were distinguished primarily by monoclonal antibodies (mAb) ED2 and ED3, but also by their ability to take up FITC-labelled Ficoll in vivo.     

Heterogeneity of macrophages evidenced by variability in their glycoconjugates

Staining rat tissues with a battery of 15 lectin-horseradish peroxidase conjugates showed that macrophages contain glycoconjugates possessing terminal alpha, beta-galactose, N-acetylgalactosamine, fucose, and N-acetylneuraminic acid, plus two terminal disaccharides. Dissimilar binding of lectins by different phagocyte populations in the same or different organs evidenced variability in glycoconjugates according to the location of the macrophages. With a group of four lectins, macrophages stained most intensely in lung, next strongest in splenic red pulp and lymph node sinuses, and weakest in skin and liver. Two populations of macrophages were newly recognized in spleen on the basis of content of fucose-rich glycoconjugate. These included a necklace-like band of macrophages at the border between marginal zone and germinal center and distinctive macrophages dispersed throughout the marginal zone. In lymph nodes, phagocytes stained strongly in the germinal centers and weakly in sinuses for glycoconjugate with N-linked oligosaccharides and conversely for glycoconjugate with terminal beta-galactose. Variable lectin binding indicated heterogeneity of thymic macrophages. Lectin cytochemistry offers increased sensitivity for detecting macrophages in tissue sections, provides selective staining that shows the prevalence and distribution of the phagocytes and differentiates macrophages into separate subtypes.     

The species-specific structure of microanatomical compartments in the human spleen: strongly sialoadhesin-positive macrophages occur in the perifollicular zone, but not in the marginal zone.

The microanatomical structure of human and rat splenic white pulp is compared, with special emphasis on the localization of the marginal zone occupied by immunoglobulin M (IgM)+ IgD-/dull B lymphocytes and its specialized macrophages. Our study reveals that in contrast to rats, the marginal zone of humans primarily exists in the vicinity of primary and secondary splenic follicles and that it is almost absent around the periarteriolar T-cell zones. We demonstrate that in humans there is an additional compartment, the perifollicular zone, located between the marginal zone and the red pulp. The perifollicular zone is a dynamic region of variable cellular and phenotypic composition, which can be regarded either as a part of the red pulp or of the follicles. In most cases the perifollicular zone appears as a compartment of the red pulp containing erythrocyte-filled spaces which differ from the typical red pulp sinusoids. Similar to the splenic cords, the perifollicular zone mostly harbours scattered B and T lymphocytes. However, sometimes B lymphocytes clearly predominate in the perifollicular area. In addition, strongly sialoadhesin-positive macrophages form sheaths around capillaries in the perifollicular zone. Such capillary sheaths are not observed in rats. In humans weakly sialoadhesin-positive macrophages are also present in the perifollicular zone and in the red pulp. In some specimens sialoadhesin is, however, strongly expressed by a large number of dispersed perifollicular macrophages. Interestingly, in striking contrast to rats, the human marginal zone does not contain sialoadhesin-positive macrophages and marginal metallophilic macrophages are also absent in humans. Thus, sialoadhesin-positive macrophages and IgM+ IgD- memory B lymphocytes both share the marginal zone as a common compartment in rats, while they occupy different compartments in humans. We show that the human splenic marginal zone does not contain a marginal sinus and assume that in humans the perifollicular region is the compartment where antigen and recirculating lymphocytes enter the organ.     

A case of splenic lymphoma with marked diffuse nodular fibrosis and calcification,complicated with severe autoimmune hemolytic anemia

A splenic lymphoma, possibly of a splenic marginal zone lymphoma, marked by small nodular splenic calcified fibrosis and complicated by intractable autoimmune hemolytic anemia, was studied by immunohistochemical, molecular genetic, and ultrastructural analyses. The patient was a 57-year-old Japanese man who had moderate splenomegaly, and who had undergone splenectomy for improvement of severe autoimmune hemolytic anemia and to rule out malignancy in the spleen. In the resected spleen, proliferative atypical lymphoid cells were observed both in the red and white pulp with diminished germinal centers and irregularly widened marginal zones with peculiar dimorphic pattern. Ultrastructural study revealed no hairy cells or villous lymphocytes. Diffuse nodular hyalinous fibrosis surrounding the small arterioles in the white pulp overlapped with frequent calcification was a unique histologic feature in this case. Degenerative connective tissue, extracellular matrix, or collagen fibers surrounding the arterial sheath in the white pulp caused by some immunological abnormalities associated with this splenic lymphoma could be assumed to be the predisposing factor for this excessive fibrosis and dystrophic calcification in the spleen.     

Human spleen microanatomy: why mice do not suffice

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.     

大网膜内植入自体脾组织与原位脾组织的结构比较

目的 :为临床应用自体脾组织植入术提供实验研究资料。方法 :大鼠分为实验组和对照组 ,前者切取 1 /2脾脏去包膜后切成 1mm× 1mm× 1mm大小均匀组织块 ,植入大网膜囊袋内。饲养 6个月后取 2组脾组织制片 ,光镜和电镜定性观察组织结构变化 ,计算机图像分析系统比较血管、红髓、白髓及胶原纤维的面密度 ;免疫组化法结合计算机图像分析测定神经肽 (NPY)阳性神经纤维密度。结果 :神经和边缘窦内皮细胞结构恢复较好 ,血管 ,白髓的面密度值较对照组减少 ,红髓与对照组相当 ,胶原纤维面密度增加。结论 :大网膜内植入的自体脾组织通过再生能恢复脾脏的主要组织结构 ,但不能完全恢复正常。     

The role of macrophages in the immunoadjuvant action of liposomes: effects of elimination of splenic macrophages on the immune response against intravenously injected liposome-associated albumin antigen.

The primary antibody response to intravenously administered and liposome-associated human serum albumin (HSA) was studied in mice under conditions where no response could be detected against the non-liposome-associated form of the antigen. The positive response against the antigen, entrapped in and/or exposed on the surfaces of liposomes, thus resulted from the adjuvant action of the liposomes. In mice intravenously injected with dichloromethylene diphosphonate (C12MDP) also entrapped in liposomes, all red pulp macrophages, marginal metallophilic macrophages and marginal zone macrophages had disappeared from the spleen 2 days after administration. Twenty-two days after such a treatment red pulp macrophages and marginal metallophilic macrophages had reappeared, but marginal zone macrophages were still absent. In mice injected with liposome-associated HSA at 2 days after treatment with the C12MDP liposomes, anti-HSA responses were severely depressed, but administration of the liposome-associated antigen 22 days after C12MDP liposomes elicited a normal response. These results point to a role of splenic macrophages in the processing of liposome-associated antigens, but marginal zone macrophages, which are located close to the open ends of the white pulp capillaries and thus are the first macrophages to meet the antigens arriving in the marginal zone are not required.     

DEC-205/CD205+ dendritic cells are abundant in the white pulp of the human spleen, including the border region between the red and white pulp

The distribution of dendritic cells (DCs) and macrophages in the human spleen has received less attention than that of lymphocytes. Here we have addressed this problem with the human DEC-205/CD205 marker ('DEC'), which is an endocytic receptor on DCs that mediates efficient presentation of antigens. DEC was abundant on dendritic profiles in the white pulp but absent from the red pulp, the latter defined with antibodies to two antigens, mannose receptor/CD206 on sinusoidal lining cells, and macrosialin/CD68 on macrophages. Double staining with anti-DEC and anti-CD3 showed the expected concentration of DEC+ cells in the relatively small T-cell areas of the human spleen. DEC+ cells were also found in other regions of the white pulp. In all regions, the DEC+ cells were positive for major histocompatibility complex (MHC) class II and the CD11c integrin but largely immature, with low expression of B7-2/CD86 costimulator and DC-lysosome-associated membrane protein (LAMP)/CD208. When we concentrated on the perifollicular region between the red pulp and the marginal zone, we found macrophages that stained with antibodies to sialoadhesin/CD169 and DC-specific ICAM-3 grabbing non-integrin (SIGN)/CD209, and just inside these cells were DEC+ profiles. The DEC+ DCs were intertwined with cells that stained for the vascular addressin mucosal addressin cell adhesion molecule (MAdCAM). Therefore, anti-DEC-205/CD205 antibodies are useful for identifying DCs in human splenic white pulp and its border region with the red pulp.     

神经营养素在小鼠脾的定位研究

为了解神经营养素与免疫系统的关系，用免疫组织化学方法对神经营养素包括神经生长因子（ＮＧＦ）、脑源性神经营养因子（ＢＤＮＦ）、神经营养素３（ＮＴ－３）进行小鼠脾的定位研究。结果表明：３种神经营养素的免疫反应均存在于脾内，但分布特点各不相同。ＮＧＦ主要分布于白髓动脉周围淋巴鞘（ＰＡＬＳ）外层、边缘区（ＭＺ）和红髓（ＲＰ）的巨噬细胞样和淋巴细胞样细胞；ＢＤＮＦ除具有与ＮＧＦ相似的分布特点外，还见于脾淋巴小结生发中心的淋巴细胞样细胞；ＮＴ－３则存在于白、红髓的网状细胞样细胞。这一结果提示，脾的免疫和非免疫细胞均可能产生神经营养素，并提示不同类型的神经营养素对免疫系统有不同的作用。     

Lymphocyte migration in the spleen: the effect of macrophage elimination.

To study the influence of macrophages on the migration and distribution of lymphocytes in the spleen, macrophages were eliminated from the spleen of mice by injection of liposomes in which DMDP was encapsulated. This leads to an elimination of macrophages in both the red pulp and marginal zone of the spleen within 1-2 days. In these animals the distribution of lymphocytes was determined by transfer of either syngeneic fluoresceinated or Ly 5 congeneic cells. It was found that after elimination of the macrophages the number of lymphocytes immigrating into the spleen had decreased, although a comparable mode of compartimentalization was found with an initial localization in the marginal zone and a subsequent distribution into the white pulp. After this elimination spleen macrophage subsets return with different kinetics, and in this way the influence of the red pulp macrophages, the marginal zone macrophages and the marginal metallophilic macrophages on lymphocyte immigration and redistribution could be investigated. A quantitative decrease of immigration was still found when red pulp and marginal metallophilic macrophages had repopulated their compartments, but was only fully restored when the last population to repopulate the spleen after treatment with DMDP-liposomes, the marginal zone macrophages, had returned. Experiments with isolated T and B cells showed that the elimination of macrophages had a profound effect on the localization of B cells in the white pulp, whereas it hardly affected T cells.     

门脉高压性脾脏形态结构的实验研究

目的：研究门脉高压性脾脏形态结构。方法：用硫代乙酰胺诱发大鼠门脉高压性脾肿大，不同的时期摘脾研究其形态结构。结果：发现早期脾组织充血水肿，８周后红髓区脾窦扩大、脾索狭窄，白髓区动脉周围淋巴鞘萎缩纤维化。形态计量测定表明随着脾肿大的发展，红髓面积百分比增加，白髓面积百分比减少。白髓区内ＰＡＬＳ、生发中心面积百分比亦进行性减少。结论：门脉高压性脾脏发挥免疫功能的区域萎缩，其免疫功能低下，此时外科保脾意义不大。     

Lymphotoxin-α-deficient and TNF receptor-I-deficient mice define developmental and functional characteristics of germinal centers

Summary: Mice deficient in LTa (LTα-/-) lack lymph nodes and Peyer's patches. This action of LTa in lymph node organogenesis appears to be mediated by the membrane form of LT using a mechanism independent of TNF receptor I (TNFR-I) or II (TNFR-II). In contrast, normal Peyer's patch development appears to require both LTa and TNFR-I, with TNFR-I-/- mice showing hypoplastic Peyer's patch structures. LTα-/- mice also fail to support the normal segregation of T-cell and B-cell zones within the splenic white ptilp. Again, this occurs via a mechanism independent of TNFR-I or TNFR-II. Additionally, follicular dendritic cell (FDC) dusters or germinal centers fail to develop in the spleen of LTα-/- animals. Mice defi-cient in either TNFα or TNFR-I also fail to develop splenic FDC dusters and germinal centers, indicating that signaling by both LTα and TNFα is required for development of these speciahzed lymphoid tissue structures. Finally, the splenic white pulp areas in LTα-/- mice lack the marginal zone of monoclonal antibody MOMA-1-staining metallophilic macrophages, whereas TNFR-I-deficient mice have preserved MOMA-1 staining. Thus, certain actions of LTa to regulate spleen white pulp architecture are medi-ated by receptors other than TNFR-I, most likely by the LTβR or a closely related receptor. We tested whether germinal centers are essential for mat-uration of T-cell-dependent antibody responses. When LTα-/- mice were immunized with low doses of NP-ovalbumin (NP-OVA) adsorbed to alum, there was dramatically impaired production of high afTmity anti-NP IgG; however, after immunization with high doses of NP-OVA adsorbed to alum, LTα-/- mice mounted a high affinity NF-specific serum IgG response similar to wild-type mice, all in the absence of germinal centers or clus-tered FDC. Thus, although germinal centers enhance the processes required for maturation of the humoral immune response, the mecha-nisms responsible for affinity maitiration are not absolutely dependent on the presence of germinal centers.     

High-dose interferon-gamma alters the distribution of B lymphocytes and macrophages in rat spleen and lymph nodes.

Continuous intravenous infusion of rat interferon-gamma (IFN-gamma) for 3 days provokes profound alterations of splenic architecture in LEW rats. The marginal zone of the white pulp is almost totally depleted of B lymphocytes and the follicles are reduced to small remnants. IgM kappa + plasmablasts and plasma cells increase substantially in the outer periarteriolar lymphatic sheath (PALS) and in the splenic red pulp. In addition, marginal metallophilic and marginal zone macrophages are augmented, partially by proliferation. It is discussed whether the activation and proliferation of these macrophages prevent replenishment of the marginal zone and follicles with recirculating B cells. Changes in B lymphocyte and medullary macrophage distribution are also present in submandibular and mesenteric lymph nodes.     

Spleen deposition of Cryptococcus neoformans capsular glucuronoxylomannan in rodents occurs in red pulp macrophages and not marginal zone macrophages expressing the C-type lectin SIGN-R1.

The fate of microbial polysaccharides in host tissues is an important consideration because these compounds are often immune modulators. Splenic marginal zone macrophages that express the C-type lectin receptor SIGN-R1, take up neutral polysaccharides such as dextran and the capsular polysaccharide of Streptococcus pneumoniae. Given that the major component of Cryptococcus neoformans capsular polysaccharide, glucuronoxylomannan (GXM), localizes in the spleen when injected intravenously, we investigated whether GXM uptake was mediated by splenic macrophages expressing the SIGN-R1 receptor in mice. No significant differences in the amount and location of GXM deposition were detected in the spleens of mice treated with a SIGN-R1 blocking antibody when compared to controls. Similarly, a blocking antibody to Dectin-1, a co-receptor of -SIGN-R1, had no effects on GXM distribution within the spleen. Histological examination of spleens from mice and rats injected with FITC-Dextran and GXM revealed no significant co-localization, with Dextran and GXM being found in marginal and red pulp macrophages, respectively. Hence we conclude that GXM was not deposited in marginal zone macrophages. However, GXM deposition was found in the red pulp. These results indicate that there is a selective localization of these polysaccharides to different receptors such as SIGN-R1 for FITC dextran in marginal zone and a to-be-identified receptor selectively expressed by red pulp macrophages for GXM.     

Ontogenic development of macrophage subpopulations and Ia-positive dendritic cells in fetal and neonatal rat spleen.

Development, differentiation, and distribution of macrophage subpopulations and Ia+ dendritic cells in the fetal and neonatal rat spleen were investigated by means of double immunohistochemical staining and immunoelectron microscopy. To characterize these cell populations, a panel of anti-rat macrophage monoclonal antibodies (RM-1, ED2, ED3, TRPM-3, Ki-M2R) and an anti-rat Ia antibody (OX6) were used. In the fetal rat spleen, macrophages were first detected by RM-1 at fetal day 15. ED2+ and/or Ki-M2R+ macrophages appeared at fetal day 16. TRPM-3+ and/or ED3+ macrophages appeared a day later. During the fetal and neonatal development, ED2+ and TRPM-3+ macrophages differentiated independently, maturing into red pulp macrophages and marginal metallophilic and marginal zone macrophages respectively. Intimate topographical relations were observed between ED2+ macrophages and hematopoietic cells and between TRPM-3+ macrophages and marginal zone lymphocytes. Ia+ cells were first observed around arterioles at fetal day 15. In the fetal and neonatal period, the number of Ia+ cells gradually increased, their shape became dendritic, and they matured into interdigitating cells in the inner periarteriolar lymphatic sheath. In ontogeny, Ia+ dendritic cells were not stained with ED2 or TRPM-3. These results suggest that ED2+ macrophages, TRPM-3+ macrophages, and Ia+ dendritic cells are distinct cell lines that pursue independent developmental process in spleen ontogeny.     

Human spleen contains phenotypic subsets of macrophages and dendritic cells that occupy discrete microanatomic locations.

Macrophages (M phi s) are an important component of the immune response and mediate numerous other functions. Phenotypic and functional subsets of circulating monocytes have been described, but few similar studies have analyzed M phi s in human tissues. By use of immunohistochemical techniques and a large number of monoclonal antibodies, the presence and distribution of phenotypic subpopulations of M phi s and dendritic cells in human spleen were assessed. The results of this study show that different subsets of M phi s and dendritic cells are present in the spleen and that some of these occupy discrete microanatomic locations. In the red pulp (RP) certain groups of antigens are expressed by different proportions of uniformly distributed M phi s in the cords. On the other hand, some antigens are present on M phi s that form clusters of variable size within the red pulp. M phi s in the splenic marginal zone (MZ) share some antigens with red pulp M phi s, but in addition express CR3, Mo-2, 61D3, and 63D3. These antigens are found on only a few RP M phi s. MZ cells expressing one antigen shared with RP M phi s (Leu-3a,b) and one present largely on the MZ cells (63D3) form clusters around small vessels; these structures resemble the so-called splenic ellipsoids that may play a role in the trapping of circulating antigens. Phagocytic M phi s (tingible body M phi s) of the white pulp follicular germinal centers were also shown to differ from RP and MZ cels with respect to the expression of the antigens detected by anti-FcR, Leu-M3, Mo-2, 25F9, and anti-CR3. The unique topographical and surface antigenic features of dendritic cells were confirmed by this study. Furthermore, these cells were found to share a number of antigens with RP, MZ, and white pulp M phi s, which suggests that they may be derived from a common progenitor. The presence of phenotypic subpopulations and variation in distribution among human splenic phagocytic cells and dendritic cells may be indicative of functional specialization.     

Are murine marginal-zone macrophages the splenic white pulp analog of high endothelial venules?

The entry of lymphocytes into the spleen, in contrast to lymph nodes, does not involve high endothelial venule (HEV) interaction. The precise point of entry, as well as the mechanism by which lymphocytes enter the lymphoid areas of the spleen, remains controversial. We examined in detail the effect of two agents, pertussis toxin (PT) and the sulfated polysaccharide fucoidan, on splenic lymphocyte entry and positioning. These have previously been shown to interfere with lymphocyte extravasation across HEV. PT prevents lymphocyte extravasation, but not binding, to HEV, whereas fucoidan prevents binding and thus subsequent extravasation. Studies presented here show that pretreatment of murine lymphocytes with PT does not numerically affect entry into spleen, but profoundly alters lymphocyte positioning within the spleen. When fluorescently labeled, PT-treated lymphocytes are injected intravenously, they initially accumulate in the marginal zone, in apparent association with the layer of marginal zone macrophages (MZM?) which form a shell around the white pulp. They fail to traverse this layer into the white pulp, and subsequently localize in the red pulp. In contrast, untreated cells initially appear in the marginal zone, then continue to migrate into the white pulp after traversing the MZM? layer. The localization of PT-pretreated lymphocytes adjacent to the MZM? layer is disrupted by intravenous administration of fucoidan. Using a flow cytometric assay of aggregation between MZM? and lymphocytes, we confirmed that fucoidan is also able to inhibit this association in vitro, whereas PT has no effect on this interaction. We propose that MZM? in the mouse are the splenic analog of HEV, forming the port of entry of lymphocytes into the white pulp of the spleen.     

The pathology of the spleen in steroid-treated immune thrombocytopenic purpura

The spleen is a central organ in the pathophysiology of immune thrombocytopenic purpura (ITP). Splenic lymphoid tissue synthesizes anti-platelet IgG, and splenic cordal macrophages destroy platelets coated with anti-platelet antibodies. The morphologic features of the spleen in this disease reflect this splenic function: hypertrophied lymphoid follicles with secondary germinal centers in the white pulp, with perivascular plasma cells in the red pulp and evidence of increased platelet phagocytosis in cords of the Billroth. Foamy macrophages and evidence of a variable degree of extramedullary hematopoiesis also have been noted. The authors have studied 17 spleens removed for therapeutic purposes in patients with proven ITP previously treated with varying amounts of corticosteroids. In all cases there was little or no morphologic evidence of follicular hyperplasia or plasmacytosis. However, platelet sequestration and phagocytosis were demonstrated easily in all cases, both in histologic sections and in touch imprints. The authors' findings indicate that morphologic evidence of lymphoid activation characteristic in spleens from patients with ITP usually is ablated by prior corticosteroid therapy but that the characteristic platelet sequestration and phagocytosis persists.     

Lymphocyte circulation in the spleen: Marginal zone bridging channels and their possible role in cell traffic

Histological evidence is presented for distinct, anatomically determined pathways in the spleen for cells in transit between the white pulp and the red pulp prior to entering the draining veins. In rats and mice these appear as narrow channels of lymphocytes which run between both the periarteriolar lymphatic sheath and the red pulp sinuses, and the peripheral white pulp and the red pulp sinuses, crossing the marginal zone in association with fine argentophilic fibres. These marginal zone bridging channels were found to contain labelled T or B cells 4 and 8 hours after injection which suggested that transit was occurring in the direction from white pulp to red pulp rather than the reverse.

Additional histological evidence is given to suggest that, after antigenic stimulation, germinal centre dissociation occurs by release of the germinal centre cells towards the periarteriolar lymphatic sheath before they are shed into the red pulp through marginal zone bridges occurring in the periarteriolar region.

The data are incorporated into a scheme of unidirectional lymphoid cell flow through the spleen. This proposes that the spleen is composed of many functionally discrete units in which the anatomical matrix, reflected by the reticulin fibre pattern, plays a major role. It further implies that the periarteriolar region of the spleen is not totally thymus dependent.