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.     

The differentiation of white pulp and red pulp in the spleen of human fetuses (72–145 mm crown-rump length)

The development of the white and red pulp in spleen from thirteen human fetuses measuring from 72 mm to 145 mm in crown-rump length (CRL) was studied using the electron microscope. This period follows the development of the primary vascular reticulum (Weiss, '73). The white pulp appears first as a periarterial sheath with variable numbers of large and medium-sized lymphocytes, monocytes, macrophages, and some granulocytes and erythrocytes. It is always rich in macrophages. At 90 to 100 mm CRL, reticular cells closely associated with collagen and having a distinctive dark hyaloplasm appeared first in the endothelium and close about blood vessels and then out in the pulp. In the white pulp they became circumferentially arranged about the central artery while in the red pulp they formed a branching reticulum. Small lymphocytes were present in increasing number in the periarterial lymphatic sheath after the development of the circumferential reticulum. The venous sinuses developed and the marginal zone stood out as an erythrocyte-rich and macrophage-rich shell of reticulum surrounding the periarterial sheath.     

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.

     

Histological,histochemical, and fine structural observations on the spleen of seals

Spleens of three species of Antarctic seals with different diving habits (Weddell seal, crabeater seal, and fur seal) have been studied with histological, histochemical, and electron microscopic methods. The spleens can be classified as nonsinusoidal, with capsule and trabeculae rich in innervated smooth muscle cells. The trabecular system is particularly well developed in the deep- and long-diving Weddell seal. As in other mammals the pulp can be divided into white and red pulp. In the white pulp, periarteriolar lymphatic sheaths and secondary lymphatic nodules occur; both are surrounded by a marginal zone rich in macro-phages and eosinophils. The nodules can be observed frequently, which is in accordance with abundance of plasma cells in the red pulp. Well-developed white pulp and numerous plasma cells and eosinophils obviously reflect a high load of nematodes, which have mainly been found in lung and stomach. Additionally, in the red pulp morphological evidence for the following functions has been found: destruction of erythrocytes, erythropoiesis, and thrombopoiesis. In respect to blood flow through the red pulp, we interpret our observations in the following way: terminal branches of arterioles open into the space between the fibroblastic reticulum cells; blood draining from here is collected into pulp veins, which are mainly found near the trabeculae. Thus, the seals have an open vascular compartment in their spleens, as also occurs in the cat. The red pulp is innervated by numerous nerve fibers that seem to include both cholinergic and adrenergic ones. The target cells of these fibers seem to be the fibroblastic reticulum cells, whose state of contraction may influence the direction of blood flow through the red pulp.     

Electron microscopic study of subcutaneous and intraperitoneal splenules in the mouse

The development of splenules derived from slices of freshly removed autologous spleen implanted subcutaneously or intraperitoneally was followed by light and electron microscopy from day 2 to day 70. Within 48 hr after transplantation, a rough space filled with blood, unlined by endothelium, formed just under the surface of the splenic fragment. The tissue central to this vascular space was disrupted and necrotic. In the outer portion of the vascular space, fibroblasts appeared and created locules which developed into a highly vascular, hematopoietic red pulp. From the inner portion, blood percolated into the central necrotic tissue. At 1 week the splenule was divisible into concentric structures. The capsule was outermost. A shell of vascularized, highly hematopoietic red pulp lay within the capsule, having replaced the vascular space. Central to the red pulp lay a band of fibroblasts and macrophages. Next was a layer of fibroblasts in a matrix of degenerating cells, and, at the center, a necrotic core. As fibroblasts and macrophages moved centrad, the red pulp moved with them, expanding and replacing the necrotic tissue. The splenule differed in character from the original spleen. Splenular red pulp, especially near the surface, was unusually hematopoietic. The circumferential reticulum of white pulp was reduced or absent, and the boundary between red and white pulp was sometimes indistinct. Some white pulp was subcapsular, and the capsule and surrounding connective tissue were infiltrated by lymphocytes. The necrotic core of the splenule was typically surrounded by a zone containing large blood vessels, connective tissue, and adipocytes.     

Immunohistochemical localization of smooth muscle myosin in human spleen, lymph node, and other lymphoid tissues. Unique staining patterns in splenic white pulp and sinuses, lymphoid follicles, and certain vasculature, with ultrastructural correlations.

The anatomic distribution of smooth muscle myosin, a contractile protein, was determined in a variety of lymphoid tissues (spleen, lymph nodes, tonsils) with the use of highly specific rabbit antibodies to human uterine smooth muscle myosin and an indirect immunoperoxidase technique. In the spleen, in addition to the anticipated immunoreactivity in the walls of arteries, veins, splenic capsule, and trabeculas, other staining patterns were observed. Smooth muscle myosin-containing cells which comprised the adventitia of the trabecular arteries appeared continuous with myosin-containing reticular cells of the white pulp. The latter cells assumed a circumferential pattern within the periarteriolar lymphoid sheaths, then blended delicately with the red pulp at the marginal zone. Ultrastructurally, immunogold techniques demonstrated that smooth muscle myosin in these cells was localized to cytoplasmic filaments. Within the red pulp, a different and distinct staining pattern was observed for the splenic sinuses. Short, regular, orderly, and repetitive bands of immunoreactivity, aligned parallel to the long axis of the sinus, extended between contiguous ring fibers. By immunoelectron microscopy these structures corresponded to distinct bundles of filaments in the endothelial lining cells of the splenic sinuses. Factor VIII associated antigen was also identified in the splenic lining cells in cryostat and paraffin sections, and ultrastructurally. Within the red pulp of the spleen, the sheaths of sheathed capillaries also revealed strong immunoreactivity for smooth muscle myosin. Other sites of immunohistochemical localization of smooth muscle myosin included dendritic reticulum cells present in reactive follicles and in nodular non-Hodgkin's lymphomas. Certain vascular structures, specifically sinus lining cells and Schweigger-Seidel capillary sheaths of the spleen and postcapillary venules of lymph nodes and tonsils, coexpressed smooth muscle myosin and Factor VIII associated antigen. The patterns of localization of smooth muscle myosin are correlated with anatomic structures and possible tissue functions.     

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.     

Monoclonal antibodies against mouse splenic stromal cells

A panel of rat monoclonal antibodies directed against mouse splenic stromal cells were isolated. These monoclonal antibodies were Immunohistochemically divided into four groups which reacted with non-lymphoid cells of the murine spleen; (1) in the white pulp, (11) at the marginal zone, (111) in the red pulp, and (IV) on the endothelium of splenic blood vessels. These monoclonal antibodies were studied Immunohistochemically In lymphoid organs by means of light and electron microscopy. Monoclonal antibodies SS-4 (group I) reacted with fibroblastic reticulum cells that were distributed only in the white pulp of the spleen and In the follicular areas of lymph nodes. The SS-4 staining cell, In clustered splenic stromal cells, formed colonies which Included a small number of Thy-1 positive lymphocytes. Therefore, we concluded that SS4 staining stromal cells comprise the lymphoid cornpartment. In contrast, monoclonal antibodies SS-1, SS-3 and SS-5 (group II) reacted with dendritic shaped cells in the marginal zone of the spleen. Examination of splenic extra-medullary hematopolesis in mice rescued by bone marrow transplantation after lethal irradiation revealed that SS-3 and SS-5 reacted with dendritic shaped stromal cells in clonal nodules of engrafted marrow in the red pulp. SS-3 and SS-5 staining cells could not be observed in physiologic hematopoiesis of non-transplanted mice. It was suggested that SS3 and SS-5 staining stromal cells are Involved in primitive hematopoiesls. Monoclonal antibodies SS2, SS-6 and SS-7 (group 111) mainly reacted with dendritic cells and macro-phages in the red pulp. Monoclonal antibodies SS-8 and SS-9 (group IV) reacted with endothelial cells of blood vessels and sinuses. These findings of heterogeneity in mouse splenic stromal cells are further evidence that specific micro-envlronments are composed by speclalired stromal cells.     

“Intermediate zone” of mammalian spleens: Light and electron microscopic study of three primitive mammalian species (platypus,shrew, and mole) with special reference to intrasplenic arteriovenous communication

The intermediate zone (IZ) of nonperfused and perfused spleens in three species of primitive mammals (shrew, mole, platypus) was studied morphologically. The IZ is a tissue zone consisting of plexiform vessels, probably venous capillaries, and is located transitionally between the white and red pulp. The IZ is separated from the white pulp by the arterial net (AN), in which the white pulp arteries terminate. Development of the IZ differs between the three species examined being distinctive in the platypus and shrew. The IZ is thin in the mole spleen. A closed type of arteriovenous (A-V) anastomosis was demonstrated in or around the IZ in the two Insectivora species examined. In the shrew spleen, peripheral arterial branches running within the IZ anastomose with the AN around the follicle. The AN anastomoses eventually with venous plexi-form vessels of the IZ around the nonfollicular area of the white pulp to form a closed system. In the mole spleen, A-V anastomoses were noted between white pulp arteries (follicular and AN) and veins of the red pulp, either by direct communication or through fenestrated IZ vessels compatible with the plexiform vessels of the shrew spleen. A-V anastomosis in the IZ is probable, but not confirmed, in the platypus spleen, as analysis was limited to a nonperfused specimen. Well-developed ellipsoids were noted around arterial terminals of the IZ in the shrew spleen. Ellipsoids were also noted around all arterial terminals of the mole spleen directed to the red pulp. Most ellipsoids of the mole spleen appeared located within the IZ. No ellipsoids were present around arterial terminals of the IZ in the platypus spleen. Closed circulation was noted in terminals of the pulp artery in spleens of all three species. All pulp arteries of the mole spleen are postellipsoid segments of white pulp (AN and follicle) arteries. No ellipsoids were found around terminals of the pulp artery (penicillar artery) in shrew and platypus spleens. The IZ is probably homologous to the perilymphatic sinusoid (vein) of the lungfish spleen and may be regarded as part of the red pulp. The IZ may be representative of primitive mammalian spleens that have closed circulation. The marginal zone (MZ) of common mammalian spllens is probably a modified IZ by differentiation (remodelling) of the intrasplenic vein. In this process, with drawal of venous vessels from the IZ occurrred, leaving a lymphoreticular zone with open circulation (MZ). The marginal sinus reported in some mammalian spleens is probably a modified AN formed durig this process. Possible morphological alterations of the spleen in vertebrate phylogeny are discussed.     

口服Ag85A DNA疫苗表达产物在脾脏的分布

目的 检测口服Ag85A DNA疫苗表达产物在脾脏内的分布,为阐明口服DNA疫苗可诱导全身性免疫应答的机制提供依据。方法 将本实验室构建的pCDNA3．1^＋-Ag85A真核表达重组质粒转化感受态大肠杆菌DH5α进行扩增,无内毒素抽提纯化,进一步用脂质体包裹制成口服重组Ag85A DNA疫苗。将C57BL／6小鼠随机分为2组,即生理盐水组和DNA疫苗组。分别将生理盐水和Ag85A DNA疫苗以灌胃方式投给各组小鼠,共免疫3次,每次间隔14d,末次免疫后14d处死小鼠,取脾,免疫组化、免疫荧光法检测Ag85A表达产物在脾脏的分布情况。结果 Ag85A重组DNA疫苗的表达产物在小鼠脾脏白髓、边缘区和红髓的脾索处有广泛分布,在边缘区及红髓的脾索处的检出强度高于白髓。免疫组化结果中边缘区与白髓比较t=3．039,P〈0．05;红髓的脾索与白髓比较t=3．068,P〈0．05;边缘区与红髓的脾索比较t=1．750,P〉0．05。免疫荧光结果中边缘区与白髓比较t=3．144,P〈0．05;红髓的脾索与白髓比较t=3．098,P〈0．05;边缘区与红髓的脾索比较t=1．369,P〉0．05。结论口服脂质体包裹的DNA疫苗的表达产物存在于脾脏,表明经口途径接种的DNA疫苗可能会在脾脏诱导全身性免疫应答的产生。     

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.     

Normal structure, function, and histology of the spleen

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.     

Phagocytosis and lymphocyte migration: evidence that lymphocyte trapping in the spleen following carbon injection is not due to direct lymphocyte-macrophage adherence.

The localization of intravenously injected labelled syngeneic lymphoid cells was studied in the spleen of mice and compared with the localization of Indian-ink-containing macrophages. To distinguish between Indian-ink-containing lysosomes of macrophages and silver grains formed in the autoradiographs over the radiolabelled cells, the latter grains were stained blue by a colour-coupling process. Labelled cells were injected 2 h after the Indian ink. Two hours after their injection the bulk of the labelled cells in the spleen was already localized in the white pulp. At this time the Indian ink had been ingested by macrophages in the marginal zone and to a somewhat lesser extent in the red pulp. Twenty-four hours after injection of the cells their concentration in the white pulp appeared constant or had decreased markedly, dependent on the source of the injected cells (spleen, lymph nodes or thymus). At this time carbon-containing macrophages were also found in the white pulp although they contained less carbon than macrophages in the marginal zone and red pulp. A positive correlation between labelled cells and carbon-containing macrophages was never seen in any part of the spleen. It is concluded that, if carbon-containing macrophages induce lymphocyte trapping, as has been supposed by other authors, this trapping must be mediated by the macrophages in an indirect way, e.g. by soluble mediator molecules released in the circulation.     

Electron microscopy of the red pulp of the dog spleen including vascular arrangements,periarterial macrophage sheaths (Ellipsoids), and the contractile,innervated reticular meshwork

The vascular and stromal arrangements of the red pulp in congested and contracted dog spleens were studied by transmission electron microscopy. Each dog had been injected intravenously with Thorotrast to label actively endocytizing cells. Only macrophages ingested Thorotrast. The proximal portion of each arterial capillary was surrounded by a “periarterial macrophage sheath” (PAMS), a term we introduce to replace the term “ellipsoid”. PAMS were composed of a fine meshwork of reticular cells and reticular fibers which held tightly-packed macrophages and interspersed blood cells. These macrophages, as well as those in the reticular meshwork of red pulp, contained Thorotrast, cell debris, and deposits of hemosiderin. The arterial capillary at the center of each PAMS was formed by parallel, rod-shaped endothelial cells and discontinuous layers of basement membrane and reticular-cell cytoplasm. PAMS were tapered at their distal ends; the terminal portion of the arterial capillary continued beyond the PAMS to end in the reticular meshwork of red pulp. Endothelial cells in the terminal arterial capillaries were separated by gaps through which blood cells passed into the spaces of the reticular meshwork of red pulp. The reticular meshwork was formed by reticular cells which appeared to be specialized for contraction. These cells were filled with thin filaments and possessed plasmalemmal dense bodies as found in smooth muscle cells. Furthermore, the reticular meshwork was innervated by unmyelinated adrenergic axons which probably were derived from nerves that followed arterioles. Axons were enclosed in surface invaginations of cells which were similar to reticular cells in shape and cytologic detail and which we called “axon-bearing reticular cells”. Axon-bearing reticular cells were inserted between the branches of the reticular cells that formed the meshwork. Venous sinuses formed an anastomosing system of vessels draining into pulp veins which then joined trabecular veins. Sinuses were formed by parallel, rodshaped endothelial cells encircled by strands of basement membrane and reticular-cell branches. Endothelial cells lay closely side by side except where inter-endothelial slits were opened by blood cells passing into the lumen or by pseudopodia of macrophages which lay outside the sinus. Cell traffic across the sinus wall was greatest in areas where blood cells were mixed with plasma. Congested spleens stored concentrated red cells in both sinuses and the reticular meshwork; contracted spleens were emptied of blood. The reticular meshwork may contract to assist trabecular and capsular smooth muscle in expelling stored red cells and effecting hemoconcentration.     

Periarterial macrophage sheaths (Ellipsoids) in cat spleen—an electron microscope study

Periarterial macrophage sheaths (PAMS), a term we introduce to replace “ellipsoids,” surround arterial capillaries in the red pulp of the spleen and are major sites for clearance of blood-borne particles. PAMS and their arterial capillaries in cat spleens in various states of congestion and contraction were studied by transmission electron microscopy. Thorotrast, a colloidal suspension of thorium dioxide, was injected to label macrophages. A PAMS consisted of a fine meshwork of reticular cells and reticular fibers which held macrophages and formed a cylindrical sheath around an arterial capillary lying in its longitudinal axis. Some PAMS were spongy due to loosening of cell associations by plasma infiltration; others were tightly compressed. Blood cells were both free in the interstices of the PAMS and attached to macrophages. Reticular cells formed a closely applied but incomplete layer adventitial to the arterial capillary and extended branches which contributed to the meshwork. Small villous processes on the major branches of reticular cells approached each other, sometimes forming intercellular junctions, and fit into complementary indentations in the surfaces of macrophages and endothelial cells. Thin filaments within reticular cells filled the villous processes and formed a border beneath the plasmalemma; intermediate filaments ran through the centers of the branches. Reticular fibers lay between reticular cells. Basement membrane fabricated of the same material as reticular fibers lay between the endothelium and reticular cells. Macrophages contained Thorotrast and abundant debris of phagocytized cells and were joined by extensive interdigitation of micropseudopodia. Endothelial cells were long rods which lay parallel and were joined along their bases by interdigitating lateral processes. Intercellular junctions were present at some points, but at others lateral processes were everted to form open interendothelial slits through which blood cells could pass. Endothelial cells possessed great numbers of randomly oriented intermediate filaments and small patches of thin filaments scattered along the basal plasmalemma and in lateral processes. Thin filaments may function to attach cells to one another and to the basement membrane and may assist in closing interendothelial slits. We believe that the endothelium responds to changes in arterial blood pressure and blood flow. It stretches to allow dilatation and recoils, probably due to the intermediate filaments, squeezing blood cells through interendothelial slits.     

Elimination of spleen and of lymph node macrophages and its difference in the effect on the immune response to particulate antigens

To study the role of macrophages in the in situ immune response to particulate antigens in spleen and popliteal lymph nodes (PLN), mice were injected with dichloromethylene diphosphonate (Cl2MDP)-containing liposomes to eliminate macrophages, followed by immunization with trinitrophenylated sheep red blood cells (TNP-SRBC). Depletion of macrophages in the spleen caused a strong decrease in the number of antibody-forming cells (AFC), which develop after intravenous (i.v.) injection of the antigen. These results strongly suggested the involvement of splenic macrophages in the processing of TNP-SRBC. In particular, the populations of marginal zone macrophages may be involved in the inductive phase of an antibody response to particulate antigens. These macrophages are strategically positioned at the end of the white pulp capillaries in the marginal zone of the spleen and they have their cell processes between the marginal zone-B cells. Elimination of macrophages in PLN had no effect on the number of AFC, which develop after subcutaneous (s.c.) injection of the antigen in the hind footpads. This indicates that the macrophages are not essential for the induction of a local immune response to the particulate antigen TNP-SRBC. After depletion of lymph node macrophages, the number of AFC developing in the spleen after s.c. footpad injection of the antigen increased and the anti-TNP serum titers were elevated. This may well be caused by the fact that more of the antigen reaches the circulation and subsequently stimulates the spleen.     

The cellular reaction of the rat spleen after extensive loss of liver parenchyma (author's transl)]

Autoradiographic studies were performed to investigate the changes in cellular proliferation of splenic cells after sham operation and 2/3 partial hepatectomy. Material and Methods: 54 female Wistar rats were hepatectomized and 27 rats were sham operated. The life span of the animals ranged between 12 hours and 31 days. One hour before sacrificing, the animals were injected intraperitoneally with 3H-thymidine (2.5 muCi/Ig). Liver and spleen were embedded in paraffin and epoxy. In strippingfilm autoradiograms (Kodak, AR 10) and in semithinsection autoradiograms (Eastman Kodak, NTB 2) labeling indices of the hepatocytes and Kupgger cells of the regenerating liver and of the cells in the white and red pulp as well as in the marginal zone of the spleen were measured. Results and Discussion: After 2/3 partial hepactectomy there is a marked increase of proliferating. DNA synthezising basophilic blasts and basophilic and polychromatic erythroblasts in the red pulp of the spleen of rats during the first 5 to 10 days. After the fifth day the labeling index of the myelopoietic cells increases too. The curve of the percentages of radioactive labeled cells in the red pulp shows two peaks. The first can be observed after 24 hours and the second after 5 to 10 days. The percentages of labeled cells in the lymphatic mantle zone of the white pulp and in the marginal zone increase also during the first two postoperative days. The first maximum of labeled cells in all the zones of the spleen shows a close relationship to the maximum of proliferating DNA synthesizing cells in the regenerating liver. The second peak in the red pulp is possible caused by a nonspecific multifactorial stimulus such as operation stress, blood loss or inflammation during woundhealing. This proliferation enhancing effect in the spleen, especially on the erythropoietic cells, after partial hepatectomy, which produces a mild splenomegaly during the first 5 days, is more distinct than after sham operation, mechanical or thermical lesions on parenchymal organs.     

The structure of the white pulp of the spleen and the peripheral blood indices in rats under increased muscle activity

The white pulp structure was studied in rat spleen white pulp in conditions of increased muscle activity as well as certain parameters of peripheral blood. 36 male outbred albino rats were involved. Experimental rats were subjected to the increased physical load (everyday swimming seances lasting from 10 to 40 hrs during 4 weeks). The number of erythrocytes, leukocytes and level of hemoglobin were determined in peripheral blood. Perimeter squares and density of lymphoid nodules (LN) and marginal zone (MZ) as well as LN minimal and maximal diameter and MZ minimal and maximal width were determined in histological sections prepared at the level of the organ portae using TV-computer set. Besides, marginal sinuses width were measured in nine sites in every LN as well as the capsule thickness in three sites and at the level of portae. The data obtained indicate that increased muscle activity causes reorganization of red and white pulp in the direction of growth of the red pulp square which is associated with the increase of erythropoietic function. The appearance of lymphoid nodules with germination centres indicates lymphocytopoietic function intensification while the significant increase of marginal zone morphometric parameters witnesses its participant in primary immune response. High correlation between certain blood indexes and the parameters of lymphoid nodules and marginal zones supports these data.     

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.     

The equine spleen: An electron microscopic analysis

The capacity of the equine spleen to store and rapidly release as much as half the circulating blood volume after adrenergic stimulation depends upon the size of the spleen, its muscular capsule, and the distinctive structure of its red pulp. The unit, or lobule, of red pulp is a cylinder of pulp spaces organized in a reticular meshwork, supplied by a peripheral ring of arterial capillaries, and drained by a central venule. Reticular cells, which make up the meshwork of the pulp, contain an extraordinarily large complement of microfilaments and intermediate filaments and are richly innervated by nerves containing both dense and lucent core vesicles typical of adrenergic nerves. The wall of the pulp venule contains large apertures. The capacious red pulp would thus appear capable both of large-scale blood storage and, by the contractile adrenergic innervated reticulum and open venous vasculature, of rapid expression of stored blood into the circulation. Antigen-presenting cells are present not only in B and T cell zones in white pulp but in the periarterial macrophage sheath of red pulp as well. The periarterial macrophage sheath is one of the first sites of antigen capture, and the presence of these cells confers on it an immunological role.