Terminating arterial vessels in red pulp of human spleen: a transmission electron microscopic study

Three-dimensional reconstruction of serial sections for analysis of the microvasculature of the white pulp and the marginal zone in the human spleen

Although a number of papers have given useful information on splenic microcirculation by light and/or scanning electron microscopy, controversies remain as to the vascular arrangement, especially in the human spleen. The present study re-examined the microvasculature of the human spleen using a three-dimensional reconstruction of immunohistochemically stained tissue sections, and showed that the central artery does not directly issue follicular arteries in the human spleen; follicular arteries are derived from penicillar arteries outside the follicle and end in the white pulp. We found that the splenic follicle is surrounded by an elaborate system of anastomosed capillaries in both the marginal zone and the superficial layer of the white pulp. Most of these capillaries are also branches of the penicillar arterioles that are issued from the central artery in the same, or a different, white pulp system. Because the endothelia of these capillaries are widely open in the marginal zone, this vascular network may play a major role in supplying blood to the marginal zone. The accumulation of sialoadhesin-positive macrophages was also observed around the vascular network, suggesting an important role for this structure as the front line of immune response.

The open microcirculation in human spleens: a three-dimensional approach

It has long been debated whether the red pulp of human spleens harbors an open or a closed microcirculation or both. To solve this issue, the authors differentially stained the endothelium in red pulp arterial microvessels and in venous sinuses using brightfield and fluorescence immunohistology with reagents against CD34 and CD141. Three-dimensional models of red pulp arterial microvessels and sinuses were derived from serial double-stained paraffin sections with the help of license-free open-access software. In each model, arterial microvascular ends were traced and verified by reference to the original serial sections. In total, 142 ends were analyzed in the specimens of three individuals. None of these ends was connected to a sinus, suggesting that the human splenic red pulp harbors an entirely open circulatory system. Thus, the spleen is the only human organ where blood passes through spaces not lined by endothelia or other barrier-forming cells.

Phenotypic differences between red pulp capillary and sinusoidal endothelia help localizing the open splenic circulation in humans

The distribution of capillaries, sinuses and larger vessels was investigated by immunohistology in paraffin sections of 12 adult human spleens using a panel of antibodies. Double staining for CD34 and CD141 (thrombomodulin) revealed that capillary endothelia in the cords of the splenic red pulp and at the surface of follicles were CD34(+)CD141(-), while red pulp sinus endothelia had the phenotype CD34(-)CD141(+). Only in the direct vicinity of splenic follicles did sinus endothelial cells exhibit both antigens. Thus, splenic sinuses do not replace conventional capillaries, but exist in addition to such vessels. The endothelium in arterioles, venules and larger arteries and veins was uniformly CD34(+)CD141(+). Anti-CD34 and anti-CD141 both additionally reacted with different types of splenic stromal cells. Differential staining of capillaries and sinuses may permit a three-dimensional reconstruction of serial sections to unequivocally delineate the "open" and "closed" splenic circulation in humans.

Variant genes and the spleen in Plasmodium vivax malaria

It is generally accepted that Plasmodium vivax, the most widely distributed human malaria, does not cytoadhere in the deep capillaries of inner organs and thus this malaria parasite must have evolved splenic evasion mechanism in addition to sequestration. The spleen is a uniquely adapted lymphoid organ whose central function is the selective clearance of cell and other particles from the blood, and microbes including malaria. Splenomegaly is a hallmark of malaria and no other disease seems to exacerbate this organ as this disease does. Besides this major selective clearance function however, the spleen is also an erythropoietic organ which, under stress conditions, can be responsible for close to 40% of the RBC populations. Data obtained in experimental infections of human patients with P. vivax showed that anaemia is associated with acute and chronic infections and it has been postulated that the continued parasitemia might have been sufficient to infect and destroy most circulating reticulocytes. We review here the basis of our current knowledge of variant genes in P. vivax and the structure and function of the spleen during malaria. Based on this data, we propose that P. vivax specifically adhere to barrier cells in the human spleen allowing the parasite to escape spleen-clearance while favouring the release of merozoites in an environment where reticulocytes, the predominant, if not exclusive, host cell of P. vivax, are stored before their release into circulation to compensate for the anaemia associated with vivax malaria.

The spleen of Zaedyus pichiy, (Mammalia, Dasypodidae): a light and electron microscopic study

Armadillos are ancient mammals important as models for biomedical, evolutive and ecological studies, because they have adaptive and primitive morpho-physiological characteristics. In this work we study the splenic microarchitecture of the 'Patagonian Piche', Zaedyus pichiy, as an attempt to understand the relationship between the organizational plan of the organ and the particular features of this species. The organ samples were classically processed for light and electron microscopic study. The microanatomy of the organ as well as its different cell types are studied. The spleen is non-sinusoidal, with the typical arrangement for storage functions. White pulp is well defined. Red pulp is a meshwork of circulating, immunocompetent and haemopoietic cells. The general structure of the organ agrees with the semi-fossorial habit and the adaptability of the species. Comparative aspects with other armadillos or other less specialized groups are discussed. Persistence of haemopoietic spleen activity in the adult suggests the existence of specific inductive functions of the stroma. Better knowledge of spleen structure and function in ancient mammals may give important information about their phylogeny.

Modulation by endogenous prostanoids of the vasoconstrictor activity of endothelin-1 in the canine isolated, perfused spleen

1. Endothelin-1 (ET-1, 0.4-200 pmol) was injected into the arterial circuit of the isolated perfused spleen of the dog in which splenic arterial perfusion pressure and spleen weight were recorded continuously. 2. Serial collection was made of splenic venous effluent before and after intra-arterial injection of ET-1 and assayed by direct radioimmunoassay for prostaglandin E2 (PGE2), 6-oxo-PGF1 alpha and thromboxane B2 (TXB2). 3. ET-1 caused graded arterial vasoconstriction of prolonged duration with small reductions in spleen weight at higher doses. 4. ET-1 cause a dose-related release of PGE2, 6-oxo-PGF1 alpha and TXB2 into the splenic venous effluent. The mean peak increase above the basal levels following 200 pmol of ET-1 was 800% for PGE2, 233% for 6-oxo-PGF1 alpha and 205% for TXB2. 5. Intra-arterial infusion of indomethacin significantly reduced the basal release of all three eicosanoids and significantly elevated the basal splenic vascular resistance. The release of all three eicosanoids in response to ET-1 and adrenaline (Ad) was significantly reduced by indomethacin and the accompanying increases in the splenic arterial vascular resistance were significantly potentiated at low doses of ET-1. The splenic arterial vascular responses to Ad were unchanged by indomethacin infusion. 6. These results indicate that the release of eicosanoids may modulate the splenic vascular responses to ET-1.

Anti-vimentin monoclonal antibodies differentiate two resident cell populations in chicken spleen

Splenic stromal cells were studied with anti-vimentin monoclonal antibody clones V9 and 3B4. V9 positive cells were widely distributed both in the red and white pulp, specifically the ellipsoid, but were absent from the germinal centers. These observations suggested that this clone recognized splenic reticular cells. The original name given to the splenic ellipsoid region by Schweigger-Seidel, "Kapillarhülse" or capillary sleeve, would be a more appropriate term than ellipsoid, at least in the chicken because the V9 positive ellipsoid covered the entire length of the penicilliform capillary. The capillary sleeve was decorated by large, 3B4 positive cells that were identical to the ellipsoid-associated cells, a dendritic-like cell. The 3B4 monoclonal antibody recognized large stellate-shaped cells in germinal centers. These data emphasized the cellular difference between ellipsoid-associated cells and the cells of the ellipsoid; the cytoskeletal similarities of the ellipsoid-associated cells and dendritic cells of germinal centers advocate their common cell lineage.

Barrier cells in the spleen

Barrier cells are a newly defined group of activated fibroblastic cells that are deployed as diverse barriers in the splenic pulp. The central capacity of the spleen, namely clearance of the blood, is normally modest but when stressed, for example by infectious disease, imperfect blood cells or the administration of interleukin 1, the clearance capacities of the spleen expand. The normally scarce barrier cells rapidly and greatly increase in number in splenic stress. By augmenting the splenic filtration beds, regulating blood flow, enveloping white pulp and enclosing hematopoietic colonies, they appear to be important in the spleen's increased capacities in clearance, phagocytosis, immunological reactivity and hematopoiesis. This review focuses on their role in the spleen, but barrier cells may appear throughout the body in response to needs for walling off and cellular induction.

Microcirculatory pathways in normal human spleen, demonstrated by scanning electron microscopy of corrosion casts

Confusion regarding microcirculatory pathways in normal human spleen has arisen due to extrapolation from pathological material and from other mammalian spleens, not to mention difficulties in tracing intricate three-dimensional routes from the study of thin sections or cut surfaces of tissue. We examined microcirculatory pathways in normal human spleens freshly obtained from organ transplant donors. A modified corrosion casting procedure was used to obtain an open view of vessels and their connections. Our results demonstrate: 1) "arteriolar-capillary bundles" within lymphatic nodules and extensive branching of arterioles in the marginal zone (MZ); 2) the marginal sinus around lymphatic nodules; 3) the peri-marginal cavernous sinus (PMCS) outside the MZ or immediately adjacent to the nodule itself; the PMCS receives flow via ellipsoid sheaths and MZ, or directly from arterial capillaries, and drains into venous sinuses; 4) fast pathways for flow into venous sinuses via ellipsoid sheaths; 5) arterial capillary terminations in the reticular meshwork of the red pulp or MZ ("open" circulation); direct connections to venous sinuses also occur ("closed" circulation), although rarely; and 6) numerous open-ended venous sinuses in the MZ, allowing a large proportion of the splenic inflow to bypass the red cell filtration sites in the reticular meshwork and at venous sinus walls.

Reticular meshwork of the spleen in rats studied by electron microscopy

The reticular meshwork of the rat spleen, which consists of both fibrous and cellular reticula, was investigated by transmission electron microscopy. The fibrous reticulum of the splenic pulp is composed of reticular fibers and basement membranes of the sinuses. These reticular fibers and basement membranes are continuous with each other. The reticular fibers are enfolded by reticular cells and are composed of two basic elements: 1) peripheral basal laminae of the reticular cells, and 2) central connective tissue spaces in which microfibrils, collagenous fibrils, elastic fibers, and unmyelinated adrenergic nerve fibers are present. The basement membranes of the sinuses are sandwiched between reticular cells and sinus endothelial cells and are composed of lamina-densalike material, microfibrils, collagenous fibrils, and elastic fibers. The presence of these connective tissue fibrous components indicates that there are connective tissue spaces in these basement membranes. The basement membrane is divided into three parts: the basal lamina of the reticular cell, the connective tissue space, and the basal lamina of the sinus endothelial cell. When the connective tissue space is very small or absent, the two basal laminae may fuse to form a single, thick basement membrane of the splenic sinus wall. The fibrous reticulum having these structures is responsible for support (collagenous fibrils) and rebounding (elastic fibers). The cells of the cellular reticulum--reticular cells and their cytoplasmic processes, which possess abundant contractile microfilaments, dense bodies, hemidesmosomes, basal laminae, and a well-developed, rough-surfaced endoplasmic reticulum, and Golgi complexes, which are characteristic of both fibroblasts and smooth muscle cells--are considered to be myofibroblasts. They may play roles in splenic contraction and in fibrogenesis of the fibrous reticulum. The contractile ability may be influenced by the unmyelinated adrenergic nerve fibers that pass through the reticular fibers. The three-dimensional reticular meshwork of the spleen consists of sustentacular fibrous reticulum and contractile myofibroblastic cellular reticulum. This meshwork not only supports the organ but also contributes to a contractile mechanism in circulation regulation, in collaboration with major contractile elements in the capsulo-trabecular system.

In vivo administration of antibodies against type I collagen in rat: the specific accumulation in spleen

[125I]-labelled rabbit antibodies against rat type I collagen and non-immune IgG were injected into rat circulation. The kinetics of their clearance and the biodistribution in different organs were studied. Both preparations showed very similar clearance rate, the kinetics fitting bi-exponential approximation with characteristic parameters t1,1/2 = 201 +/- 20 min before 320 min and t2,1/2 = 1350 +/- 450 min at times over 320 min for antibodies and 258 +/- 45 min and 890 +/- 140 min for IgG. The specific affinity of the circulating antibodies did not decrease within 24 hours. The antibodies were specifically accumulated in spleen, where their accumulation was 5-fold higher than that of non-immune IgG. Accumulation of antibodies was maximal 3 hours after the injection. The localization ratio (i.e. the ratio of the amount of the antibodies per g of tissue to that per g of blood) reached a maximum 24 hours after the injection and remained stable for 120 hours. Immunofluorescent staining of spleen sections resulted in a bright fluorescence of dense collagenous structures in the trabeculae and in the wall of the central follicular arterium, bright spot fluorescence in the marginal zone of the follicle, and diffuse fluorescence in the red pulp. These findings suggest an unusually high accessibility of collagen type I in spleen to circulating blood plasma components.

Kinetics of red blood cell passage through interendothelial slits into venous sinuses in rat spleen, analyzed by in vivo microscopy

Sequential photomicrographs of RBCs passing through interendothelial slits (IES) in walls of venous sinuses in rat spleen were obtained by video recording in vivo microscopic views. Kinetics of RBC passage were analyzed by slow-motion playback of recordings. An inverted microscope and oblique lighting from a water-cooled fiber optic light source were key elements in obtaining images of sufficient quality for analysis. The direction of RBC passage through IES was, invariably, from reticular spaces of the red pulp into venous sinuses. RBC flow through an individual IES occurred as brief discontinuous bursts, separated by periods of zero, or near zero, flow. Mean rates of RBC flow through six IES analyzed in normal relaxed spleen ranged from 1.4 to 9.1 cells/15 sec, the total RBCs studied being 1523 and the total combined period of observation 98 min. The maximum instantaneous rate was 10 RBCs/sec. RBC transit times ranged from 0.02 to 60.5 sec, even for a single IES; the distribution was highly skewed: median 0.23 sec, mean 1.7 sec. Analysis of RBC flow through two closely adjacent IES simultaneously, for 30 min, showed that most bursts were asynchronous. The results indicate that changes in caliber of IES are primarily responsible for the observed pattern of flow. It was estimated that only 19% of the total IES present anatomically actually allowed passage of RBCs during any 5-min period.

Distribution of blood-borne particles of two different sizes in rat spleens

Polystyrene microspheres 5 micron in diameter and Indian ink, consisting of particles 0.03 micron in diameter, were injected into the splenic artery in rats. The distribution of the polystyrene microspheres and the ink particles in the spleen were examined microscopically and morphometrically. The polystyrene microspheres appeared mainly in the red pulp, and the Indian ink particles mostly in the marginal zone, which functions as an immunological filter. (No arteries opened into the lymphoid follicles.)

Mechanisms of splenic control of murine malaria: reticular cell activation and the development of a blood-spleen barrier

By complex stromal responses, the spleen controls the course of nonlethal Plasmodium yoelii murine malaria. The course of disease may be divided into four phases. In the immediate postinfective phase, lasting several days, the filtration beds of the spleen are open. Parasitized and nonparasitized erythrocytes, many plasma cells, lymphocytes, and macrophages are sequestered from the blood; and most or all of the parasitized erythrocytes are phagocytized. In the following precrisis phase, approximately 1 week long, there is increasing parasitemia and anemia. The filtration beds of the spleen support large-scale erythropoiesis, lymphopoiesis, plasmacytopoiesis, and monocyte-to-macrophage differentiation. Reticular cells, the stromal cells which form the splenic filtration beds, become activated, showing signs of intense protein secretion and increased branching and mitosis. The locules of the filtration beds appear sealed off from the blood by branches of activated reticular cells. A blood-spleen barrier is thereby formed, protecting splenic hematopoiesis from the parasite. Factors are produced, moreover, which damage intraerythrocytic parasites, producing crisis forms. Crisis follows. It may occur over several days, presaged by the appearance of circulating crisis forms. The filtration beds are opened to the blood. Circulating crisis forms are trapped within the locules of the filtration beds and phagocytized while the stores of reticulocytes produced there in the precrisis period are released to the blood. The malaria, as a result, is no longer patent and the anemia is relieved. In the fourth or postcrisis phase, lasting many months, the normal structure of the spleen is approached. We postulate that reticular cells, normal and activated, have the following functions: to fabricate the locules of the filtration beds; to control the migration of free cells through these beds; to trap free cells, including parasitized erythrocytes, by cell-surface adherence; to open or close the filtration locules, creating a dynamic blood-spleen barrier; to control the circulation of the spleen--by contraction and alignment in normal spleens and by activation and closing of locules in enlarged spleens; to synthesize collagen III; to synthesize factors which influence marrow release of monocytes; and to secrete antiplasmodial substances.