A study of some histochemical and phagocytic reactions of the reticuloendothelial system of the rabbit spleen

Cells in the marginal zone of the spleen

The marginal zone of the spleen forms an intriguing area in which a variety of cell types are combined. Several of these cell types seem to have a fixed position in the marginal zone, such as the marginal zone macrophages, the marginal metallophilic macrophages at the inner border, and, to a lesser extent, the marginal zone B cells. For other cell types--T lymphocytes, small B cells, and dendritic cells--the marginal zone is only a temporary residence. It is this combination of relatively sessile cell populations and the continuous influx and passing of bloodborne immunocompetent cells that turn the marginal zone into a dynamic area, particularly apt for antigen processing and recognition. In no other lymphoid organ can such a unique combination of cells and functions be found. The opening of the arterial blood stream in the marginal sinuses results in a reduction of the velocity of the blood stream, and antigens are initially screened in the marginal zone. To this, extremely potent phagocytic cells, the marginal zone macrophages, are present which can take up and phagocytize large foreign particles, such as bacteria and effete red blood cells. Further filtration of the blood takes place in the filtration beds of the red pulp. The marginal zone macrophages express membrane receptors for bacterial polysaccharides which lead to efficient phagocytosis, probably even in the absence of prior opsonization. Antigenic fragments produced this way can be taken up by dendritic cells that enter the spleen by the blood as part of a mobile surveillance immune system. Dendritic cells present antigen to T cells in the outer area of the T cell-dependent PALS, leading to clustering and enrichment of antigen-specific T cells. Antigens in the marginal zone can also directly associate with memory B cells thought to reside here for longer times, having intimate contact with the marginal zone macrophages. B memory cells then migrate into the PALS and present antigen to T cells. The marginal zone therefore functions not only as an area of initial filtration and phagocytosis of antigens from the blood, but also as a site of lymphocyte emigration. Some of the incoming T and B lymphocytes in the recirculating pool enter the white pulp from the marginal zone. The underlying force and selective molecular mechanisms that guide this migration are unknown. Both B and T lymphocytes recirculate through the outer PALS area on their way to the follicles and the inner PALS, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Sites of action of soltriol (vitamin D) in hamster spleen, thymus, and lymph node, studied by autoradiography

Siberian hamsters (Photopus sungorus) were injected with 3H dihydroxycholecalciferol (vitamin D, soltriol). Autoradiograms of spleen, thymus, and lymph nodes revealed nuclear concentration of the hormone in a select population of cells in all of these organs. In the spleen, labeled cells were abundant in the red pulp, but sparse in the white pulp. In the periarterial lymphatic sheath (PALS) labeled cells were found predominantly at the outer rim, with a few scattered labeled cells in the inner PALS and in the marginal zone. Lymphocytes, including pyronin-positive plasma cells, did not display nuclear labeling. In the red pulp, some of the labeled cells contained pigmented inclusions in the cytoplasm, while most of the labeled cells did not appear phagocytic under the conditions of the experiment. In the thymus, labeled cells were most numerous in the medulla, but sparse in the cortex. Many of the thymic target cells were larger than the unlabeled lymphocytes, with a large and pale nucleus, sometimes containing a distinct nucleous, and with large and dendritic cytoplasm, having the appearance and distribution of epithelio-reticular cells. In lymph nodes, scattered labeled cells were conspicuous in or near the subcapsular sinus, while other cells did not concentrate radioactivity in their nuclei. The results indicate that nuclear receptors and direct genomic actions for soltriol exist in certain cell populations of lymphatic tissues that probably include reticular cells and a subpopulation of macrophages. These target cells may mediate effects of the steroid on lymphocytes that appear to have no or only very low numbers of nuclear receptors.

In-vivo effects of lipopolysaccharide on lymphoid and non-lymphoid cells in the mouse spleen. Migration of marginal metallophils towards the follicle centres

In mice marginal metallophils are located at the periphery of the white pulp along the inner border of the marginal sinus. These cells have a weak phagocytic capacity but their function is still unclear. In the present study evidence is given that marginal metallophils migrate from the periphery of the follicle towards the follicle centres after administration of at least 7 micrograms lipopolysaccharide (LPS). This migration is most significant after 24 and 48 h and appears to be a specific effect of LPS. In the follicle centre marginal metallophils take up cell debris and may become tingible body macrophages. The similarity between these two cell types is discussed. The possible effects of several other polyclonal B-cell mitogens on marginal metallophils have also been studied. Dextran sulphate also induces migration of marginal metallophils but this compound triggers a migration and accumulation of these cells at the periphery of the follicles.

The significance of the subcompartments of the marginal zone for directing lymphocyte traffic within the splenic pulp of the rat

These studies were designed to gain more detailed information on the sites of lymphocyte migration into the marginal zone, on lymphocyte segregation within this area and on subsequent migration of the cells in individual compartments of the rat spleen. A lymphocyte population enriched in T-lymphocytes was obtained from rat lymph nodes and was labeled with 5-(3H)uridine in vitro. Observations on localization of labeled lymphocytes at short time intervals following their intravenous transfusion into syngeneic recipients, indicate that the sites of emigration from the blood are the internal and external layers of the marginal zone. From here, the cells migrate towards the periarteriolar lymphoid sheath and the red pulp.

Species variation in the structure and function of the marginal zone--an electron microscope study of cat spleen

The marginal zone in the cat spleen consisted of a characteristic mixture of lymphocytes and other blood cells located mainly between the several layers of circumferential reticulum around white pulp. A region of fine-meshed reticulum between white pulp and red pulp, as present in some species, was absent from the cat spleen. Arterial capillaries to the marginal zone were few. Some were continuations of white pulp capillaries, whereas others were red pulp capillaries that likely were continuations of axial capillaries of periarterial macrophage sheaths (PAMS) (ellipsoids). Blood cells deposited in the marginal zone could reach red pulp by passing through the numerous openings in each layer of circumferential reticulum. Lymphocytes appeared to migrate across the marginal zone both toward and away from white pulp. Macrophages lying on the circumferential reticulum of the marginal zone phagocytized cells but did not ingest Thorotrast, although it coated their surfaces. Because of the scarcity of arterial endings and the lack of a macrophage-charged reticular meshwork, the marginal zone in cat spleen is not a major site of blood clearance and phagocytosis. These functions are better served in PAMS and red pulp.

Platelets, endothelial cells and macrophages in the spleen. An ultrastructural study on perfusion-fixed organs

Rabbit spleens have been examined after perfusion fixation with and without prior washing with various fluids. The platelets were stored in the splenic sinuses and in the cord spaces as single platelets, or in loosely packed aggregates which appeared to be anchored to the endothelium by one or a few platelets. After washing prior to fixation most of the platelets disaggregated and regained their normal shape. Some platelets adhered to morphologically normal endothelium even after prolonged perfusion. Occasionally, platelets were observed inside splenic endothelial cells. Others were closely associated with macrophages, many of which also contained engulfed platelets. There was no morphological evidence of a particular platelet population being retained in the spleen after washing. In the sinuses special granule-rich cytoplasmic structures were observed. They were interposed between ordinary endothelial cells and contained a large number of small lysosome-like granules. Nuclei were never observed in these structures, probably because they consisted of pseudopod-like protrusions. Their origin and function are discussed. They may represent actively phagocytizing elements.