Electron microscopic appearance of the parenchyma of lymph nodes

Infectious Diseases and the Lymphoid Extracellular Matrix Remodeling: A Focus on Conduit System

The conduit system was described in lymphoid organs as a tubular and reticular set of structures compounded by collagen, laminin, perlecan, and heparin sulfate proteoglycan wrapped by reticular fibroblasts. This tubular system is capable of rapidly transport small molecules such as viruses, antigens, chemokines, cytokines, and immunoglobulins through lymphoid organs. This structure plays an important role in guiding the cells to their particular niches, therefore participating in cell cooperation, antigen presentation, and cellular activation. The remodeling of conduits has been described in chronic inflammation and infectious diseases to improve the transport of antigens to specific T and B cells in lymphoid tissue. However, malnutrition and infectious agents may induce extracellular matrix remodeling directly or indirectly, leading to the microarchitecture disorganization of secondary lymphoid organs and their conduit system. In this process, the fibers and cells that compound the conduit system may also be altered, which affects the development of a specific immune response. This review aims to discuss the extracellular matrix remodeling during infectious diseases with an emphasis on the alterations of molecules from the conduit system, which damages the cellular and molecular transit in secondary lymphoid organs compromising the immune response.

The microanatomy of T-cell responses

The priming of a T cell results from its physical interaction with a dendritic cell (DC) that presents the cognate antigenic peptide. The success rate of such interactions is extremely low, because the precursor frequency of a naive T cell recognizing a specific antigen is in the range of 1:10(5)-10(6). To make this principle practicable, encounter frequencies between DCs and T cells are maximized within lymph nodes (LNs) that are compact immunological projections of the peripheral tissue they drain. But LNs are more than passive meeting places for DCs that immigrated from the tissue and lymphocytes that recirculated via the blood. The microanatomy of the LN stroma actively organizes the cellular encounters by providing preformed migration tracks that create dynamic but highly ordered movement patterns. LN architecture further acts as a sophisticated filtration system that sieves the incoming interstitial fluid at different levels and guarantees that immunologically relevant antigens are loaded on DCs or B cells while inert substances are channeled back into the blood circulation. This review focuses on the non-hematopoietic infrastructure of the lymph node. We describe the association between fibroblastic reticular cell, conduit, DC, and T cell as the essential functional unit of the T-cell cortex.

The Conduit System Transports Soluble Antigens from the Afferent Lymph to Resident Dendritic Cells in the T Cell Area of the Lymph Node

Resident dendritic cells (DC) within the T cell area of the lymph node take up soluble antigens that enter via the afferent lymphatics before antigen carrying DC arrive from the periphery. The reticular network within the lymph node is a conduit system forming the infrastructure for the fast delivery of soluble substances from the afferent lymph to the lumen of high endothelial venules (HEVs). Using high-resolution light microscopy and 3D reconstruction, we show here that these conduits are unique basement membrane-like structures ensheathed by fibroblastic reticular cells with occasional resident DC embedded within this cell layer. Conduit-associated DC are capable of taking up and processing soluble antigens transported within the conduits, whereas immigrated mature DC occur remote from the reticular fibers. The conduit system is, therefore, not a closed compartment that shuttles substances through the lymph node but represents the morphological equivalent to the filtering function of the lymph node.

The organization of cell populations within lymph nodes: their origin, life history and functional relationships

The normal lymph node comprises a superficial cortex, a deep cortex or paracortex and a medulla. In each of these regions there are three kinds of spaces: an intralymphatic space, an intravascular space and an extravascular space or interstitium. Both the vascular endothelium and the lymphatic endothelium are specialized in these different regions. The cell types in lymph nodes comprise lymphoid cells, accessory or non-lymphoid cells and stromal cells, and within these cell types a number of different sub-types can now be identified by means of enzyme- and immunocytochemistry. Based predominantly on experimental studies, the origin, migratory patterns, localization, inter-relationships and interactions between these various cells are reviewed.

Lectin I of Ulex europaeus as a marker for a subset of histiocytic tumours of the lymph node

We describe four lymph node based tumours in which numerous neoplastic cells and some mitotic figures were characterized by staining affinity for Lectin I of Ulex europaeus (UEA-I). The patients had no vascular or epithelial tumours and presented symptoms suggestive of a systemic lymphoproliferative disease. Histologically, the tumours were composed of large, cohesive, cells which were mainly located in the paracortex. UEA-I reactivity was more evident in the Golgi area and was present in large mononucleated cells often arranged to delimit vascular-like spaces. The neoplastic cells were weakly muramidase-positive in one case, and were ANAE+/AP+ in two other cases. Large dots of UEA-I reactivity were detected in S-100+/muramidase-negative Langerhans-like cells present in one case of Letterer-Siwe disease. UEA-I staining was consistently negative in 20 cases of B cell- or T cell lymphoma and in 9 other cases of histiocytic lymphoma. It is suggested that UEA-I+ tumours of the lymph nodes are part of a distinct subset of histiocytic malignancies whose neoplastic cells present some morphological and phenotypic properties normally associated with endothelial cells.

Postnatal development of the non-lymphoid elements in the rat lymph node. Connective reticulum cells, macrophages and postcapillary venules

The present study analyzes ultrastructurally the non-lymphoid elements occurring in the diffuse deep cortex and medulla of the postnatal developing lymph nodes of the rat. The supporting meshwork of the organ consist of connective reticulum cells joined together themselves by intercellular junctions. Two morphologically different macrophage cell types can be described in the developing lymph nodes. The first one consists of free-migrating macrophages homing the cortical parenchyma, apparently related with phagocytosis of necrotic cells. In contrast, macrophages located in the lumen of the medullary lymphatic sinuses are anchored to connective reticulum cell processes. They form lymphocyte-macrophage clusters and are involved also in erythrophagocytosis phenomena. Postcapillary venules lined by a low cuboidal endothelium, which allow lymphocyte diapedesis, appear in the diffuse deep cortex of the neonatal lymph nodes. The possible significance of all these cellular elements is discussed with respect to their role as constituents of the local lymph node microenvironments.

An electron microscopic study on the reticuloendothelial cells in the lymph nodes

In an attempt to clarify the cytological characteristics of the RES cells in the lymph nodes and their embryological correlations, lymph nodes and lymph node anlages of germ-free rats, nude mice, and human fetuses were light and electron microscopically examined. On the basis of differences of intracellular organelles, their behaviors for reticulum fibers and of endogeneous peroxidatic activity, histiocytes should be reasonably distinguished from the cells conventionally called reticulum cells. Reticulum cells and histiocytes respectively are destined to differentiate in different directions from the early stage of development of the lymph node anlage. Sinus endothelial cells are ontogenetically originated and differentiated from the endothelial lining cells of lymphatic vessels. Primitive reticular cells are differentiated into mature reticulum cells in the lymph nodes, they transform into the lympho-reticular cells, further into lymphoblasts, and finally develop into medium-sized lymphocytes.

Ultrastructural and histochemical investigations of spindle-shaped corpuscles in phagocytic cells of lymph nodes

In connection with various diseases, spindle-shaped or roundish 2-15 micrometer large corpuscles with a yellowish-green, partly brown color are found in phagocytic cells of lymph nodes of various localisation. There are three groups according to staining and ultrastructural characteristics. I. Strongly Fe+++ positive corpucles only, 2. corpuscles with a positive PAS-reaction, Ziehl-Neelsen and Sudan black B stain and slight iron adsorption, 3. particles with irregular stainability. Ultrastructurally, the spindle bodies of Group I have a uniform membrane, double in some sections and a narrow, finely granular rim, in which 50 A large iron micelles are stored. The electrondense center contains structures which may correspond to remainders of cell organelles. The similarly shaped particles of Group 2 have a homogeneous, electron-dense center and sometimes half-moon-shaped, homogeneous, less dense appositions. A dense round center with finely honey-combed translucencies and marginal vacuoles characterizes the irregularly shaped, in part spindly corpuscles of Group 3. The energy-dispersion X-ray analysis indicates only iron for the particles of Group I. A relation of the corpuscles to specific diseases, especially to sarcoidosis and certain therapeutic measures, is not evident.

Nodular lymphoma: an ultrastructural study of its relationship to germinal centers and a correlation of light and electron microscopic findings

This study describes the ultrastructural findings in 16 cases of nodular lymphoma, and compares these with normal germinal centers in order to determine the possible germinal center origin of the lymphomas. Long branching desmosome-associated dendritic reticulum cells, characteristic of germinal centers, were found in all 16 cases of nodular lymphoma. Desmosomes were observed only between dendritic cells and not between lymphoid cells, and they were seen in only 1 of 7 cases of diffuse lymphoma. Cells comprising the nodular lymphomas share cytologic features, such as nuclear blebs and marked nuclear indentations, with germinal center cells. It appears that nodular lymphomas have a close anatomical relationship to germinal centers, although their actual origin from these sites is not established by this study. They are composed of lymphoid cells; we have no evidence to confirm the view that nodular lymphomas are derived from dendritic cells. Light and electron microscopic findings have been compared in order to establish the nature of large "histiocyte-like" cells in the nodular lymphomas. Many of these cells appear to represent transformed lymphocytes rather than histiocytes. They are more numerous in so-called mixed "histiocytic" lymphocytic lymphomas than in poorly differentiated lymphocytic lymphomas, which otherwise are ultrastructurally similar. This study casts doubt upon the existence of a mixed lymphocytic and histiocytic lymphoma within the spectrum of the nodular lymphomas, and indicates that a cytologic continuum exists between cells interpreted as poorly differentiated lymphocytes and those heretofore identified as histiocytes.

The fine structure of lymphosarcoma in cattle

Nine cases of bovine lymphosarcoma were classified by light microscopy as lymphoid or reticulum according to the predominant cell type, or as lymphoreticular when there was a heterogeneous mixture of lymphoid and reticulum tumor cells. Typical ultrastructural features of these cells were as follows. Lymphoid tumor cells had smoothly contoured nuclear and cytoplasmic membranes and few cytoplasmic organelles. Reticulum tumor cells had large interchromatin spaces and irregular nuclear and cytoplasmic outlines. They often had multivesicular bodies, lysosomal vesicles, and much smooth-surfaced endoplasmic reticulum in the cytoplasm and were frequently surrounded by extracellular microfibrils and collagen libers. Nuclear inclusion-like masses of cytoplasm were common in both types of tumor cells. No structure was observed which could be recognized as virus.

Antigens in immunity. XIV. Electron microscopic radioautographic studies of antigen capture in the lymph node medulla

Details of antigen trapping and processing in the rat lymph node have been investigated by the technique of high resolution radioautography. A series of 24 adult rats was injected with 20 microg of (125)I-labeled Salmonella adelaide flagella, given as either a primary or a secondary stimulus into one hind foot-pad. At intervals ranging from 3 min to 3 wk, rats were killed and the popliteal nodes were processed for electron microscopic radioautography using Kodak NTE emulsion. The present paper deals with events in the lymph node medulla, and an accompanying report describes the radically different behavior of antigen in the cortical follicles. In the medulla, lightly labeled granulocytes were transiently encountered, but by far the greatest bulk of antigen was in macrophages. Antigen entered these cells in two ways: by direct penetration of the plasma membrane; and by pinocytosis. In either case, the antigen rapidly became surrounded by tiny vesicles which may have represented Golgi-derived "protolysosomes." Vacuolar fusion ensued and a series of progressively larger and more complex antigen-containing "phagolysosomes" was formed. Substantial amounts of antigen could be detected in such bodies for at least 3 wk. The antigen injection, as expected, caused extensive plasma-cytopoiesis. No evidence of label in plasma cells was obtained. No special anatomic relationship between plasma cells and antigen depot sites was discovered. These results are briefly discussed in relation to current theories of immune induction.

Electron microscopic autoradiography of germinal center cells in mouse spleen

The fine structure of tritiated thymidine-labeled cells in antigen-stimulated mouse spleen germinal centers is described. In studies on the ultrastructural level, two labeled cell types found in germinal centers are observed. Large lymphocytes are characterized by their very numerous free ribosomes, a paucity of endoplasmic reticulum, relatively few mitochondria, and a poorly developed Golgi region. The nuclei are large and vesicular, and large nucleoli are present. A second labeled cell type appears to contain more mitochondria and has a higher development of the Golgi area. The nucleus contains large, numerous blocks of chromatin, indicative of a more differentiated cell type. Reticular cells, both phagocytic and non-phagocytic, were not observed to be labeled in the germinal centers.