Aging involves morphological and functional alterations within the microenvironment of the thymus where heterogenous populations of thymic epithelial cells (TEC) play the main roles. The studies performed to date on thymic involution signalize a disturbed interaction between individual thymic compartments that disrupt thymocyte-TEC interactions and, as a sequele, disturb differentiation of both TEC and thymocytes. The process of aging affects the various subsets of TEC at different periods of life. Changes in different subsets of TEC are documented on the basis of their phenotypical characteristics, involving morphological analysis and immunocytochemistry. The character and kinetics of changes in TEC are typical for individual subsets and probably sex-dependent. In the course of life, the involutionary changes, expressed by disorganised thymic structure and function, are accompanied by changes in medullary TEC, manifested by alterations in the differentiation process of the cells. In parallel, at the same stage of individual life, the aging process induces increased proliferative and secretory activity of subseptal TEC, which seem to functionally replace medullary TEC. Structural and phenotypic modifications of TEC are locally controlled by complex sets of different factors and seem to represent a morphological adaptation of the gland to the process of aging. Microsc. Res. Tech. 62:488-500, 2003.

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Background Acidic pH and the presence of glucose degradation products (GDP) are believed to compromise the biocompatibility of peritoneal dialysis fluids (PDF). The present study examines the effects of long-term exposure to GDP and low pH by comparing conventional PDF and a new, neutral pH, low GDP solution. Methods All experiments were performed using a chronic infusion model of dialysis in nonuremic rats. The animals were treated for 6 weeks with 2 daily injections of 4.25% glucose-containing PDF. The following PDF were tested: CAPD3 (single-chamber bag, low pH, high GDP), CAPD3 pH 7.4 (single-chamber bag, neutral pH, high GDP), CAPD3-Balance (double-chamber bag, neutral pH, low GDP). All test solutions were obtained from Fresenius Medical Care, Bad Homburg, Germany. Results After 6 weeks of exposure, peritoneal permeability to water, urea, creatinine, glucose, and sodium, assessed by peritoneal equilibration test, was similar in all groups. However, compared to other PDF, dialysis with CAPD3-Balance was associated with reduced concentrations of protein and hyaluronan in the dialysate, decreased peritoneal eosinophilia, and reduced dialysate levels of chemokines CCL2/MCP-1 and CCL5/RANTES. Morphologic changes in the peritoneal membrane of CAPD3-Balance-treated animals were much less pronounced and included reduced vascular density, preservation of the mesothelial monolayer and intercellular junction, and no reduplication of the submesothelial basement membrane. Conclusions A new generation of PDF with physiologic pH and low GDP level produce less irritation to the peritoneal membrane and better preserve its structural integrity. This effect seems to be related predominantly to minimized GDP concentrations.

Male Wistar rats were injected intravenously with 5-(3H)uridine-labeled lymphocytes isolated from lymph nodes of syngeneic donors and enriched in T cells. After short periods of time (3 to 120 min after injection), labeled lymphocytes were localized in spleen compartments using autoradiography to identify routes of lymphocyte movement from blood into splenic parenchyma and to follow migration pathways of recirculating lymphocytes within the periarterial lymphoid sheath (PALS). Topographical analysis of labeled lymphocytes was performed in specific planes of PALS characterized by the diameter of the arterial vessel and termed PALS large, PALS medium, and PALS small (PALS L, PALS M, PALS S, respectively). Attention was also paid to accumulations of labeled lymphocytes close to the arterial vessel wall. Initially, labeled lymphocytes were localized in PALS S and PALS M near the terminal branching of arterial vessels and in the marginal zone (MZ). We conclude that lymphocytes emigrate from blood into splenic parenchyma within two white pulp compartments: in MZ, and directly within PALS through the wall of capillary vessels. The sequential accumulation of labeled cells near arterial vessels of increasing diameter suggests that the recirculating pool of lymphocytes migrates into the central part of PALS L by two routes: from MZ, and along arterial vessels from PALS S and PALS M.

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.

Structural alterations in thymuses of female rats during the first 2 years of life were evaluated by morphometric analysis and, then, correlated with organization of epithelial cells in various thymic compartments, examined for their cytokeratin immunoreactivity. With an advancing age, the thymuses demonstrated morphological modifications related to maturation and senescence, the dynamics of which varied between particular thymic compartments, and involved subpopulations of thymic epithelial cells. In the entire period of life the most dynamic changes were found in the cortex while the medulla was demonstrated to be a rather "stable" region. Morphometric studies revealed a negative correlation between the volume of thymic cortex and medulla and age of rats and a linear, positive relationship between the volume of connective tissue compartment and age. Changes in organization of epithelial network in the medulla preceded those observed in the cortex. Decreased proliferative activity of subset of medullary cells, which probably represented a self-renewable population, was accompanied by alterations in the immunocytochemically characterized (cytokeratines) differentiation process. At the same period of life, hypertrophy and hyperplasia of superficial epithelial cells seems to functionally replace medullary cells. This process begins around 3rd month of life and expands on all thymic compartments. The first changes in the cortex appeared around 8th month and were connected with reduced cytokeratin immunoreactivity. The involution observed in older animals was preceded by age-related alterations in epithelial network pattern which, in the course of stable morphometric parameters (between 5th and 12th month), showed character of a structural and functional adaptation.

This study attempted to define reciprocal positions of cell types within the thymus. Random or non-random contacts between specific cell types were analyzed by means of graph theory. For analysis, thymus blocks were sectioned serially and, then, thymus cells were categorized into types, based on morphological criteria. The distribution of individual cell types within the cortex, cortico-medullary zone and medulla was presented in form of a map. In the analysis, three types of epithelial cell, characteristic of each thymus zone, macrophages, Langerhans-like cells and lymphocytes were found in non-random relations to one another. Moreover, characteristic groups of cells associated with one another were also demonstrated.

The relationship between unbound and bound proteins prepared during labelling with colloidal gold (Au) was investigated. For this aim, labelled 125I-bovine serum albumin and 125I-rabbit immunoglobulin were employed. The procedures associated with the washing of the Au labelled proteins (i.e. albumin) had a marked influence on the dissociation of the bound ligand. This was most evident when the concentration of albumin that was used for labelling was too high (0.1 or 1.0 mg/ml Au sol). We suggest that purification of labelled proteins be conducted shortly before use so as to avoid a significant amount of dissociation during the time when the solution is coming to equilibrium.