Cortical Reticular Cells Research Articles

Histological and cytological studies on the developing thymus of sharpsnout seabream, Diplodus puntazzo.

The structure of the developing thymus of the marine teleost, Diplodus puntazzo, was studied by light and transmission electron microscopy. The first anlage of the thymus developed by d 20 postfertilisation (p.f.) as a group of undifferentiated cells dorsal to the epithelium of the branchial chamber. The organ increased significantly in size around d 51-66 p.f. and differentiation of cortex and medulla occurred concomitantly. On the basis of their localisation, 4 main types of epithelial cell were distinguished: (1) limiting, adjacent to the connective capsule; (2) medullary and cortical reticular cells; (3) nurse cells, located in the corticomedullary boundary; (4) Hassall-like corpuscles. The majority of medium to large blast-like lymphoid cells were localised in the medulla, while small lymphocytes were housed in the cortical region. These morphological features were maintained at later stages. However, in juveniles in the medulla we observed reticular epithelial cells with cysts and rare Hassall-like corpuscles. The study was designed to obtain more information concerning the histology of the developing thymus of sharpsnout seabream and give a concise description of the differentiation of epithelial cells and lymphoid cells in the thymic parenchyma.

Histological and cytological studies on the developing thymus of sharpsnout seabream, Diplodus puntazzo

The structure of the developing thymus of the marine teleost, Diplodus puntazzo, was studied by light and transmission electron microscopy. The first anlage of the thymus developed by d 20 postfertilisation (p.f.) as a group of undifferentiated cells dorsal to the epithelium of the branchial chamber. The organ increased significantly in size around d 51–66 p.f. and differentiation of cortex and medulla occurred concomitantly. On the basis of their localisation, 4 main types of epithelial cell were distinguished: (1) limiting, adjacent to the connective capsule; (2) medullary and cortical reticular cells; (3) nurse cells, located in the corticomedullary boundary; (4) Hassall-like corpuscles. The majority of medium to large blast-like lymphoid cells were localised in the medulla, while small lymphocytes were housed in the cortical region. These morphological features were maintained at later stages. However, in juveniles in the medulla we observed reticular epithelial cells with cysts and rare Hassall-like corpuscles. The study was designed to obtain more information concerning the histology of the developing thymus of sharpsnout seabream and give a concise description of the differentiation of epithelial cells and lymphoid cells in the thymic parenchyma.

Parvalbumin and calretinin in the avian thymus

The avian thymic hormone, known to support the maturation of T-lymphocytes, is biochemically similar to parvalbumin. However, its exact cellular distribution in the thymus is unknown. We have therefore studied the occurrence of parvalbumin and other related calcium-binding proteins in this organ by immunohistochemistry during development and after hatching. Parvalbumin immunoreactivity appears in the epithelial cells on embryonic day 9, and is expressed in the cortical reticular cells in the adult. Calretinin is found in the clusters of medullary epithelial cells from embryonic day 11, whereas calbindin D-28k is absent from this organ. Thus, calcium binding-proteins are excellent markers for different compartments of the avian thymus in embryonic life and after hatching, and their expression seems to coincide with their functional maturation.

Phenotypically diverse mouse thymic stromal cell lines which induce proliferation and differentiation of hematopoietic cells.

The heterogeneity of the thymic stroma has made careful characterization of particular thymic stromal cell types difficult. To this end, we have derived a panel of cloned thymic stromal cell lines from simian virus 40 T antigen (SV40-T antigen) transgenic mice. Based on their analysis with monoclonal antibodies that distinguish among subsets of thymic stroma cells, and on the morphology and ultrastructural features of the different clones, we suggest that our panel includes representatives of the thymic subcapsular cortex or thymic nurse cells (427.1), the deep cortex or cortical reticular cells (1308.1) and the medulla including medullary interdigitating (IDC)-like cells (6.1.1) and medullary epithelial cells (6.1.7). A fifth cell type of undesignated but apparent medullary origin (6.1.11) was also isolated. All of the cell lines constitutively express the SV40 T antigen transgene and the class I antigens of the major histocompatibility complex (MHC), and they can be induced to express MHC class II antigens upon stimulation with recombinant interferon-gamma (IFN-gamma). These cell lines elaborate a factor(s) that induces the proliferation of cells from the fetal liver and bone marrow, but not from the neonatal thymus. A factor(s) elaborated by the 1308.1 cell line also induces the proliferation of fetal thymocytes in the absence of mitogens, phorbol esters or calcium ionophore which is augmented with the addition of recombinant interleukin-2 (IL-2). Analysis by reverse transcription polymerase chain reaction with primers for some mouse cytokines reveals that each of these cell lines contain granulocyte-macrophage colony-stimulating factor (GM-CSF) transcripts and that 1308.1, 6.1.1 and 6.1.7 produce IL-6 mRNA. Cell lines 1308.1 and 6.1.1 also produce IL-7; 6.1.1 produces IL-1 beta and tumor necrosis factor (TNF)-alpha while the 427.1 cell line produces IL-5 and IFN-gamma mRNA. None of the cell lines tested express the IL-2 receptor, IL-2, IL-3, IL-4, TNF-beta or macrophage inflammatory proteins mRNA. Conditioned medium (CM) from 1308.1 and 6.1.11 induced differentiation of cells purified from the mouse fetal liver into granulocytes; 1308.1 CM also induced differentiation of the mouse hematopoietic stem cell line 32DCl3(G) suggesting that the CM contains granulocyte (G)-CSF activity. Each cell line produces GM-CSF but the greatest activity is associated with 1308.1 and 6.1.11 CM. The availability of these well-characterized, functional, cloned thymic stromal cells will allow a more detailed analysis of the role of each cell type in both myeloid and T cell development.

Thymic nurse cells in culture: morphological and antigenic characterization.

Epithelial monolayers were derived from thymic nurse cells (TNC), and were seeded onto collagen-coated dishes immediately after their isolation from young adult C3H-murine thymuses. Different media and supplements were tested in order to obtain cultures that were as pure as possible. Primary cultures were enriched in epithelial cells but always contained non-epithelial components among which fibroblasts predominated. Immunodetection of keratins, and repeated light- and electron-microscopic observations established the epithelial nature of the elongated cells derived from TNC; these elongated cells were cortical reticular cells, and were different from medullary globular cells that immediately adopted a mosaic pattern in vitro. At the beginning of the culture, the necrosis of cortical lymphocytes appeared to be toxic for epithelial cells; when epithelial cells survived, they showed a temporary lipid accumulation. After a 5-day culture, they still synthesized DNA but lost this capacity thereafter and dedifferentiated. The lympho-epithelial symbiosis appeared to be necessary to maintain some epithelial characteristics of the cultured cells, such as the clear vesicles and the expression of Ia antigens. In sub-cultures, the monolayers were almost purely epithelial in nature but growth was no longer observed. The cells remained reticular in shape, as they were in vivo, but their cytoplasm and their nucleus became larger and numerous cells were multinucleated. Confluence was not obtained with classical media even after mitogenic stimulation. The frequent observation of strongly keratinized areas suggested a process of terminal differentiation; this could not be avoided by using low serum concentration.

Thymic nurse cells: morphological study during their isolation from murine thymus

Thymic nurse cells (TNC), which are multicellular complexes composed of epithelial cells and thymocytes, were obtained from C3H-mice thymuses. They were described by means of light and electron microscopy. The morphology of epithelial cells forming isolated TNC compared to that of small tissue fragments obtained by enzymatic digestion revealed that TNC could be derived from all parts of the thymus: cortex, corticomedullary junction and medulla, the cortex being their principal source. This variety of origin, the presence of several epithelial cells inside a single TNC, the presence of non-lymphoid cells, and the various locations of cleaved desmosomes confirmed that their aspect "in vitro" as round and sealed structures can be considered to be an artifact due to the isolation technique used. Indeed, during this procedure, they are formed by a process of wrapping of the epithelial cytoplasm around the tightly associated thymocytes. All three epithelial cell types: cortical reticular cells, medullary reticular cells, and medullary globular cells can form TNC.

Enzymatic and morphological response of the thymus to drugs in normal and zinc-deficient pregnant rats and their fetuses.

In the thymus of normally fed pregnant rats the plasma membrane enzymes dipeptidyl peptidase IV (DPP IV) and alkaline phosphatase (alP) were found in cortical and medullary lymphocytes (thymocytes). Plasma membrane aminopeptidase A (APA) and adenosine monophosphate hydrolysing phosphatase (AMPP) were present in cortical reticular cells. In medullary reticular cells, aminopeptidase M (APM), gamma-glutamyl transferase (GGT), adenosine triphosphate (ATPP) and thiamine pyrophosphate (TPPP) cleaving phosphatases were detected. Medullary reticular cells did not contain APA. Lysosomal DPP I and II, acid phosphatase, acid beta-D-galactosidase, beta-D-N-acetyl-glucosaminidase, beta-D-glucuronidase and non-specific esterases occurred especially in macrophages at the corticomedullary junction. The 21-day-old fetal thymus showed a similar reaction pattern as the maternal organ except for APA which was absent before birth. After treatment of the pregnant rats with valproic acid (VPA), salicylic acid (SA), streptozotocin (ST) and retinoic acid (RA) APA showed an increase in activity in the thymic cortex. In addition, ST and RA induced AMPP, ATPP and TPPP activity in cortical reticular cells up to the same pattern as in medullary reticular cells. After ethanol (ET) administration severe damages occurred. The thymic cortex was free of DPP IV-positive lymphocytes; the medullary reticular cells showed reduced or no GGT and occasionally an increased APM activity. Dexamethasone (DEXA) given to normal or zinc-deficient rats produced the most severe lesions; thymocytes with DPP IV activity were completely absent in the cortex and medulla. In Zn-deficient pregnant rats similar alterations were observed as after ET.(ABSTRACT TRUNCATED AT 250 WORDS)

The stroma of the thymus of the rat: Morphology and antigen diffusion, a reconsideration

This work reconsiders aspects of the morphology of the capsule, of the blood vasculature, of the distribution of reticular fibers, and of the diffusion of intramediastinally injected antigens in the stroma of the thymus of the rat. This was done by an analysis of standard sections of normal thymuses, of sections of thymuses perfused with colloidal carbon, of silver-impregnated sections, and of sections of thymuses of rats injected intramediastinally with a fluorescent antigen or intravenously with Trypan blue, and by electron microscopy of the thymic capsule. The capsule consisted of two layers: an outer layer covering the entire periphery of a thymic lobe, and an inner layer which outlined the entire convoluted peripheral cortex of a lobe. Cortical vessels entered the capsule and septa in which they formed a capillary network. These capsular capillaries were fenestrated and leukocytes were often present near them. Adipocytes were also seen near these vessels in some areas of the capsule, and often at the bases of septa and trabeculae. Furthermore, much of the medulla had a dense network of coarse reticular fibers, whereas the remainder of the medulla and the cortex contained a loose network of fine fibers stretching out from the capsule, septa, and trabeculae. Intramediastinally injected fluorescent antigens were observed to spread in the capsule and septa and to diffuse in the fiber networks stretched across the cortex and the medulla. Fluorescence also highlighted cortical reticular cells but not the thymocytes. Intravenously injected Trypan blue stained the capsule, the septa, the cortical reticular cells, and the autofluorescent cells outlining the corticomedullary junction of each lobule. The unusual penetration of capillaries from the thymic parenchyma into the thymic capsule suggested that the capsular capillaries participate in peculiar thymic events, such as the recruitment of blood stem-cells. It is concluded that small amounts of blood antigens normally exude from capsular capillaries and diffuse into the fibers extending from the capsule across the cortex. The phenomenon would be increased under conditions causing thymic involution. An explanation is proposed to account for the development of involution which involves the exudation of antigens from the capsular capillaries. A comparable mechanism could also account for the development of a particular experimental immune tolerance.

Enzyme cytochemistry combined with electron microscopy, pharmacokinetics, and clinical chemistry for the evaluation of the effects of steady-state valproic acid concentrations on the mouse.

A number of organs from adult female mice were investigated after continuous application of the anticonvulsant drug valproic acid (VPA) by enzyme cytochemistry, light and electron microscopy, pharmacokinetics and clinical chemistry. VPA plasma levels were maintained between 55 micrograms/ml and 67 micrograms/ml for three days following subcutaneous implantation of drug reservoirs. Effects detectable by enzyme cytochemical or electron microscopical means were mainly observed in liver, kidney, thymus and spleen. A strict concentration-dependency of drug effects could not be found. In the liver, the activities of some surface-membrane hydrolases were increased at the biliary pole; the activities of other hydrolases were decreased or unchanged. Electron microscopically, number and length of microvilli of hepatocytes were increased and many of them showed fat inclusions, mitochondrial swellings and autophagic vacuoles. In some of the proximal convoluted tubules of the kidney, the reaction product originating from microvillous and lysosomal hydrolases was diffusely distributed and its amount lowered. This was paralleled by tubular cells with an increased number of fat droplets and swollen mitochondria or destroyed tubular cells, as demonstrated by electron microscopy. Additionally, peritubular endothelial cells were arranged in a garland-like pattern. Alkaline phosphatase was activated in the straight portion of the proximal tubules. Increased glucose, creatinine and total protein concentrations and increased gamma-glutamyl transpeptidase and alkaline phosphatase activities in the urine reflected well the damage of the proximal renal tubules. Cortical and medullary morphology varied considerably in the thymus. In extreme cases, the cortical zone was either reduced in size or the medulla showed a cortex-like structure or vice versa (inverted type of thymus). The thymic cortical reticular cells showed increased aminopeptidase A activity accompanied by a generalized aminopeptidase M and alkaline phosphatase reaction. Our data indicate that--in addition to the liver--also the kidney, thymus and spleen are target organs of VPA-induced toxicity in the mouse.