Evolution, development and involution of the thymus

We studied the appearance of the thymus primordia of main representatives of the vertebrate evolution, observed these anlages in different places of the branchial region and followed their gradual reduction in phyletic line. On the assumption that the thymus potency has been encoded in evolution, we studied the human thymus ontogeny and described the entodermal thymus primordia not only in the 3rd but also in the 2nd and 4th pouch. We describe a very important contribution of the ectodermal canaliculus, projecting from the 3rd cleft, as well as the participation of the hypobranchial neural placode to the definitive thymus formation. Even though the entodermal thymus primordium in the 2nd pouch has reached an advanced stage, the thymus cannot complete its development because the ento-ectodermal relation is missing. In addition to this dysgenesis, we describe thymic involution in a later fetal period in connection with some malformations and confirm the close relation between the central immune organ and the neuro-endocrine system.     

Seasonal and age changes in the thymus gland of the Red fox, Vulpes vulpes,

Seasonal and age changes in thymus weight and histological structure were examined in the Red fox ( Vulpes vulpes ). Growth in the fox thymus slowed down after birth compared with the last third of foetal existence, but the gland still grew rapidly to reach a peak first year weight when the cubs were 20 weeks of age. From this point the thymus in both sexes decreased markedly in weight to reach a low point by the beginning of the first breeding season. During this involution lobule structure broke down and adipose tissue and connective tissue was laid down in the gland. Recovery of the thymus towards the second year weight maximum was accompanied by the regaining of lobule structure and the gland resembled that of the juvenile again. The male thymus increased in weight from the middle of the mating season, but recovery in the female thymus was delayed until the end of lactation. Involution occurred prior to the second breeding season. Thereafter, the gland never attained the high weights seen in the first two years of life, but histological changes still occurred even in old animals. The thymus gland of animals infected with sarcoptic mange is described.     

Mutations affecting thymus organogenesis in Medaka, Oryzias latipes

The thymus is an organ for T lymphocyte maturation and is indispensable for the establishment of a highly developed immune system in vertebrates. In order to genetically dissect thymus organogenesis, we carried out a large-scale mutagenesis screening for Medaka mutations affecting recombination activating gene 1 (rag1) expression in the developing thymus. We identified 24 mutations, defining at least 13 genes, which led to a marked reduction of rag1 expression in the thymus. As thymus development depends on pharyngeal arches, we classified those mutations into three classes according to the defects in the pharyngeal arches. Class 1 mutants had no or slight morphological abnormalities in the pharyngeal arches, implying that the mutations may include defects in such thymus-specific events as lymphocyte development and thymic epithelial cell maturation. Class 2 mutants had abnormally shaped pharyngeal arches. Class 3 mutants showed severely attenuated pharyngeal arch development. In Class 2 and Class 3 mutants, the defects in thymus development may be due to abnormal pharyngeal arch development. Those mutations are expected to be useful for identifying the molecular mechanisms underlying thymus organogenesis.     

WDR55 is a nucleolar modulator of ribosomal RNA synthesis, cell cycle progression, and teleost organ development

The thymus is a vertebrate-specific organ where T lymphocytes are generated. Genetic programs that lead to thymus development are incompletely understood. We previously screened ethylnitrosourea-induced medaka mutants for recessive defects in thymus development. Here we report that one of those mutants is caused by a missense mutation in a gene encoding the previously uncharacterized protein WDR55 carrying the tryptophan-aspartate-repeat motif. We find that WDR55 is a novel nucleolar protein involved in the production of ribosomal RNA (rRNA). Defects in WDR55 cause aberrant accumulation of rRNA intermediates and cell cycle arrest. A mutation in WDR55 in zebrafish also leads to analogous defects in thymus development, whereas WDR55-null mice are lethal before implantation. These results indicate that WDR55 is a nuclear modulator of rRNA synthesis, cell cycle progression, and embryonic organogenesis including teleost thymus development.     

Isolation,Identification, and Purification of Murine Thymic Epithelial Cells

The thymus is a vital organ for T lymphocyte development. Of thymic stromal cells, thymic epithelial cells (TECs) are particularly crucial at multiple stages of T cell development: T cell commitment, positive selection and negative selection. However, the function of TECs in the thymus remains incompletely understood. In the article, we provide a method to isolate TEC subsets from fresh mouse thymus using a combination of mechanical disruption and enzymatic digestion. The method allows thymic stromal cells and thymocytes to be efficiently released from cell-cell and cell-extracellular matrix connections and to form a single-cell suspension. Using the isolated cells, multiparameter flow cytometry can be applied to identification and characterization of TECs and dendritic cells. Because TECs are a rare cell population in the thymus, we also describe an effective way to enrich and purify TECs by depleting thymocytes, the most abundant cell type in the thymus. Following the enrichment, cell sorting time can be decreased so that loss of cell viability can be minimized during purification of TECs. Purified cells are suitable for various downstream analyses like Real Time-PCR, Western blot and gene expression profiling. The protocol will promote research of TEC function and as well as the development of in vitro T cell reconstitution.     

Upregulation of TrkA neurotrophin receptor expression in the thymic subcapsular,paraseptal, perivascular,and cortical epithelial cells during thymus regeneration

Neuroimmune networks in the thymic microenvironment are thought to be involved in the regulation of T cell development. Here, we report upon an examination of the expression of the TrkA neurotrophin receptor, the high affinity receptor for nerve growth factor, during regeneration following acute involution induced by cyclophosphamide in the rat thymus. Light and electron microscopic immunocytochemistry demonstrated enhanced expression of the TrkA receptor in the subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration. In addition, various morphological alterations, suggestive of a hyperfunctional and dynamic state, of the subcapsular, paraseptal, and perivascular epithelial cells were also observed. The presence of TrkA protein in extracts from the control and regenerating rat thymus was confirmed by western blot. Furthermore, RT-PCR analysis supported these results by demonstrating that thymic extracts contain TrkA mRNA at higher levels during thymus regeneration. Thus, our results suggest that the TrkA receptor located on the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells could play a role in the development of new T cells to replace T cells damaged during thymus regeneration.     

Hemopoietic tissue in the adult newt,Notopthalmus viridescens

The presence of developmental stages of lymphocytes and their precurors, as revealed by serial and thin sections of hemopoietic organs of normal adult newts (Notopthalmus viridescens) suggests that lymphopoiesis is limited to the thymus, medulla of the spleen and, to a lesser degree, the intestine. Stromal cells, small lymphocytes, granulocytes, mature erythrocytes and melanocytes were observed either within or near the parenchyma of the thymus. The urodele thymus differs from the thymus of anurans and higher vertebrates in that it lacks a cortex and a medulla, myoid cells and Hassall's corpuscles.     

Carboxypeptidase E,an essential element of the regulated secretory pathway,is expressed and partially co-localized with chromogranin A in chicken thymus

Although the functions of hormones and neuropeptides in the thymus have been extensively studied, we still do not know whether these intra-thymic humoral elements are released in a stimulated manner via the regulated secretory pathway or in a constitutive manner. Carboxypeptidase E (CpE) and chromogranin A (CgA) are functional and structural hallmarks of the regulated secretory pathway in (neuro)endocrine cells. Whereas we have previously shown a CgA-positive neuroendocrine population in the chicken thymus, the current study assesses the expression of CpE in the thymus, both at the mRNA and the protein level. Our immunohistochemical studies provide evidence for the co-existence of CgA and CpE in identical neuroendocrine cells in the thymus. CpE and CgA dual-positive cells have primarily been found in the transition zone between the cortex and medulla of the thymus, an area known to contain numerous arterioles and to be innervated by the autonomic nervous system. Our findings suggest that the diffuse neuroendocrine system serves as a relay for nervous stimuli delivered by the sympathetic and/or parasympathetic nervous system. Thus, these newly defined neuroendocrine cells might play an important role in the immuno-neuro-endocrine cross-talk in the thymus, potentially enabling thymopoiesis to be fine-tuned via the regulated secretory pathway by a variety of physical and environmental factors.     

Effect of pentoxyl, ibuprofen and acetylsalicylic acid on the thymus, spleen and adrenals in an experiment]

The effect of acetylsalicylic acid, ibuprofen and pentoxyl on the histological and morphometric pattern of the thymus and the weight of the thymus and spleen was studied in rats. There was decreased function of the thymus and its atrophy with acetylsalicylic acid and ibuprofen. Pentoxyl increased the secretory activity of the thymus. The effect of the drugs on the thymus and spleen was unidirectional.     

Reviewing old concepts at the start of a new millenium: growth restriction,adrenal hypoplasia,and thymomegaly in human anencephaly

BACKGROUND: Anencephaly has been associated frequently with intrauterine growth retardation (IUGR), consistently with adrenal hypoplasia, and occasionally with an enlarged thymus. Few studies have analyzed the relationship between gestational age (GA), IUGR, associated anomalies and thymomegaly in anencephaly. The aims of our study were to evaluate this relationship and to highlight the usefulness of anencephaly as a model when investigating immune-endocrine interactions. METHODS: Fifty-two anencephalics' autopsies were reviewed retrospectively. Body weight, adrenal, and thymus weights were compared to prenatal, postnatal, and stillborn control values, and between associated and isolated anencephalic cases (presenting with and without other unrelated anomalies). Comparisons of adrenal and thymus weights were done by GA and by body weight. Thymus weight:body weight (TW:BW) ratios were compared to expected values. RESULTS: Anencephalics' body and adrenal weights were lower than their control values, whereas thymus weights did not differ. Body and thymus weights were twice as high in isolated than in associated anencephaly, whereas adrenal weights did not differ. Anencephalics TW:BW ratios were higher than their control values, higher in cases with IUGR, and higher in isolated rather than associated cases. When distributed by GA, thymus weights in anencephaly increased at a higher-than-expected rate. CONCLUSIONS: Our results suggest that adrenal hypoplasia is invariably present in anencephaly, and depending on an underdeveloped pituitary gland, seems to be independent of its etiology. On the contrary, IUGR mainly exists in associated cases and thymus enlargement mainly exists in isolated cases, suggesting a relationship with the underlying cause.     

Fine structure of the thymus in the adult cling fish Sicyases sanguineus (Pisces, Gobiesocidae)

The structure of the thumus in adult specimens of a marine teleost, the cling fish Sicyases sanguineus, has been studied by light and transmission electron microscopy. Most cling fishes have an outer thymus located beneath the opercular epithelium. A few of them, however, have a large inner thymus besides a poorly developed outer thymus. In the well-developed outer thymus of cling fish there are three different zones: outer cortex, inner cortex, and medulla. The inner cortex is similar to the cortical region of the thumus in other vertebrates, whereas the outer cortex is a specialized lympho-epithelial zone containing cystic cells (also present in medullary region) and true Hassall's corpuscles. In accordance with the development of the thymic parenchyma, the medullary or basal region may appear either like a true thymic medulla or like a subcapsular region. In the inner thymus, a subcapsular or peripheral "medullary" region and a central area (inverted cortex) show structural features like those of the medullary (basal) and deep cortical regions of the outer thymus, respectively. In addition to the above regions, sometimes there is a lymphomyeloid perithymic infiltration that often extends along connective tissue septa into the perivascular spaces of the gland. Reticuloepithelial, mesenchymal, and unidentified types of stromal cells within the thymus are described. Some erythrocytes, granulocytes, and monocytoid cells are found, but no plasma cells nor erythropoietic foci are evident. The probable significance of these findings is discussed.     

The restoration of the T-lymphoid system of nude mice: lower efficiency of nonlymphoid, epithelial thymus grafts.

The T-cell deficiency of nude mice is due to an abnormal differentiation of the thymus epithelium; it can be persistently corrected by grafting a neonatal thymus. However, grafted adult thymuses or epithelial thymuses are not repopulated by large numbers of host-derived lymphocytes, as is the case when a whole neonatal thymus is grafted. Furthermore, the repopulation of the spleen and lymph nodes by T cells is less pronounced than after whole neonatal thymus transplantation, and the restoration of the reactivity to T-cell mitogens is irregular. Therefore, the integrity and the age of the thymus graft are important for a good restoration of the T-lymphoid system of congenitally athymic animals.     

Effects of raloxifene, a selective estrogen receptor modulator, on thymus, T cell reactivity, and inflammation in mice

Raloxifene is a selective estrogen receptor modulator approved for prevention of osteoporosis in postmenopausal women. It is selective by virtue of having estrogen agonistic effects in bone, vessels, and blood lipids, while it is antagonistic with mammary and uterine tissue. The aim of the study was to examine whether the raloxifene analogue LY117018 (LY) has estrogenic effects on the thymus, T cell responsiveness, and inflammation. Oophorectomized normal mice were treated with subcutaneous injections of equipotent antiosteoporotic doses of LY (3 mg/kg) and 17beta-estradiol (E2) (0.1 mg/kg) or vehicle as controls. Effects on thymus were studied by analyses of thymus weight, cellularity, and CD4 and CD8 phenotype expression and histology, while inflammation was determined as T-cell-mediated delayed-type hypersensitivity (DTH) and granulocyte-mediated footpad swelling. LY lacked the suppressive properties of E2 on DTH and granulocyte-mediated inflammation. Furthermore, LY induced only minor thymus atrophy compared with E2 and did not, in contrast to E2, alter the thymic CD4/CD8 phenotypes. These results clearly demonstrate that raloxifene principally lacks the modulatory effects of estrogen on T cell responsiveness and inflammation. Our data are discussed in the context of recent findings in estrogen receptor biology and also with respect to estrogen-mediated alteration of autoimmune rheumatic diseases.     

Proliferation, apoptosis and thymic involution.

The thymus reaches its maximal size at the age of 1 month in ICR mice and thereafter, the thymic cortex undergoes an exponential decline. This study was designed to compare the proliferation and apoptosis of thymocytes in different parts of the thymus of ICR female mice at the beginning and after the rapid phase of decline of the thymic cortical cellularity. The pattern of proliferation and apoptosis of the thymus was studied in situ in 1-month-old ICR female mice (10 mice) compared to mice at 7 months of age (10 mice). Staining for argyrophylic nucleolar organizer region by histochemistry was used to determine the proportion of type 2 thymocytes, which are considered as cells at S phase of the cell cycle. The mean number of type 2 cells in four random samples of 50 cells in each part of the thymus was defined as the proliferation index of this part of the thymus. In situ detection of apoptosis of thymocytes was carried out using the Apoptag kit, which can detect a single cell apoptosis. The mean number of apoptotic cells in five randomly selected fields of each part of the thymus was defined as the apoptotic index of this part of the thymus. The proliferation index of the peripheral cortex of the 1-month-old mice was 3.6 times higher than the proliferation index of the deep cortex and 5.8 times higher than the proliferation index of the medulla (P < 0.0001). The proliferation index of the peripheral cortex of the 7-month-old mice was reduced by 45% compared to the 1-month-old mice (P < 0.005). The apoptotic index of the corticomedullary junction of the 1-month-old mice was six times higher than the apoptotic index of the cortex and 18 times higher than the apoptotic index of the medulla. The apoptotic index of the thymic cortex was elevated by 66% in the 7-month-old mice compared to the 1-month-old mice (P < 0.0001). We conclude that there is a reduction of the proliferation index and an elevation of the apoptotic index of the thymic cortex in adult mice compared to young mice. These changes might account for the reduction of thymic cortical cellularity during thymic involution.     

cytophotometric,flow cytofluorometric and morphometric demonstration of the existence of an antigen dependent thymus lymphocyte subpopulation

Summary Using absorption cytophotometry and flow cytofluorometrical DNA and protein estimation of single thymus lymphocytes we were able to establish that after injection of a large dose of antigen (ovalbumin) a subpopulation of lymphocytes arises in the thymus with high protein contents above that of those lymphocytes normally present, however, in small quantities in the thymus. By morphometrical analysis it was established that these lymphocytes are situated in the outermost cortex.     

中华鳖胸腺显微和亚显微结构及其在进化上的意义

应用透射电子显微镜观察了中华鳖胸腺的显微和亚显微结构。发现中华鳖胸腺从结构上可分为皮质和髓质,皮质富含淋巴细胞,髓质富含上皮性网状细胞。在各发育期中华鳖胸腺皮质、髓质交界处和髓质区有明显的囊状胸腺小体和交错突细胞。在２００ｇ以上的成年鳖胸腺内发现有同心圆状胸腺小体。电镜下,胸腺内淋巴细胞分为大、中、小３型。上皮性网状细胞从亚显微结构上分为支持型和分泌型。胸腺囊包括细胞内囊和细胞间囊,以细胞内囊居多     

The specificity of the syngeneic mixed leukocyte response, a primary anti-I region T cell proliferative response, is determined intrathymically

In previous studies, the syngeneic MLR of peripheral T cells was shown to be predominantly an I region-restricted function. In this report we show that adult thymocytes are also capable of responding to syngeneic irradiated stimulator cells in a syngeneic MLR, provided that TCGF is added to the culture system. Using this assay, it was possible for the first time to examine the pattern of I region restriction within the thymus itself. Analysis of the thymocyte syngeneic MLR in thymuses from radiation-induced bone marrow chimeras demonstrated that the MHC preference seen in the peripheral T cell population also existed in cells resident within the thymus. Experiments utilizing congenitally athymic mice transplanted with allogeneic thymic grafts demonstrated that both peripheral T cells and thymocytes from such animals displayed a strong preferential proliferation toward stimulator cells bearing thymic-type MHC determinants. The results in the nude model thus demonstrate that the thymus by itself is sufficient to impart such restriction specificity on a developing T cell repertoire. These results are consistent with the notion that the thymus exerts selective pressure on maturing T cell populations that results in a skewing of the T cell repertoire toward the recognition of thymic-type I region products, and that this MHC preference exists before expansion of T cells in the periphery.     

The human thymus: A new perspective on thymic function,aging, and hiv infection

Shortly after birth, the human thymus begins a life long process of involution, whereby the net size of the thymus is not altered but the organ is replaced by adipose tissue. As a result, it has long been believed that thymic involution is indicative of a nonfunctional organ. Recently, however, with the use of computed tomography analysis and innovative molecular approaches that measure T-cell receptor circles, indicative of recent thymic emigrants, doubt has been placed on that dogma. The thymus appears to be active in thymopoiesis throughout the adult life, albeit inversely correlated with age. Being faced with diseases that deplete T-cells such as the acquired immunodeficiency syndrome (AIDS), this recent finding has the potential to exploit novel approaches that enhance thymic output as a mechanism to reconstitute the immune system. In this review, we will revisit the role of T-cells in immunity, the relationship between thymic function and age, and closely examine the impact of HIV-mediated thymic dysregulation on thymopoiesis.     

Demonstration of steroid 17 alpha-hydroxylase activity in human fetal kidney, thymus, and spleen

In previous investigations, steroid 21-hydroxylase activity was demonstrated in human fetal kidney, thymus, and spleen tissues. In the present investigation, steroid 17 alpha-hydroxylase activity also was found in microsome-enriched preparations of human fetal kidney, thymus, and spleen tissues.     

Interactions of an antitumor platinum compound with deoxyribonucleic acid, histones, L-amino acids, poly-L-amino acids, nucleosides and nucleotides

Interaction of cis-dichloro(dipyridine)platinum(II) (cis-PPC) with calf thymus DNA, calf thymus histone, l-amino acids, poly-l-amino acids, nucleosides, and nucleotides has been evaluated by equilibrium dialysis technics. At least a 28 % decrease in the association of cis-PPC with DNA occurs when the platinum compound is pre-incubated with l-amino acids. The greatest decrease in association is seen upon pre-incubation of the platinum compound with the free amino acids. Glut, Asp, Lys, Arg, and CySH, before the addition of a sack containing a solution of DNA. The low level of association between DNA and the amino acids tends to rule out competition between cis-PPC and amino acids for DNA association sites. cis-PPC was repelled from sacks containing positively charged poly-l-Lys, poly-l-Arg, and calf thymus histone; however, in the presence of poly-l-Glut and poly-l-Asp, cis-PPC associated with these negatively charged polymers to a considerable degree. Enhanced chloride dissociation from cis-PPC was observed in the presence of all of the amino acids and the nucleotides GMP, CMP, UMP, and TMP, but not in the presence of AMP or the nucleosides rG and dG. In the presence of calf thymus histone, the association of cis-PPC with calf thymus DNA was reduced by more than 50% at histone/DNA ratios of 0.8–1.0.These data suggest that cis-PPC or cis-Pt(II) may associate with electron-rich areas of not only nucleic acids and proteins but also with body pools of free nucleotides and amino acids. The presence of positively charged histones shielding DNA strands in vivo suggests that the most probable point of platinum-DNA association would be at de-repressed areas of DNA which are undergoing RNA synthesis. The aquated form of the platinum complex may also associate with acidic proteins which appear to be involved in the positive control of RNA synthesis and, as a result, this interaction may be of pharmacological significance.