Cell Death In Thymus Research Articles

Protective Effect of Poria Cocos Polysaccharides on Fecal Peritonitis-Induced Sepsis in Mice Through Inhibition of Oxidative Stress, Inflammation, Apoptosis, and Reduction of Treg Cells.

This study was conducted to investigate the potential pharmacological effects of Poria cocos polysaccharides (PCPs) on fecal-induced peritonitis (FIP) mice. Consequently, the fecal peritonitis (FP)-induced septic mice with the higher levels of tumor necrosis factor-α (TNF-α), interleukin 6 (IL-6), IL-1β, malondialdehyde (MDA), myeloperoxidase (MPO), histopathological lesion and bacterial burden, and lower levels of superoxide dismutase (SOD) and glutathione (GSH). Interestingly, PCP pre-treatment reduced inflammatory cytokines and oxidative stress in plasma and spleen and improved the resistance to FIP. Inflammatory infiltration and cell death in thymus or splenic tissue were alleviated with PCP pretreatment. Furthermore, Treg cells were moderated in the spleen with PCP pre-administration. In addition, PCP pretreatment downregulated Annexin-V in the thymus of FP-induced septic mice, and apoptosis of splenic cells was dose-dependent. In conclusion, PCPs have pharmacological and biological effects on FP-induced septic mice, and its molecular mechanism is related to antioxidative, anti-inflammation, anti-apoptosis, and the reduction of Treg activity in splenic cells.

Self Nano-Emulsifying Curcumin (SNEC30) attenuates arsenic-induced cell death in mice.

Several precedents have confirmed numerous infirmities caused by arsenic poisoning, including immune suppression and cancer. Exposure to arsenic leads to alterations of the cellular machinery and eventually cell death, depending on the dose and duration of exposure. Oxidative stress induced by arsenic is the major mechanism by which it inflicts cellular toxicity, challenging the survival-support - autophagy and culminating in apoptosis in the thymus and spleen of mice. Curcumin, a potent dietary anti-oxidant with known anti-apoptotic and anti-inflammatory properties, was assessed for therapeutic benefits. However, the major caveat of this polyphenol is its low water solubility and limited bioavailability. Therefore, Self Nano-Emulsifying Curcumin (SNEC30) was used to treat mice exposed to arsenic. When administered, SNEC30 effectively ameliorated the adverse effects of arsenic in mice, by restoring structural alterations and reducing ROS-mediated cell death, thereby endorsing the importance of nutraceuticals in counteracting heavy metal-induced cellular toxicity.

Hypothyroidism impairs the host immune response during the acute phase of Chagas disease

Diseases associated with thyroid hypofunction have been the subject of studies in infectious models, since several authors have demonstrated a pivotal role of iodinated hormones (thyroxine and triiodothyronine) in the modulation of immune effector responses. Using a model of hypothyroidism induced by anti-thyroid drug, we investigated the influence of hypothyroidism in the course of acute Trypanosoma cruzi infection. For this, male Hannover Wistar rats were challenged with methimazole for 21 days (0.02% in drinking water), and water for control counterparts. After confirmation of the hypothyroidism, rats were intraperitoneally challenged with 1x105 blood trypomastigotes of the Y strain of T. cruzi. Our findings suggest that hypothyroidism impairs animal weight gain, but does not affect the health of essential organs. Interestingly, infected hypothyroid animals had a significant increase in thymic cell death, with consequent drop in lymphocyte frequency in whole blood (evaluated on the 11th day of infection). Analyzing the percentage of immune cells in the spleen, we found a strong influence of hypothyroidism as a negative regulator of B cells, and antigenic ability of macrophages (RT1b expression) in the course of the experimental chagasic infection. Enhanced serum IL-17A concentration was induced by T. cruzi infection, but hypothyroidism impaired the production of this mediator as seen in infected hypothyroid animals. Taken together, our work suggests for the first time that hypothyroidism may adversely interfere with the modulation of effective immunity in the early phase of Chagas' disease.

Targeting Bortezomib to Bone Increases Its Bone Anabolic Activity and Reduces Systemic Adverse Effects in Mice.

Bortezomib (Btz) is a proteasome inhibitor approved by the FDA to treat multiple myeloma. It also increases bone volume by promoting osteoblast differentiation and inhibiting osteoclastogenesis in mice. However, Btz has severe systemic adverse effects, which would limit its use as a bone anabolic agent. Here, we designed and synthesized a bone-targeted form of Btz by conjugating it to a bisphosphonate (BP) with no antiresorptive activity. We report that BP-Btz inhibited osteoclast formation and bone resorption and stimulated osteoblast differentiation in vitro similar to Btz. In vivo, BP-Btz increased bone volume more effectively than Btz in three mouse models: untreated wild-type mice, mice with ovariectomy, and aged mice with tibial factures. Importantly, BP-Btz had significantly less systemic side effects than Btz, including less thymic cell death, sympathetic nerve damage, and thrombocytopenia, and it improved survival rates in aged mice. Thus, BP-Btz represents a novel anabolic agent to treat conditions, such as postmenopausal and age-related bone loss. Bone targeting is an attractive approach to repurpose approved drugs to treat skeletal diseases. © 2019 American Society for Bone and Mineral Research. © 2019 American Society for Bone and Mineral Research.

Thy28 protects against anti-CD3-mediated thymic cell death in vivo.

Apoptotic cell death plays a pivotal role in the development and/or maintenance of several tissues including thymus. Deregulated thymic cell death is associated with autoimmune diseases including experimental autoimmune encephalomyelitis (EAE), a prototype murine model for analysis of human multiple sclerosis. Because Thy28 expression is modulated during thymocyte development, we tested whether Thy28 affects induction of EAE as effectively as antigen-induced thymocyte deletion using Thy28 transgenic (TG) mice. Thy28 TG mice showed partial resistance to anti-CD3 monoclonal antibody (mAb)-induced thymic cell death in vivo, as assessed by annexin V-expression and loss of mitochondrial membrane potential. The resistance to anti-CD3 mAb-induced cell death in Thy28 TG mice appeared to correlate with a decreased c-Jun N-terminal kinase phosphorylation and reduced down-regulation of Bcl-xL. Moreover, thymic hyperplasia was detected in Thy28 TG mice, although thymocyte development was unaltered. Development of peripheral lymphoid tissues including spleen and lymph nodes was also unaltered. Thy28 TG spleen T cells showed an increased production of IFN-γ, but not IL-17, in response to both anti-CD3 and anti-CD28 mAbs. Finally, Thy28 TG mice displayed accelerated induction of EAE as assessed by disease incidence, clinical score, and pathology following immunization with myelin oligodendrocyte glycoprotein compared with control WT mice. These findings suggest that modulation of Thy28 expression plays a crucial role in the determination of thymic cell fate, which may contribute to the development of EAE through proinflammatory cytokine production.

Alterations in stromal cell compartment during thymus atrophy induced by P. brasiliensis infection.

Event Abstract Back to Event Alterations in stromal cell compartment during thymus atrophy induced by P. brasiliensis infection. Thiago Alves Da Costa1, Rosaria Di Gangi1, Carolina Francelin1, Rodolfo Thomé1, Ronei L. Mamoni2 and Liana Verinaud1* 1 State University of Campinas, Department of Structural and Functional Biology; Institute of Biology, Brazil 2 State University of Campinas, Department of Clinical Patology; Medical Sciences Faculty, Brazil Thymus is the development site of T lymphocytes, which are important cells of the specific immune response. To perform its function, the thymic microenvironment must be preserved (Ciofani & Zuniga-Pflucker, 2007). Despite its importance, the thymus is a target organ for many infectious diseases, including paracoccidioidomycosis, the most prevalent systemic mycosis in Latin America, caused by the fungus Paracoccidioides brasiliensis (Pb) (Brito et al., 2003). We have already demonstrated in experimental models that Pb invades the thymic microenvironment, inducing severe atrophy characterized by organ weight decrease and loss of corticomedullary delimitation (Brito et al., 2003, Souto et al. 2003). We have also reported that such atrophy is related to the increased death of immature lymphocytes and to the premature migration of these cells to the periphery. However, the possibility that thymic atrophy during the paracoccidioidomycosis infection is also associated to alterations in non-lymphoid cells, which include dendritic cells (DCs), thymic cortical (cTECs) and medullary epithelial cells (mTECs) cannot be discarded. In this study, we examined the stromal cell compartment of thymuses from BALB/c mice infected with the Pb virulent isolate (Pb18) during the acute phase of the disease. By immunofluorescence technique, our results show an altered pattern of distribution of DCs as well as mTECs (Keratin-5+ cells) and cTECs (Keratin-8+) in thymuses from Pb-infected animals. Also, by flow cytometry technique, we observed phenotypic alterations in DCs and a reduction in the number of mTECs and cTECs. Altogether, these findings show that the stromal compartment is also deeply affected during thymic atrophy caused by Pb infection, complementing previous studies of our laboratory. Acknowledgements Supported by FAPESP(#2012/22131-7 & #2013/08194-9) and CAPES. References Brito, V. N., Souto, P. C. S., Cruz-Höfling, M. A., Ricci, L. C., Verinaud, L. (2003). Thymus invasion and atrophy induced by Paracoccidioides brasiliensis in BALB/c mice. Medical Mycology 41, 83-87. Ciofani, M., Zuniga-Pflucker, J. C. (2007). The thymus as an inductive site for T lymphopoiesis. Annu Rev Cell Dev Biol 23, 463-93. Souto, P. C. S., Brito, V. N., Gameiro, J., Cruz Höfling, M. A., Verinaud, L. (2003). Programmed cell death in thymus during experimental paracoccidioidomycosis. Med Microbiology and Immunology 192, 225-229. Keywords: Thymus Gland, thymus atrophy, stromal cell compartment, Paracoccidioides brasiliensis, Paracoccidioidomycosis Conference: 15th International Congress of Immunology (ICI), Milan, Italy, 22 Aug - 27 Aug, 2013. Presentation Type: Abstract Topic: Host-pathogen interactions Citation: Alves Da Costa T, Di Gangi R, Francelin C, Thomé R, Mamoni RL and Verinaud L (2013). Alterations in stromal cell compartment during thymus atrophy induced by P. brasiliensis infection.. Front. Immunol. Conference Abstract: 15th International Congress of Immunology (ICI). doi: 10.3389/conf.fimmu.2013.02.01002 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 29 Jun 2013; Published Online: 22 Aug 2013. * Correspondence: Prof. Liana Verinaud, State University of Campinas, Department of Structural and Functional Biology; Institute of Biology, Campinas, SP, Brazil, verinaud@unicamp.br Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Thiago Alves Da Costa Rosaria Di Gangi Carolina Francelin Rodolfo Thomé Ronei L Mamoni Liana Verinaud Google Thiago Alves Da Costa Rosaria Di Gangi Carolina Francelin Rodolfo Thomé Ronei L Mamoni Liana Verinaud Google Scholar Thiago Alves Da Costa Rosaria Di Gangi Carolina Francelin Rodolfo Thomé Ronei L Mamoni Liana Verinaud PubMed Thiago Alves Da Costa Rosaria Di Gangi Carolina Francelin Rodolfo Thomé Ronei L Mamoni Liana Verinaud Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Protection from brain damage and bacterial infection in murine stroke by the novel caspase-inhibitor Q-VD-OPH

Infarction size and infections are important determinants of stroke outcome in humans. Bacterial infections are promoted by stroke-induced immunodeficiency which in experimental stroke is mainly characterized by extensive lymphocyte apoptosis and dysfunction. Pharmacological inhibition of caspases may improve stroke outcome not only by reducing apoptotic brain damage but also by attenuating stroke-induced immunodeficiency. We investigated the effects of systemic administration of the novel, non-toxic caspase-inhibitor quinolyl-valyl- O-methylaspartyl-[-2,6-difluorophenoxy]-methyl ketone (Q-VD-OPH) on stroke-induced neuronal and lymphocyte apoptosis, susceptibility to infections, and mortality in a murine model of stroke induced by middle cerebral artery occlusion (MCAO). Q-VD-OPH reduced ischemic brain damage and stroke-induced programmed cell death in thymus and spleen, decreased susceptibility to post-stroke bacteremia, and improved survival. Therefore, Q-VD-OPH may be a promising therapeutic agent in stroke.

DRB1*0402 protects while DRB1*0401 is permissive for inducing collagen-induced arthritis in transgenic mice (130.45)

Abstract DRB1*0401 has been associated with predisposition to rheumatoid arthritis while DRB1*0402 is associated with resistance. We have generated transgenic mice lacking endogenous H2-A and E molecules (AEo) but expressing DRB1*0401 or DRB1*0402 molecules. All transgenic mice expressed functional DR molecules which could positively and negatively select various Vβ T cell repertoire profile. Transgenic mice were studied in vivo for induction of arthritis following immunization with type II collagen. DRB1*0401 mice are susceptible to collagen-induced arthritis while DRB1*0402 mice are resistant. Presence of DR4 leads to arthritis predominantly in female mice. CIA susceptible AEo.DQ8 mice have been shown to develop arthritis without a gender bias. However, AEo mice expressing DRB1*0401.DQ8 develop arthritis in female mice with a ratio of 3F:1M, similar to that in humans. DRB1*0402 mice did not show any defect in humoral response though they demonstrated a lower cellular response to CII and an increased activation-induced cell death compared to susceptible mice. In vitro T cell response with superantigen staphylococcal enterotoxin B and Concovalin A showed that DRB1*0402 mice are not defective in generating T cell response. The resistant mice showed an increased cell death in thymus suggesting deletion of autoreactive T cells at thymic level might be one reason for protection. The data suggest that DRB1 polymorphism lead to selection of TCR in thymus thus controlling the selection of autoreactive T cells available in periphery. Thus these transgenic mice may be useful to study the reasons for gender-bias observed in human patients develop therapeutic strategies.

Programmed cell death in thymus during experimental paracoccidioidomycosis.

Many works have shown that immunosuppressive effects induced by systemic mycosis can be related to primary lymphoid organ damage. Previous studies in our laboratory showed that Paracoccidioides brasiliensis was able to invade the thymus, inducing a severe atrophy with significant reduction of cortical area along with a loss of cortico-medullary boundary. The objective of the present study was to investigate whether thymic atrophy is caused by programmed cell death (PCD) and to examine the ultrastructural characteristics of the thymus in experimentally infected BALB/c mice. The results revealed an eightfold increase in the apoptotic index occurring by day 5 post infection, i.e., during early stages of the infection, shown by immunohistochemistry. In addition, typical cell alterations of autophagic PCD were observed by transmission electron microscopy. Taken together, these results reinforce the idea that thymic alterations may be involved in the immunosuppressive phenomenon frequently associated with paracoccidioidomycotic infection.

Glucocorticoid-induced apoptosis in lymphoid organs is associated with a delayed increase in circulating deoxyribonucleic acid.

Pathological processes like cancer, chronic inflammation and autoimmune phenomena, all of which involve massive cell death, are associated with significant increases in circulating DNA. In order to clarify whether massive apoptosis occurring under physiological circumstances also causes DNA release into the circulation, we correlated the time-course of dexamethasone-induced intra thymic cell apoptosis with plasma DNA dynamics in rats. Animals were given 10 mg/l dexamethasone in their drinking water for up to 7 days. Sequential plasma samples were obtained during the treatment and DNA was quantitated by a micro fluorometric assay. Thymus and spleen weight as well as apoptotic cell levels were assessed at different times. Seven days of glucocorticoid treatment reduced thymic and spleen mass by 82 and 31%, respectively. Intra thymic apoptosis was maximal 24 h after the beginning of glucocorticoid treatment, declining markedly by 48 h. Very little apoptosis was observed in the spleen. Plasma DNA increased steadily during the first 4 days of glucocorticoid treatment (11.8 +/- 1.2 microg/ml on day 0; 24.2 +/- 1.6 microg/ml on day 4) beginning to decline afterward. Thymectomy but not splenectomy, drastically reduced the glucocorticoid-induced increase in plasma DNA. It is concluded that hormone-induced massive intra thymic cell death is followed by a delayed release of nucleosomal DNA into the circulation.

Modulation of Cell Death in the Rat Thymus: Light and Electron Microscopic Investigations

In mammals, the thymus is the primary central organ of the lymphoid system; after birth, it progressively diminishes in size, undergoing gradual atrophy. Physiological maturation and/or involution of the thymus may be accelerated by endogenous or exogenous factors. Exposure to extremely low frequency EMF seems to interfere with thymic cell death. Data suggest that, in the rat model, a prolonged exposure to 50 Hz electric and magnetic fields, independently from field strength, seems to affect thymic cell death and possibly thymic physiology, since alterations in the balance of cell death and other parameters such as mitoses might interfere with the positive and negative selection of thymocytes and with the immunosurveillance properties of the thymus.

Early Maturation of T-Cell Progenitors in the Absence of Glucocorticoids

In the present work, we demonstrated that both fetal liver and thymic T-cell precursors express glucocorticoid receptors (GRs) indirectly suggesting a role for glucocorticoids (GCs) in the earliest events of T-cell differentiation. To evaluate this issue, we analyzed the thymic ontogeny in the progeny of adrenalectomized pregnant rats (Adx fetuses), an in vivo experimental model, which ensures the absence of circulating GCs until the establishment of the fetal hypothalamus-pituitary-adrenal (HPA) axis. In the absence of maternal GCs, T-cell development was significantly accelerated, the process being reversed by in vivo GC replacement. Mature single positive thymocytes (both CD4 and CD8) appeared in 16-day old fetal Adx thymus when in the control fetuses, most thymocytes still remained in the double-negative (DN) CD4(-)CD8(-) cell compartment. In addition, emigration of T-cell receptor (TcR)alphabeta positive cells to the spleen also occurred earlier in Adx fetuses than in control ones. In vitro recolonization of cultured deoxiguanosine-treated mouse fetal thymus lobes with 13-day-old fetal liver cell suspensions from both Adx and control fetuses demonstrated changes in the developmental capabilities of fetal liver T-cell precursors from embryos grown in the absence of GCs. Furthermore, a precocious lymphoid colonization of the thymic primordium from Adx fetuses was evidenced by ultrastructural analysis of both Adx and Sham early thymus. Both findings accounted for the accelerated T-cell differentiation observed in Adx fetuses. Together, these results support a role for GCs not only in the thymic cell death, but also in the early steps of T-cell differentiation.

Relatively low-dose cyclophosphamide is likely to induce apoptotic cell death in rat thymus through Fas/Fas ligand pathway

Cyclophosphamide (CPA) is widely used as an efficiently antineoplastic drug, but also causes immunosuppression as its adverse-side effect. To understand the effect of low- or relative low-dose CPA on the immune system, apoptotic cell death in rat thymus, either exposed to different doses of CPA (0, 2, 7, 20 and 70 mg/kg) for 12 h or exposed to 70 mg/kg for different times (4-48 h), was investigated by DNA fragmentation (DNA ladder) detection and in situ morphological examination using hematoxylin and eosin (H and E) staining. Immunohistochemical staining for Fas protein expression in the thymus of rats exposed to CPA was performed. Results showed that exposure of rats to CPA 0–70 mg/kg for 12 h did not cause significant decrease in the ratio of thymus weight to body weight. However, the ratio of thymus weight to body weight was decreased significantly at 48 h after exposure to 70 mg/kg CPA. Exposure to 20 and 70 mg/kg CPA for 12 h caused a visible DNA ladder in gel electrophoresis. DNA ladder formation was increased progressively in the groups from 8 h to optimal magnitude at 12–24 h and then disappeared at 48 h after 70 mg/kg CPA. This pattern was confirmed by a quantitative evaluation of the apoptotic cells using H and E staining. Expression of Fas protein was enhanced in the thymus of rats exposed to 70 mg/kg CPA for 4–8 h as compared to control rats. These results are different from previous studies on high dose CPA and the induction of the apoptotic cell death in thymus by low or relative low doses of CPA might be a result of Fas/Fas–ligand interactions.

Apoptotic cellular damage in mice after T-2 toxin-induced acute toxicosis.

By histopathologic, electron microscopic, and immunochemical observation, the mechanism of cellular death was investigated in thymus, spleen, and liver of mice given intraperitoneally sublethal doses of T-2 toxin, a trichothecene mycotoxin. In the thymus and spleen of mice given 5.0 mg/kg body weight of T-2 toxin and killed 12 hours later, a massive cellular destruction characterized by chromatin condensation was evident, and electron microscopy analysis revealed the presence of apoptotic bodies. In the liver of mice given 2.5 mg/kg of T-2 toxin and killed 2 hours later, the induction of apoptotic cellular lesions was observed by electron microscopy, and Kupffer cells phagocytosed the apoptotic bodies. Such lesions were not observed in the mice killed 12 hours after receiving the toxin. In situ nick translation analysis (Tunel method) revealed DNA fragmentation in thymus, spleen, and liver shortly after administration of T-2 toxin. As previously observed in vitro, these findings indicated that T-2 toxin is a potent inducer of apoptotic cell death in thymus, spleen, and liver in vivo; especially in liver, apoptosis is induced rapidly as compared with the other tissues observed, and Kupffer cells play an important role for clearance of apoptosis.

Melatonin decreases mRNA for histone h4 in thymus of young rats

The antiproliferative properties of melatonin have been previously demonstrated for several normal and tumoral tissues. In a recent report we have shown that melatonin is able to inhibit programmed cell death in thymus both, in vivo and in vitro. Given that other authors have related programmed cell death and cell proliferation and that no previous reports on melatonin and cell division exist on thymus, we decide to study the possible antiproliferative effect of melatonin in this organ measured as the levels of mRNA for the histone H4. We found that melatonin inhibits cell division on thymus when administered chronically both, at high (500 μg/body weight) and low (50 μg/body weight) dose. We also found a circadian rhythm of the mRNA for histone H4, opposed to the one previously described for melatonin, supporting the negative regulation by this hormone of cell division on thymus. A single dose of melatonin (50 μg/body weight) was not able to decrease the levels of mRNA for H4 in the time-points studied but after two hours of its administration. Finally, we report the inhibitory effect of melatonin in the cell proliferation of Harderian gland, brain, lung and kidney.

Apoptotic cellular damage in mice after T‐2 toxin‐induced acute toxicosis

AbstractBy histopathologic, electron microscopic, and immunochemical observation, the mechanism of cellular death was investigated in thymus, spleen, and liver of mice given intraperitoneally sublethal doses of T‐2 toxin, a trichothecene mycotoxin. In the thymus and spleen of mice given 5.0 mg/kg body weight of T‐2 toxin and killed 12 hours later, a massive cellular destruction characterized by chromatin condensation was evident, and electron microscopy analysis revealed the presence of apoptotic bodies. In the liver of mice given 2.5 mg/kg of T‐2 toxin and killed 2 hours later, the induction of apoptotic cellular lesions was observed by electron microscopy, and Kupffer cells phagocytosed the apoptotic bodies. Such lesions were not observed in the mice killed 12 hours after receiving the toxin. In situ nick translation analysis (Tunel method) revealed DNA fragmentation in thymus, spleen, and liver shortly after administration of T‐2 toxin. As previously observed in vitro, these findings indicated that T‐2 toxin is a potent inducer of apoptotic cell death in thymus, spleen, and liver in vivo; especially in liver, apoptosis is induced rapidly as compared with the other tissues observed, and Kupffer cells play an important role for clearance of apoptosis. Nat. Toxins 5:141–145, 1997. © 1997 Wiley‐Liss, Inc.

An improved method for combined autoradiography and histochemistry: the simultaneous detection of 6-3H thymidine and acid phosphatase activity in cryostat sections of mouse thymus and duodenum.

A new method is described that enables the simultaneous detection of 6-3H thymidine incorporation and acid phosphatase activity in the same tissue section. Histochemically, naphthol AS B1 released by tissue based acid phosphatase activity from the substrate naphthyl AS B1 phosphoric acid is coupled with a range of diazonium salts to produce insoluble azo dyes. The azo dye tests result in a particulate localization of lysosomal acid phosphatase and also label diffuse sources associated with cell death. The tests selected permit the application of photographic emulsion without the necessity of an inert barrier layer to separate the emulsion from the histochemically treated cryosections. The localization of 6-3H thymidine incorporation and cell death in mouse thymus and duodenum is demonstrated and comparative counts estimating the distribution of 6-3H thymidine incorporation and hydrolase labelled cell death in the thymus are presented. Young mouse thymus (5 weeks) was found to contain 1.36 +/- 0.12% dying cells and 6.78 +/- 0.03% thymidine incorporating cells, whilst old mouse thymus (53 weeks) was found to contain 2.34 +/- 0.6% dying cells and 5.29 +/- 0.37% thymidine incorporating cells.

FATE OF THYMOCYTES: STUDIES WITH 125I‐IODODEOXYURIDINE AND 3H‐THYMIDINE IN MICE

Cortical thymocytes of young adult mice were labeled in situ with radioactive DNA precursors. As a result of cell emigration and cell death, total thymic radioactivity decreased within 8 days to 10% or less of that present on day 1. Accumulation of thymic migrants in peripheral lymphoid organs was estimated by computing the net thymus-derived radioactivity in these tissues. Thymic cell death was assessed by comparing values obtained with 125I-UdR to those acquired with 3H-TdR; The results indicate that cortical thymocytes migrate to the spleen, mesenteric lymph node, femurs and intestine; nevertheless, only a small fraction of the activity originally present in the thymus was recovered in these organs; the vast majority of newly formed cortical thymocytes apparently die after a relatively short life span. Exclusive of the fraction which dies in situ, evidence for thymocyte death is seen in bone marrow; however, most migrants appear to terminate in the intestine.