Spatiotemporal distribution and pollution assessment of trace metals in the Buriganga River, Bangladesh.

The Buriganga River plays a key role in the socioeconomic structure of Dhaka, the capital of Bangladesh. However, this river is severely polluted and is considered one of the most polluted in the world. Therefore, this study aimed to assess the concentrations of various metals in the Buriganga River. A study was conducted from August 2019 to February 2020 to determine the concentrations of 16 metals in water samples (n = 210) collected from 10 distinct sites in the Buriganga River. The mean values for the concentrations of Cr, Mn, Ni, Zn, As, Se, Cd, Sb, and Pb in river water were above the guideline values prescribed by the WHO, Japan, and Bangladesh. Moreover, the fraction ratios of Be, Cr, Co, Ni, Cu, Zn, Se, Mo, Ag, Cd, Sb, and Pb were high (>0.85); consequently, these metals could accumulate at high concentrations in river sediments. Assessment using the single-factor pollution index allowed the classification of the pollution level as 'serious pollution' for Sb and 'heavy pollution' for Cd, Ni, and Pb. The trace metal concentrations in this river imply that crops cultivated along the river using river water may also be contaminated with trace metals.

Occurrence, potential source, and cancer risk of PM2.5-bound polycyclic aromatic hydrocarbons and their halogenated derivatives in Shizuoka, Japan, and Dhaka, Bangladesh.

Because of their unintentional formation and low vapor pressure, polycyclic aromatic hydrocarbons (PAHs) and their halogenated derivatives (XPAHs) in the atmosphere are distributed primarily to aerosolized particles with an aerodynamic diameter less than 2.5 µm (PM2.5). However, no information is available regarding the occurrence of PM2.5-bound PAHs and XPAHs in Bangladesh, one of the most highly PM2.5-polluted regions worldwide. In this study, we investigated the occurrence of PM2.5-bound PAHs and XPAHs in the atmospheres of Dhaka in Bangladesh and Shizuoka in Japan (as a reference) and estimated their incremental lifetime cancer risks (ILCRs). In addition, we statistically estimated the potential sources of PM2.5-bound PAHs and XPAHs by using principal component analysis and positive matrix factorization. The median concentration of total PM2.5-bound PAHs and XPAHs in Bangladesh was 24.2 times that in Japan. The estimated potential sources of PAHs clearly differed between Japan and Bangladesh, whereas those of XPAHs were largely (>80%) unknown in both countries. The median ILCR in Bangladesh was 2.81 × 10-3, which greatly exceeded the upper limit of acceptable risk (10-4). These results indicate that comprehensive monitoring and control of atmospheric PM2.5-bound PAHs and XPAHs are needed urgently, especially in highly polluted countries.

Risk assessment of polycyclic aromatic hydrocarbons and their chlorinated derivatives produced during cooking and released in exhaust gas.

Cooking exhaust gas includes polycyclic aromatic hydrocarbons (PAHs) that are unintentionally generated during cooking, which exposes the cook and others in the vicinity to these toxic compounds. However, information on the occurrence of PAHs, particularly their chlorinated derivatives (ClPAHs), in cooking exhaust gas is limited. Here, we determined the concentrations of 12 PAHs and 20 ClPAHs in cooking exhaust gas emitted during gas-grilling of a Pacific saury using a typical Japanese fish grill in an indoor kitchen. The total concentrations of PAHs and ClPAHs in the cooking exhaust gas were 3400 and 19 ng m-3, respectively. All 12 PAHs were detected in the cooking exhaust gas, with phenanthrene (2100 ng m-3), fluorene (630 ng m-3), and anthracene (200 ng m-3) detected at the highest concentrations. Four of the 20 ClPAHs were detected, with 9-monochlorinated phenanthrene detected at the highest concentration (12 ng m-3). The exposure rates for the cook to the PAHs and ClPAHs in the cooking exhaust gas, estimated using the National Institute of Advanced Industrial Science and Technology - Indoor Consumer Exposure Assessment Tool (AIST-ICET), were in the range of 7.2-72 ng-BaPeq kg-1 day-1 (toxic equivalent concentrations relative to the toxicity of benzo[a]pyrene), which was comparable with that for dietary ingestion of cooked foods (54 ng-BaPeq kg-1 day-1). A risk assessment of exposure to PAHs and ClPAHs in cooking exhaust gas in the indoor environment revealed that this gas may pose a health risk to the cook (incremental lifetime cancer risk: 2.1 × 10-6 to 2.1 × 10-5), indicating that further investigations are warranted.

Mitigation of the inhibitory effects of co-existing substances on the Fenton process by UV light irradiation.

Co-existing substances (substances not targeted for degradation) can negatively affect wastewater treatment process performance. Here, we quantitatively evaluated the effects of propanal, a common co-existing substance, on the degradation of the azo-dye Orange II, a common pollutant, by the Fenton process to provide data for the development of measures to reduce the effects of co-existing substances on this wastewater treatment process. Inhibition rate (IR; ratio of the reaction rate constants obtained in the absence and presence of propanal) was calculated to examine the effects of propanal on the degradation of Orange II. The IRs for the Fenton process in the first phase and the second phase were 1.6 and 4.2, respectively. However, addition of ultraviolet irradiation to the Fenton process (i.e., the photo-Fenton process) resulted in a comparable IR for the first phase but a markedly lower IR for the second phase. We attributed this to the improvement of the photo-reduction reaction rate due to complexation of propanal with ferric ions, which compensated for the scavenger effects (the trapping of OH radicals) of propanal. Thus, ultraviolet irradiation reduced the inhibitory effects of propanal on the degradation of Orange II by the Fenton process.

Simultaneous determination of polycyclic aromatic hydrocarbons and their chlorinated derivatives in grilled foods.

Polycyclic aromatic hydrocarbons (PAHs) are unintentionally generated in foods that are cooked, and dietary ingestion of these PAHs is regarded as the dominant route of exposure to PAHs. Some chlorinated PAHs (ClPAHs) are more toxic than their corresponding parent PAHs and can also be generated in food during cooking. Knowledge of the concentrations of ClPAHs in cooked foods has been limited by the lack of an adequate analytical method for measuring ClPAH concentrations in cooked foods. In this study, we developed an analytical method for simultaneous determination of PAHs and ClPAHs in lipid-rich foods. The combination of a potassium hydroxide silica gel column and an activated carbon cartridge enabled us to simultaneously measure PAH and ClPAH concentrations in raw and grilled fish and meats. Twelve kinds of PAHs (e.g., fluorene [Fle], phenanthrene [Phe], fluoranthene [Flu], and pyrene [Pyr]) were detected in grilled foods. The concentrations ranged from below the limit of quantitation (

Comparison of rates of direct and indirect migration of phosphorus flame retardants from flame-retardant-treated polyester curtains to indoor dust.

In this study, the pathways for migration of phosphorus flame retardants (PFRs), tris(1,3-dichloroisopropyl) phosphate (TDCPP) and tricresyl phosphate (TCsP) which were detected from curtains often, from flame-retardant-treated polyester curtains to indoor dust were investigated. Two possible migration pathways were compared quantitatively: (1) an indirect pathway in which the PFRs in the curtains first evaporate from the curtains and are then adsorbed onto indoor dust and (2) a direct pathway in which the PFRs are directly transferred to dust placed on the curtains. The contribution of the indirect pathway was evaluated by means of emission cell tests, which showed that the area-specific emission rates from curtains treated with PFRs were 0.044 (TDCPP, Curtain 5), 0.17 (TDCPP, Curtain 8), and 0.060 (TCsP, Curtain 12) µg m-2 h-1 at 20 °C (averaged during 24 h). The contribution of the direct pathway was evaluated by measurement of the time dependence of PFR concentrations on the indoor dust placed on the curtains. These measurements indicated that PFR concentrations on the dust increased with time and that the direct migration rates of PFRs from curtains treated with PFRs were 4.4 (TDCPP, Curtain 5), 12 (TDCPP, Curtain 8), and 7.0 (TCsP, Curtain 12) µg m-2 h-1 at 20 °C (averaged during 24 h), or 71-120 times the indirect migration rate. This result suggests that the direct pathway can be expected to predominate.

Methods for the analysis of organophosphorus flame retardants-Comparison of GC-EI-MS, GC-NCI-MS, LC-ESI-MS/MS, and LC-APCI-MS/MS.

Organophosphorus flame retardants (PFRs) are extensively used as alternatives to banned polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD). In this study, we analyzed 14 PFRs by means of four mass-spectrometry-based methods: gas chromatography combined with electron-impact mass spectrometry (GC-EI-MS) or negative-chemical-ionization mass spectrometry (GC-NCI-MS) and liquid chromatography combined with tandem mass spectrometry using electrospray ionization (LC-ESI-MS/MS) or atmospheric pressure chemical ionization (LC-APCI-MS/MS). The limits of quantification (LOQs) for LC-ESI-MS/MS and LC-APCI-MS/MS (0.81-970 pg) were 1-2 orders of magnitude lower than the LOQs for GC-EI-MS and GC-NCI-MS (2.3-3900 pg). LC-APCI-MS/MS showed the lowest LOQs (mean = 41 pg; median = 3.4 pg) for all but two of the PFRs targeted in this study. For LC-APCI-MS/MS, the lowest LOQ was observed for tributyl phosphate (TBP) (0.81 pg), and the highest was observed for tris(butoxyethyl) phosphate (TBOEP) (36 pg). The results of this study indicate that LC-APCI-MS/MS is the optimum analytical method for the target PFRs, at least in terms of LOQ.

Mechanism of Formation of Chlorinated Pyrene during Combustion of Polyvinyl Chloride.

Chlorinated polycyclic aromatic hydrocarbons (ClPAHs) are an emerging class of environmental contaminants, but the sources of these chemicals in the environment are not well-known. In this study, we developed a kinetic model describing the chlorination of PAHs to elucidate the mechanism of formation of ClPAHs during the combustion of organic waste containing chlorinated compounds and/or chlorine in an incinerator. Pyrene (Pyr) and polyvinyl chloride (PVC) were selected as a model PAH and a model organic substrate, respectively. All combustion experiments were carried out using a model furnace operated under similar experimental conditions. Combustion of PVC in the model furnace produced 1-ClPyr, 1,3-Cl2Pyr, 1,6-Cl2Pyr, 1,8-Cl2Pyr, 1,3,6-Cl3Pyr, and 1,3,6,8-Cl4Pyr. The developed model supported the experimental data on the sequential chlorination of pyrene. The rate constants for the formation of mono- to trichlorinated pyrenes were over 30 times of those for the formation of tetra- and penta-chlorinated pyrenes. A qualitative analysis of the formation of highly chlorinated pyrenes based on the comparison of theoretical and empirical isotopic patterns of the mass spectrum revealed that penta- and hexa-chlorinated pyrenes, whose analytical standards were not available, were also produced by the combustion of PVC.

Rate of hexabromocyclododecane decomposition and production of brominated polycyclic aromatic hydrocarbons during combustion in a pilot-scale incinerator.

Here, we examined the incineration of extruded polystyrene containing hexabromocyclododecane (HBCD) in a pilot-scale incinerator under various combustion temperatures (800-950°C) and flue gas residence times (2-8sec). Rates of HBCD decomposition ranged from 99.996% (800°C, 2sec) to 99.9999% (950°C, 8sec); the decomposition of HBCD, except during the initial stage of combustion (flue gas residence time<2sec), followed a pseudo-first-order kinetics model. An Arrhenius plot revealed that the activation energy and frequency factor of the decomposition of HBCD by combustion were 14.2kJ/mol and 1.69sec-1, respectively. During combustion, 11 brominated polycyclic aromatic hydrocarbons (BrPAHs) were detected as unintentional by-products. Of the 11 BrPAHs detected, 2-bromoanthracene and 1-bromopyrene were detected at the highest concentrations. The mutagenic and carcinogenic BrPAHs 1,5-dibromoanthracene and 1-bromopyrene were most frequently detected in the flue gases analyzed. The total concentration of BrPAHs exponentially increased (range, 87.8-2,040,000ng/m3) with increasing flue gas residence time. Results from a qualitative analysis using gas chromatography/high-resolution mass spectrometry suggest that bromofluorene and bromopyrene (or fluoranthene) congeners were also produced during the combustion.

Development of a Dehumidification System for a Passive Sampler for Determining 1,3-Butadiene.

The passive sampler is a simple device for collecting gaseous compounds. It is affected by environmental conditions such as relative humidity, but a dehumidification system (DHS) has not been developed for it. We have developed a dehumidification device for the passive sampler for determining 1,3-butadiene. It consists of a passive sampler surrounded by a DHS consisting of a basket filled with granular magnesium perchlorate as the dehumidification agent. The passive sampler consists of a porous polytetrafluoroethylene tube packed with Carboxen 1000 adsorbent. We compared the amounts of water adsorbed on the Carboxen 1000 adsorbent in the presence and in the absence of the DHS; the results showed that the DHS prevented water from entering the sampler. Volatile organic compounds that entered the sampler were collected by the Carboxen 1000 adsorbent. In the case of 1,3-butadiene, much higher amounts were collected by the passive sampler with the DHS than without it. This may be due to interference of adsorption or acceleration of 1,3-butadiene degradation by the water-soluble oxidant on the surface of the Carboxen 1000 adsorbent.

Comparison study on observed and estimated concentrations of perfluorooctane sulfonate using a fate model in Tokyo Bay of Japan.

Perfluorooctane sulfonate (PFOS) was quantified in seawater and sediment samples collected from Tokyo Bay, Japan, ranging from 2.0 to 7.3 ng/L for surface seawater, 2.2 to 5.7 ng/L for intermediate seawater, 1.5 to 5.7 ng/L for bottom seawater, and 0.3 to 0.9 ng/g dry wt. for sediment. In addition, a 3-D chemical fate prediction model (National Institute of Advanced Industrial Science and Technology - Risk Assessment Model: AIST-RAM) has been applied to determine the accuracy of estimating PFOS in Tokyo Bay. A decreasing trend of PFOS from the northwestern part (i.e., the closed-off section of Tokyo Bay) to the mouth of the bay was observed, and the trend obtained from the actual surveys in this study was accurately represented by simulation with appropriate input parameters for AIST-RAM. From the comparison of observed concentrations with estimated concentrations, AIST-RAM showed a high degree of accuracy in estimating the concentrations of PFOS in seawater, whereas the estimations of PFOS concentrations in sediment had a relatively larger margin of error than those in seawater. However, our results indicate that AIST-RAM allows estimation of PFOS concentrations in seawater and sediment in Tokyo Bay with satisfactory accuracy.

Development of a safety analysis method for volatile organic compounds using 2-phenoxyethanol as solvent.

Indoor volatile organic compounds (VOCs) are usually sampled using active carbon samplers and subsequently analyzed using gas chromatography-mass spectrometry (GC-MS) to assess the exposure risk to workers. Therefore, selection of a suitable solvent for VOC extraction is crucial. However, reports on the use of 2-phenoxyethanol-known for its low vapor pressure and low toxicity-as a solvent for extracting VOCs from activated carbon are lacking. Here, we show that 2-phenoxyethanol is a suitable alternative solvent with low toxicity and can extract a wide variety of VOCs without overlapping with target VOCs on the gas chromatogram. The recoveries of 2-phenoxyethanol were 57% (styrene) to 83% (methyl n-butyl ketone), which were higher than those of CS2, acetone, and n-hexane at room temperature. The recoveries improved to 67% (styrene) to 102% (isopentyl acetate) under 50 °C. Optimization of the GC conditions showed that a viscosity delay time of 3 s was required to avoid producing bubbles in the injection syringes. We selected DB-HeavyWAX as a column because it could be heated above the boiling point of 2-phenoxyethanol (247 °C), allowing the removal of 2-phenoxyethanol from the column. This study contributes to the development of analysis methods for VOCs under safe operating conditions.

Determination of potential dermal exposure rates of phosphorus flame retardants via the direct contact with a car seat using artificial skin.

Dermal exposure to phosphorus flame retardants (PFRs) has received much attention as a major alternative exposure route in recent years. However, the information regarding dermal exposure via direct contact with a product is limited. In addition, in the commonly used dermal permeability test, the target substance is dissolved in a solvent, which is unrealistic. In this study, a dermal permeability test of PFRs in three car seats was performed using artificial skin. The PFR concentrations in the car seats are 0.12 wt% tris(2-chloroethyl) phosphate (TCEP), 0.030-0.25 wt% tris(2-chloroisopropyl) phosphate (TCPP), 0.15 wt% triphenyl phosphate (TPhP), 0.89 wt% cresyl diphenyl phosphate (CsDPhP), 0.074 wt% tricresyl phosphate (TCsP), and 0.46-4.7 wt% diethylene glycol bis [di (2-chloroisopropyl) phosphate (DEG-BDCIPP). The mean skin permeation rates for a contact time of 24 h are 14 (TCEP), 5.4-160 (TCPP), 0.67 (CsDPhP), 0.38 (TPhP), and 3.3-58 ng cm-2 h-1 (DEG-BDCIPP). The concentrations of TCsP in receptor liquid were lower than the limit of quantification at the contact time of 24 h. The skin permeation rates were significantly affected by the type of car seat (e.g., fabric or non-fabric). The potential dermal TCPP exposure rate for an adult via direct contact with the car seat during the average daily contact time (1.3 h), which was the highest value assessed in this study, was estimated to be 16,000 ng kg-1 day-1, which is higher than that related to inhalation and dust ingestion reported as significant exposure route of PFRs in previous studies. These facts reveal that dermal exposure associated with direct contact with the product might be an important exposure pathway for PFRs.

Evaluation of the potentials of rice varieties and water management practices for reducing human health risks associated with polluted river water irrigated rice in Bangladesh.

The consumption of arsenic and trace-metal-contaminated rice is a human health concern worldwide, particularly in Bangladesh. In this study, the effects of rice varieties and water management practices on the concentrations of arsenic and trace metals in rice grains were investigated to reduce human health risks related to rice consumption. In addition, the performance of risk reduction using the optimum combination of rice variety and water management practices was quantitatively assessed using Monte Carlo simulation, in which non-carcinogenic and carcinogenic risk distributions under the status quo and the optimum combination were compared. The experimental results revealed that Dular and BRRI dhan45 (rice varieties) cultivated under alternate wetting and drying (AWD) and continuous flooding (CF) conditions showed the lowest hazard quotient (HQ) values for copper, cadmium, and arsenic and the lowest target cancer risk (TR) for arsenic. In Dular and BRRI dhan45 (AWD and CF) varieties, the proportion of the population for which HQs exceeded 1.0 (the reference value) tended to decrease (except for arsenic), compared with populations for which the rice varieties and water management practices were not specified. These results suggest that the use of optimum combinations of rice varieties and water management practices could reduce non-carcinogenic and carcinogenic risks associated with arsenic and trace metals uptake via rice grain consumption by the Bangladeshi people.

Development of an analytical method for indoor polycyclic aromatic hydrocarbons and their halogenated derivatives by using thermal separation probe coupled to gas chromatography-tandem mass spectrometry.

Polycyclic aromatic hydrocarbons (PAHs) and their halogenated derivatives (XPAHs) have been a concern because of their high toxicity. Monitoring indoor PAHs and XPAHs concentrations is important for risk assessment because humans typically spend >90 % of their time indoors. However, the background levels of indoor PAHs and XPAHs concentrations are unknown because of the low sensitivity of conventional analytical methods. In this study, we developed a highly sensitive analytical method using a thermal separation probe (TSP) coupled to a gas chromatograph with a triple quadrupole mass spectrometer method for 26 PAHs and 40 XPAHs. The method quantification limit (MQL) values of the TSP method were 1.1 (3,8-dichlorofluoranthene)-906 (dibenzo[a,e]pyrene) times lower than those of the conventional method. The regression line comparing the TSP and conventional methods was y = (0.944 ± 0.0401)x, which was in good agreement. These results demonstrate that the TSP method can be applied to indoor air analysis. The total concentrations of PAHs and XPAHs were 944 and 73.5 pg m-3 for the house and 735 and 0.924 pg m-3 in the office, respectively. Among the detected compounds, 13 PAHs and XPAHs could not be detected using conventional methods because of their high MQL values. The composition of total toxicity equivalency values in the house was dominated by dibenzo[a,i]pyrene (DBaiP: 43.2 %) and dibenzo[a,h]pyrene (DBahP: 27.1 %), which could not be detected using the conventional method. Therefore, the TSP method can improve the risk assessment of indoor PAHs and XPAHs.

Enhancement of photodegradation of polyethylene with adsorbed polycyclic aromatic hydrocarbons under artificial sunlight irradiation.

The photodegradation of plastic waste produces microplastics (MPs) in marine environments. Plastics can adsorb hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and can be transported over long distances. However, the impact of adsorbed pollutants on the photodegradation remains unknown. Here, we show that adsorbed PAHs act as photocatalysts that promote the photodegradation of polyethylene. Upon light irradiation, coloration and surface degradation of the PAH-adsorbed polyethylene sheets were observed, indicating that the PAH-adsorbed polyethylene sheets are less resistant to light. Furthermore, fluorene, phenanthrene, anthracene, benzo[a]anthracene, benzo[a]pyrene, and indeno[1,2,3-cd]perylene adsorbed on polyethylene MP exhibited lower photodegradation rates than the aqueous phase. These results indicate that these PAHs can act as photocatalysts; their role of PAHs may have two adverse effects on marine environment. First, enhanced photodegradation of plastic waste increased the production of MPs. Second, the lifetime of PAHs is extended, thereby enhancing PAHs pollution in marine environments.

Polycomb group gene Bmi1 plays a role in the growth of thymic epithelial cells.

Polycomb group gene Bmi1 plays an essential role in HSCs and the BM microenvironment. Recent reports also pointed to the importance of Bmi1 in thymocyte development. However, little is known about its role in the development of the thymic microenvironment. Here, we examined the function of Bmi1 in thymic epithelial cells (TECs) by using the engraftment of fetal thymus (FT) lobes under the kidney capsule. The engrafted Bmi1(-/-) FT lobes were clearly smaller and the number of thymocytes in these lobes was significantly decreased compared with control FT lobes. Analysis of the cell cycle status of TECs in the reconstituted lobes revealed that the reduction of thymus size in Bmi1(-/-) FT grafts was caused by less proliferation of TECs during the early expansion stage. Unlike cases with hematopoietic stem cells or thymocytes, the deletion of p16Ink4a and p19Arf could not restore the defects in Bmi1(-/-) TEC, indicating a distinct role for Bmi1 in TECs. In conclusion, epigenetic regulator polycomb group gene Bmi1 plays a role in the thymic microenvironment in a regeneration process by supporting TEC growth, and thereby contributes to the control of thymus size for T-cell growth in mice.

Analysis of N(ε) -ethyllysine in human plasma proteins by gas chromatography-negative ion chemical ionization/mass spectrometry as a biomarker for exposure to acetaldehyde and alcohol.

BACKGROUND: N(ε) -ethyllysine (NEL) is a major stable adduct formed by the reaction of acetaldehyde (AA) with lysine residues in proteins. However, its occurrence and levels in biological specimens and its relationship with AA/alcohol exposure-associated disorders have not been fully elucidated. In this study, we have developed a sensitive and specific method to quantitate NEL levels in human plasma proteins. METHODS: The method consists of (1) purification of the protein fraction of interest by Sephadex G-15 to remove low molecular substances, (2) hydrolysis of proteins with Pronase E in the presence of stable isotope-labeled internal standards, (3) derivatization of amino acids with pentafluorobenzyl (PFB) bromide, and (4) quantification of the PFB derivatives of NEL and l-lysine using gas chromatography-negative ion chemical ionization/mass spectrometry in a selected ion monitoring mode. RESULTS: Using the above method, the NEL levels in human plasma proteins obtained from 10 each of control subjects and alcoholic patients were measured. NEL was detected in all samples analyzed, the average level of NEL in the plasma proteins of alcoholic patients (1.17 ± 0.36 NEL/1,000 l-lysine) being significantly higher than that of control subjects (0.26 ± 0.07 NEL/1,000 l-lysine). CONCLUSIONS: The method could be applied to molecular epidemiological studies to investigate possible associations between the NEL levels in human tissue proteins and human diseases associated with exposure to AA and alcohol.

Postnatal tissue-specific disruption of transcription factor FoxN1 triggers acute thymic atrophy.

The transcription factor FoxN1 is essential for differentiation of thymic epithelial cell (TEC) progenitors during thymic organogenesis. However, limited information is available on the postnatal contribution of FoxN1 to thymic maintenance. To address this question, we generated a loxP-floxed FoxN1 (fx) mouse with three different promoter-driven inducible CreER(T) transgenes. Postnatal ubiquitous deletion of FoxN1 caused dramatic thymic atrophy in 5 days and more severe deterioration in medullary TECs (mTECs) than in cortical TECs (cTECs). Induction of FoxN1 deletion selectively in K5 promoter-driven somatic epithelial cells (mostly mTECs and possibly some adult epithelial stem cells) was sufficient to cause significant thymic atrophy, whereas FoxN1 deletion in K18 promoter-driven somatic epithelial cells (mostly cTECs) was not. Thymic atrophy resulted from increased apoptosis and was associated with activation of the p53 gene in mature mTECs. Although FoxN1 is required for the development of both mTECs and cTECs in thymic organogenesis, it is most important for the maintenance of mTECs in the postnatal thymus, which are in turn necessary to prevent thymic atrophy.

Gene expression profile of the third pharyngeal pouch reveals role of mesenchymal MafB in embryonic thymus development.

The thymus provides a microenvironment that induces the differentiation of T-progenitor cells into functional T cells and that establishes a diverse yet self-tolerant T-cell repertoire. However, the mechanisms that lead to the development of the thymus are incompletely understood. We report herein the results of screening for genes that are expressed in the third pharyngeal pouch, which contains thymic primordium. Polymerase chain reaction (PCR)-based cDNA subtraction screening for genes expressed in microdissected tissues of the third pharyngeal pouch rather than the second pharyngeal arch yielded one transcription factor, MafB, which was predominantly expressed in CD45(-)IA(-)PDGFRalpha(+) mesenchymal cells and was detectable even in the third pharyngeal pouch of FoxN1-deficient nude mice. Interestingly, the number of CD45(+) cells that initially accumulated in the embryonic thymus was significantly decreased in MafB-deficient mice. Alterations of gene expression in the embryonic thymi of MafB-deficient mice included the reduced expression of Wnt3 and BMP4 in mesenchymal cells and of CCL21 and CCL25 in epithelial cells. These results suggest that MafB expressed in third pharyngeal pouch mesenchymal cells critically regulates lymphocyte accumulation in the embryonic thymus.