| Background and objectives:2,3,7,8-tetrachlorinated-dibenzo-p-dioxin (TCDD) is one of the persistent organic pollutants.It is difficult to be degraded under natural conditions. Because of its high fat solublility andstrong accumulation in the food chain, it threatens the human health and the environmentalsafety. TCDD is harmful to human growth and development,immune system,endocrine system,and reproduction system. It is also a human carcinogen. Under higher concentrations, with adultor in utero/lactation exposure, TCDD can damage the thyroid function in animals, induce thyroidhyperplasia, such as, the decreased thyroid volume, the increased number of small follicles, andthyroid dysfunction. However , there is little confirmed information on the effects of verylow-dose exposure to TCDD on thyroid morphology and function in parental and first filialgeneration rats. The present research was designed to do so.Materials and methods:One hundred and twenty one-month-old SD rats were allocated randomly into 4 groups with 30rats in each (male: female =1:1). Animals were treated daily with three does of TCDD (i.e. 2ng/kg, 10 ng/kg, 50 ng/kg), and one control of corn oil. Five weeks later, 5 female rats wereselected randomly for reproduction. Offsprings from each female rat were then separated fromthe mother after weaning and live dependently (pregnancy/lactation exposure). In the 13th and18th week, the parental rats and offspring rats were killed, animal's thyroid was collected toobserve the thyroid morphology, conduct thyroid stereological analysis; and serum was used tomeasure the thyroid hormones. Data were analyzed by one-way ANOVA and nonparametric test.P≤0.05 was regarded as statistical significance.Results:1. The general situation of SD ratsIn general, no significant toxicity were found among the parental and first filial generation SDrats with subchronic low-dose exposure to TCDD. In the parental SD rats: the thyroid absoluteweight and relative weight and body weight was not statistically different among groups (P>0.05). In the first filial generation SD rats: the absolute weight was lower in TCDD treatedgroups than that in the control (P<0.05). 2. The thyroid outcomes of parental SD rats2.1. The thyroid morphologyIn control group: thyroid tissues were pale pink and soft; most of thyroid follicles weremedium-sized, single flat or cubic epithelium, follicular cavities were filled with colloid. InTCDD treated groups: thyroid tissues were dark red, soft, clearly hyperemic and intumescent;most of thyroid follicles were found hyperplastic with small follicles, lower thyroid folliclesvolume, more thyroid follicle numbers, cubical, high columnar or stratified epithelium, andreduced or absent follicular colloid.2.2. Stereological analysis of thyroid follicleThere were significant differences in the thyriod follicular volume density(Vv) level andsurface density(Sv) level among four groups( P<0.05), Vv level and Sv level were bothsignificantly lower in 10 ng/kg and 50 ng/kg TCDD treatment groups than controlgroup(P<0.05), and the Sv and Vv in 10 ng/kg and 50 ng/kg TCDD treatment groups alsosignificantly lower than 2 ng/kg TCDD treatment group (P<0.05). Numerical density(Nv) leveland specific surface(S/V) level were both significantly higher in the 10 ng/kg and 50 ng/kgTCDD treatment groups than control group(P<0.05), and they showed significant differencesbetween 2 ng/kg and 50 ng/kg TCDD treatment group(P<0.05). A significant decrease in thethyroid follicular mean volume(V) level was observed in TCDD treatment groups ( P<0.05);There was no difference in the thyroid follicular mean surface area(S) among fourgroups( P>0.05).2.3. The serum thyroid hormoneIn the female rats: the concentration of FT3 was the lowest in the groups of 2 ng/kg; theconcentration of FT4 was the lowest in the groups of 50 ng/kg; there was no significance in TT3among groups.In the male rats: there was no significance in FT3 among groups; the concentration of FT4was the highest in the groups of 2 ng/kg; the concentration of TT3 was the highest in the groupof 50 ng/kg.There were no significant differences in the gender and groups in the concentration of TT4.There was no statistical significance in TSH in gender, but statistical significance among groups(P<0.05). 3. The thyroid outcomes of the first filial generation SD Rats3.1. The thyroid morphologyThyroid tissues were pale pink and soft; lots of thyroid follicles were medium-sized, singleflat epithelium, and follicular epitheliums are in a relatively state, follicular cavities were filledwith colloid.3.2. Stereological analysisThere was no difference in the thyroid follicular VV,Sv,S/V and V among fourgroups( P>0.05). Nv was lower in the TCDD treatment groups than which in control group(P<0.05), but the S was lower in the 2 ng/kg and 10ng/kg groups than which in control group(P<0.05).3.3. The serum thyroid hormoneIn the female rats: There were no significant differences in groups in the concentration of FT3,FT4 and TT3.In the male rats: The concentration of FT3 was higher in the group of 50 ng/kg than which incontrol (P<0.05); The concentration of FT4 was higher in control than which in the groups of 10ng/kg and 50 ng/kg (P<0.05); The concentration of TT3 was lower in control than which in thegroups of 2 ng/kg and 50 ng/kg (P<0.05).There were no significant differences in the gender and groups in the concentration of TSH.There was no statistical significance in TT4 and in gender, but statistical significance amonggroups (P<0.05).Conclusions:Subchronic low-dose TCDD exposure has a certain influence on thyroid morphology andfunction, results in thyroid hormone disorders on parental SD rats. TCDD exposure duringpregnancy and lactation, although there were no significant differences in the thyroid glandmorphology, the thyroid hormone may be disrupted in the male offspring. Therefore, subchroniclow-dose TCDD exposure have hazardous effects on thyroid morphology and function ofparental and first filial generation SD rats. |
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