Histological and biochemical study on the toxic effects of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin [TCDD] on the follicular cells of thyroid gland of rats

ABSTRACT

2,3,7,8, TCDD and related substances are ubiquitous environmental pollutants causing a wide variety of pathological alterations. They are capable of altering endocrine homeostasis. This work was carried out to study the histological and biochemical changes of acute and chronic administration of TCDD on the follicular cells of thyroid gland of rats. Thirty six male Sprague-Dawley rats were used in this study. They were divided into three groups. Group I was given corn oil and served as a control group. Group II was given 10 micro g/Kg TCDD as an acute single oral dose. Group III was further subdivided into subgroup IIIa and subgroup IIIb that were given 0.125 micro g /Kg/ day TCDD for 4 weeks and 16 weeks respectively. At the end of the experiment, animals were sacrificed and thyroid glands were removed and examined by light and electron microscope [EM]. Biochemical estimation of serum thyroxin [T[4]], triiodothyronine [T[3]] and thyroid stimulating hormone [TSH] were also done to all animals. Histologically, Thyroid gland of group II showed beginning of follicular hypertrophy and hyperplasia in some follicles. These follicles were lined by high cuboidal epithelium with appearance of follicles lined by more than one layer of cells. Dilated rough endoplasmic reticulum [rER], well developed Golgi apparatus and numerous lysosomes were evident. Microvilli of some follicular cells were short and blunted. Subgroup IIIa and IIIb showed signs of hyperactivity affecting most of the follicles. These changes were more pronounced in animals of subgroup IIIb that received TCDD for 16 weeks. Most of the follicles were small and irregular. Many of them were lined by tall columnar epithelium with appearance of papillary projections in their lumens that appeared empty or with sparse scalloped colloid. EM examination revealed marked dilatation of rough endoplasmic reticulum and prominent well developed Golgi complexes with accumulation of numerous lysosomes and membrane bound colloid droplets. Some follicles of this group revealed degenerative changes due to direct toxic effect of TCDD. These histological changes were accompanied by a significant decrease in serum level of T[4] and a significant increase in TSH level. These biochemical changes were more pronounced in animals that received TCDD for 16 weeks. TCDD had deleterious effect on thyroid gland which is time and dose dependent