Missing secretory granules, dilated endoplasmic reticulum, and nuclear dislocation in the thyroid gland of rdw rats...

Previous studies on the rdw rat have suggested that its dwarfism is caused primarily by dysfunction of the thyroid gland. In this study, rat thyroid glands were analyzed endocrinologically and morphologically to clarify the primary cause of dwarfism in the rdw rat. The rdw rat showed lowered thyroid hormone (T4 and T3) levels but elevated TSH in serum. The rdw thyroid gland was almost proportional in size and it was not goiter in gross inspection. Our histological investigation produced three results that may lend important evidence in understanding the problem in the thyroid gland of rdw rats. First of all, secretory granules could not be detected in the follicular epithelial cells of the rdw. Secondly, thyroglobulin was found at very low levels in the follicular lumen by immunohistochemical analysis. In contrast, it could be detected in a substantial quantity inside the dilated rER and in the huge vacuoles that are formed by swelling of the rough endoplasmic reticulum (rER) at the basal side of the follicular epithelial cells. Additionally, the nucleus of the follicular epithelial cells was pressed to the luminal side by the enlarged rER. These morphological changes would indicate that the transport of thyroglobulin is stopped at or before the formation of the secretory granules and thyroglobulin is not secreted into the follicular lumen. The rdw characterization strongly supports that rdw dwarfism is induced by hypothyroidism due to some defect(s) in the thyroid gland.