Morphology of the thyroid gland and oxidoreductases in the liver of mithuns (Bos frontalis) and yaks (Bos grunniens)

Thyroid glands and livers from 3 adult mithuns 3 adult yaks were studied. The topography, gross anatomical features and microscopic features of the thyroid gland and histochemical localization of oxidoreductases in the liver were recorded. The gross appearance of both the thyroid glands of mithuns was elongated and elliptical, whereas in yaks the right lateral lobe was triangular in shape and the left one cashewment shaped. The thyroids of mithun extended from the level of cricoid cartilage up to the tracheal ring 4, while in yak it was located just caudal to the larynx up to tracheal ring 2 or 3. The isthmus was more distinct and glandular in mithuns than in yaks. Thyroids of mithuns revealed comparatively higher biometrical characteristics in length, width, thickness and weight than of yaks. The capsule of the gland in both the species was double-layered with predominance of collagenous fibres in it and trabeculae. The average mean diameter of the thyroid follicles did not show remarkable variations between mithuns (89.60 +/- 7.06 mu m) and yaks (92.80 +/- 6.60 mu m). The follicular lining epithelium was simple squamous with PAS-positive basement membranes and colloid. The para follicular cells were marked as single or double in both the species. The oxidoreductases in the liver of mithuns and yaks comprised of succinate-, lactate-, glucose-6-phosphate dehydragenases, cytochrome- and monomine oxidase. These were localized in biliary epithelium, vascular endothelium and hepatocytes. The hepatocytes in the liver of these large ruminants contained all above enzymes with a relatively higher concentration in mithuns.