Thyroid follicles in the head-kidney of teleosts

Out of five species of fishes, selected for this investigation, only two have shown the presence of thyroid follicles in their head-kidneys. In Tor tor the thyroid follicles are larger than those of Cyprinus carpio. They are elliptical in shape in T. tor while spherical in C. carpio. The follicular epithelium in T. tor is thick and prominently built while in C. carpio it is weak in its appearance and structure. The colloid of the follicles in both the cases is uniformely homogeneous, non-vacuolated ans acidophilic in nature. The follicles are almost always located in close proximity to the posterior cardinal veins and their branches. It is, for this reason, argued that they might have migrated along these blood vessels. there is no any regular distribution and proper arrangement of these follicles.