Histological Changes in the Activity of the Thyroid Follicles and Its Impact on Ovarian Tissues in Mystus Vittatus (bloch, 1794) during Growth, Maturation and Spawning Phases

The thyroid gland of Mystus vittatus was unencapsulated, composed of follicles scattered mainly around the ventral aorta in the sub pharyngeal region. Each follicle provided with agranular eosinophilic colloid materials in the central lumen was encircled by a single layer of epithelial cells. The follicular cells varied according to the secretary activity in harmony with the reproductive cycle. The low active condition of the thyroid follicles during the growth phase i.e., December to February was well in coincidence with the increase of early and late perinucleolar oocytes. The most active condition of the thyroid follicles were noted during spawning phase i.e., June to August as revealed by the maximum follicular cell height and also correlated with the highest frequency percentage of oocyte V and oocyte VI stages. Thereafter, the thyroid activity gradually proceeded at a low level up to December and revealed an abrupt increase in April i.e., active maturation phase. It was concluded that the peaks of thyroidal and ovarian activities correlate well and run parallel.