Cibtech Journal of Zoology ISSN: 2319–3883 (Online) An Open Access, Online International Journal Available at http://www.cibtech.org/cjz.htm 2014 Vol. 3 (1) January-April, pp.27-36/Chakrabarti and Chatterjee Research Article © Copyright 2014 | Centre for Info Bio Technology (CIBTech) 27 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 *P Chakrabarti and N Chatterjee Fisheries Laboratory, Department of Zoology, The University of Burdwan, Golapbag, Burdwan-713 104, West Bengal, India *Author for Correspondence ABSTRACT 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. Keywords: Thyroid, Ovary, Mystus vittatus INTRODUCTION The gross morphological structures of the teleostean thyroid are extremely variable from scattered follicles to encapsulated structures occurring in the sub-pharyngeal region in few cases (Chavin, 1972; Gorbman et al., 1983). In teleosts, seasonal changes in the activity of the thyroid gland have been described in several species and often been correlated with the gonadal maturation (Singh, 1968; Leatherland 1982). It is now a well established fact that the seasonal changes in thyroid gland of vertebrates including the fishes are influenced by several environmental factors (Dodd and Matty, 1964; Osborn and Simpson, 1978). On the contrary, Belsare (1971) remarked that the cyclical changes of thyroid activity in Channa punctatus have got a close relationship with the breeding activity of the fish instead of the temperature of the habitat. Majority of the teleost fishes are seasonal breeders and histological studies provide an opportunity to understand the cellular kinetics of the gonads, recruitments, development and resorption of gonadal cells and finally in staging the maturity stages of the gonads (Lee et al., 2008). It has however, been reported by many investigators that since large amount of gonadal steroid hormones are secreted during gonadal maturation in teleosts, some of the apparent thyroid activity during these times might be related to be thyrotrophic effects of the steroid hormones themselves (Higgs et al.,1976; Singh and Raizada, 1979). The aim of the present work is to examine more precisely the correlative changes between different functional states of thyroid follicles with the changes in the ovary during different reproductive phases in Mystus vittatus (Bloch). This small indigenous fish species has a nutritional value in terms of proteins, micronutrients, vitamins and minerals (Ross et al., 2003). MATERIALS AND METHODS Specimen Collection and Measurements Adult female M. vittatus (Bloch) of length 10 to 12 cm and weight 30 to 50 g were procured fortnightly throughout the year from a particular stocking pond located in Burdwan in order to avoid ecological