The endocrine system or internal secretion system is a set of organs and tissues of dissimilar origin, structure and location, which secretions called hormones, enter the bloodstream directly without a duct so they can reach distant organs. Hormones are signalling molecules that regulate many physiological and behavioural functions including growth, reproduction and water and mineral balance, among others.
There is a close relationship between the endocrine and the nervous system and often they act jointly to control and regulate different body functions and processes (neuroendocrine integration). Perhaps one of the best known examples being the hypothalamic-hypophyseal-interrenal axis, stimulated by a wide range of factors that are perceived as aggressive to the organism.
The endocrine system mediates four types of effects: kinetic (e.g., pigment migration), metabolic (e.g., carbohydrate balance), morphogenic (e.g., sexual differentiation) and behavioural (e.g., reproductive, migrations).
However most authors recognize 16 organs and tissues with endocrine activity in teleosts distributed in different parts of the body: epiphysis (or Pineal gland), hypophysis (or Pituitary, including neurohypophysis and adenohypophysis), saccus vasculosus, thymus, pseudobranch (opercular or hyoid hemibranch), ultimobranchial bodies (sub oesophageal gland, supra pericardial bodies), thyroid, interrenal tissue, Chromaffin tissue, renal juxtaglomerular cells, corpuscles of Stannius, gonads, Langerhans or pancreatic islets, stomach, enterochromaffin cells and urophysis (or caudal neuro secretory organ) (Fig. 1). From these, the corpuscles of Stannius, urophysis and pseudobranch are exclusive to teleosts. Unlike superior vertebrates, fish lack discrete parathyroid organs, although parathyroid hormone has been found in some species.
Pathological changes have rarely been reported for the endocrine system, included granulomas, cysts, oedema and adenoma. Like other organisms fish are also susceptible to the action of exogenous endocrine disruptors, chemical substances that can interfere with marmoreal functions, homeostasis and regulation of developmental processes.
Further reading Pait, A.S. and J. O. Nelson. 2002. Endocrine Disruption in Fish: An Assessment of Recent Research and Results. NOAA Tech. Memo. NOS NCCOS CCMA 149. Silver Spring, MD: NOAA, NOS, Center for Coastal Monitoring and Assessment 55pp.