ABSTRACT
Endocrine systems regulate body homeostasis by hormones affecting reproduction, metabolism, osmomineral regulation, stress, development (metamorphosis), color change, and behavior. Hormones are chemical messengers, synthesized in specific cells and released internally to cause physiological reactions in distant cells or organs at very low concentrations. The biological actions act due to the principle of key and key hole by specific receptors of target cells binding the hormone to elicit a cellular response by activation or inactivation of signal transduction pathways ranging from short lasting metabolic to long lasting genomic responses. Lipophilic hormones (steroids, thyroid hormones) act mainly via genomic effects whereas hydrophilic ones (catecholamines, peptide hormones) affect target cells via membrane bound receptors affecting physiological cellular activities. The number of new hormones detected, especially concerning peptide hormones, is exponentially increasing because of the application of modern molecular genetic techniques. Their functions, however, are mostly unknown.
The endocrine system is organized, together with the central nervous system, in a hierarchical manner. The highest hierarchic level is the hypothalamus triggering the pituitary by releasing (or inhibiting) hormones to stimulate the classical endocrine glands: thyroid gland, ovary and testis, adrenal gland, or the liver being a source of hormones (insulin like growth factor-I) as well as a target organ. The classical endocrine glands secrete their hormones into the blood circulation to affect their target organs. At each hierarchic level like the hypothalamus, the pituitary, the endocrine glands and the target organs further endogenous and exogenous factors can influence their regulation.
In this chapter, we restrict our description to the endocrine system of teleosts. They possess, apart from the classical endocrine organs of higher vertebrates, additional organs like the corpuscles of Stannius, the urophysis, the ultimobranchial body, and the pseudobranch.
