Breaking Dogma on the Hypothalamic-Pituitary Anatomical Relations in Vertebrates.

Abstract

The hypothalamus and pituitary are present in all vertebrates from agnathans (jawless fishes) to mammals. The hypothalamus is located below the thalamus, just above the brain stem and forms the ventral part of the diencephalon. The appearance of the pituitary was a seminal event in the evolution of vertebrates (1, 2). The pituitary is not present in protochordates or other invertebrates. The pituitary consists of the same 2 principal divisions, the neurohypophysis and adenohypophysis. The neurohypophysis develops from the floor of the diencephalon as an infundibular extension, whereas the adenohypophysis develops from the oral epithelium that comes in contact with this infundibulum. In vertebrates, the evolution of a complex pituitary with dual developmental origin along with the more highly developed tripartite brain added another layer of control leading to the neuroendocrine control of many complex physiological functionssuchasgrowth,reproduction,developmentand metabolism among others. These functionally adaptive conditions may then have contributed to the expansion of vertebrates into new environments. The acquisition of the vertebrate pituitary probably resulted from wholegenome duplications that occurred early in vertebrate evolution (3). The adenohypophysis of the pituitary gland secretes a number of protein hormones that regulates a variety of the physiological processes of vertebrates. The adenohypophysialhormonescanbeclassified,onthebasis of structural and functional similarity, into 3 groups, the proopiomelanocortin family, the GH/prolactin/somatolactin family, and theglycoprotein hormone family (gonadotropins, thyroid-stimulating hormone, and a novelhormonecalledthyrostimulin).Somatolactinisonly found in teleosts. Each family is believed to have evolved fromanancestralgenebyduplicationandsubsequentmutations (4). During the evolution of the vertebrates, structural features of the pituitary and hypothalamus also evolved that perhaps optimized the communication between these tissuesasvertebratesbecamelargerandmorecomplicatedin form and distance between the hypothalamus and pitu