The pineal gland: structural and functional diversity

Abstract

The article reviews the work carried out on pineal structure and function in vertebrates. The pineal has undergone striking changes in structure during the course of evolution from lamprey to mammals. In the primitive vertebrates, structure of the pineal is very much similar to that of the retina of the eyes and it acts as a direct photosensory organ. It acts as a photosensory and secretory organ in a number of species. Interestingly, the pineal complex among the reptiles presents the full spectrum of its morphological possibilities. There is a gradual regression of light sensitive pineal structure during the course of avian line of evolution. Further, it has been unequivocally accepted that mammalian pineal is an endocrine organ. The pineal is a prominent secretory organ, in mammals, which synthesises and secretes a number of exocrine and endocrine substances, such as indoles, peptides, various enzymes, amino acids and their derivatives, lipids, carbohydrates, and inorganic constituents. Pineal plays an important role in regulation of seasonal breeding in various vertebrate species. It is also considered as one of the most important components of the vertebrate circadian system and is a principal source of rhythmically produced melatonin. Apart from the above mentioned functions, pineal also influences thermoregulation, electrolyte metabolism, intermediary metabolism, hemopoiesis, immune system and behaviour.