The Pineal Gland and Melatonin
This chapter presents the diversity of pineal research, but focuses on basic aspects of structure and function, with emphasis on the mammalian pineal gland. While the literature in this field portrays a dynamic area of research, it also reflects an area of research in which fundamental questions concerning the pineal gland remain unanswered. The vertebrate pineal gland, or epiphysis, develops as an evagination of the roof of the diencephalon. A phylogenetic relationship between the pineal gland and retina is further suggested by the presence of proteins in the pineal gland that are associated with phototransduction in the retina. The mammalian pineal gland is a solid parenchymal structure surrounded by a capsule of connective tissue and pia mater. Unlike other endocrine glands, the pineal gland shows little ultrastructural evidence that it is storing a hormonal product. Its known secretory product, melatonin, appears to be released almost immediately after it is synthesized. Storage mechanisms for melatonin, if they exist, are short term.