The Pineal Gland

The pineal gland was identified as a distinct structural component of the vertebrate brain for several centuries prior to its description in the human by Galen of Pergamon (c. 130–200). It is now understood that the main function of the pineal is to receive information about the current state of the light–dark cycle from the environment and convey this information through the secretion of the hormone melatonin to the internal physiological systems of the body. Relative to other endocrine and neuroendocrine systems, the pineal gland has undergone a great deal of change during its evolutionary development. In cold-blooded vertebrates, the cells of the pineal gland include photoreceptors that contact neurons to communicate with other organs in the body. The avian pineal gland is also a directly photosensory organ and secretes melatonin in response to this signal. In mammals, pinealocytes producing melatonin have replaced photoreceptors and the gland receives its information about light and darkness from the retina through multiple neuronal connections. Melatonin is secreted during the dark period of the day and, through its elevated blood levels, informs, through specific cell receptors, the peripheral organs and tissues regarding the light/dark cycle. In addition to its function of sending information to the periphery regarding light and dark, melatonin from the pineal gland also contributes to the entrainment of the central SCN oscillator to the light–dark cycle. This chapter covers the anatomical features of the pineal gland, the synthesis and secretion of melatonin, the biological actions of melatonin, and various clinical aspects related to the pineal gland.