Sensory and endocrine characteristics of the avian pineal organ

The avian pineal organ represents a transitional type between a photosensory organ of lower vertebrates and the endocrine gland of mammals and shows remarkable changes in its innervation and structure during ontogeny. In the avian pineal organ the progressive reduction of the pinealofugal component and the spectacular increase in pinealopetal sympathetic innervation occur in parallel. In domestic fowl the number of intrapineal AChE-positive (afferent) neurons decreases rapidly during ontogenetic development, whereas the sympathetic innervation becomes more prominent. Furthermore, the end vesicle of the pineal organ is an anatomical entity fully separated from the brain in the adult domestic fowl, as observed in some mammalian pineals. The avian pineal organ contains several types of photoreceptors with different photopigments and the synthesis of melatonin, the pineal hormone, is controlled by light. Immunoreactivity for photopigments is reduced during the posthatching development of chicken, whereas neuron-specific enolase (NSE)-immunoreactive pinealocytes increase remarkably in number in the end-vesicle of the domestic fowl with age, followed by a gradual expansion toward the proximal portion. NSE is the most acidic isoenzyme of the glycolytic enzyme enolase and is useful as a cytoplasmic marker of neurons and neuroendocrine tissue. The above-mentioned findings reflect the sequence of changes leading from pineal sense organs to pineal gland. The demonstration of melatonin receptors in a variety of avian peripheral tissues suggest a possible direct action of melatonin on the physiological functions of different organ systems in response to internal and external stimuli.