It is only in recent years that the pineal gland has emerged from being thought of as non-functional and unimportant. The rise from obscurity has been the result of the interest of investigators of multidisciplinary origins; such approaches, whilst clearly advancing understanding, also tend to leave knowledge fragmentary. In the last decade, a new neuroendocrine axis known as the APUD system has been delineated and scientifically accepted. The essence of the concept has been to link a physically widely dispersed and apparently unconnected series of glandular structures by a common embryological derivation and their secretion of closely similar functional products. The present proposal collates evidence for the inclusion of the pineal gland within the APUD system. In addition, because of the proven connections and response of the pineal gland to environmental changes, it is hypothesized that pineal products may provide the link between the environment and other members of the widely dispersed APUD system and thereby temper body homeostasis.

Leong AS, Matthews CD. The pineal gland as an APUD organ: supporting evidence and implications. Med Hypotheses. 1979 Feb;5(2):265-73. doi: 10.1016/0306-9877(79)90127-0. PMID: 37417.