any structural anomaly of the unpaired epithalamic structure that is shaped like a tiny pine cone and located above the posterior commissure near the level of the habenular complex and the sylvian aqueduct; it is both a circumventricular organ and an endocrine organ that secretes melatonin, a serotonin-derived hormone involved in the regulation of circadian rhythms; the pineal parenchyma, consisting of pinealocytes, interstitial (glial-like) cells, phagocytes and capillaries, is arranged into large folliculi separated by septae of connective tissue and blood vessels