pineal gland

The pineal gland is a small, pinecone-shaped endocrine gland located in the epithalamus, near the posterior commissure and the roof of the third ventricle, between the two thalami. It arises embryologically from neuroectoderm of the diencephalon. Its primary function is the synthesis and secretion of melatonin from serotonin, using the enzymes serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase (HIOMT). Melatonin secretion is stimulated by darkness and suppressed by light via a pathway involving the retina, suprachiasmatic nucleus (SCN) of the hypothalamus, superior cervical ganglion, and sympathetic innervation to the pineal gland—this regulates circadian rhythms and sleep-wake cycles. The pineal gland is notable for lacking a complete blood-brain barrier and for calcifying with age, making it a useful radiographic midline marker on CT/skull X-ray (used to detect midline shift). Pathologically, the pineal region is a common site for germ cell tumors (especially germinomas, more common in males), pineoblastomas, and pineocytomas. Tumors in this region can compress the superior colliculi and cerebral aqueduct, leading to Parinaud syndrome (paralysis of upward gaze, pupillary light-near dissociation, convergence-retraction nystagmus) and obstructive hydrocephalus due to aqueductal compression. The pineal gland is also historically referred to as the 'seat of the soul' by Descartes, though this holds no scientific basis. Clinically, pineal gland dysfunction has been implicated in circadian rhythm disorders, and melatonin supplementation is used therapeutically for jet lag and insomnia.