Pinealis

Pinealis refers to the pineal gland, a small endocrine organ located in the center of the brain. Also known as the epiphysis cerebri, it sits within the epithalamus near the dorsal third ventricle. In humans it measures about 5 to 8 millimeters and develops from the neuroectoderm of the diencephalon.

Anatomy and histology: The gland is composed mainly of pinealocytes and supportive glial cells. Microcalcifications, often called brain sand, are common with age. The pineal gland receives a rich blood supply from the posterior choroidal arteries and drains venously via the internal cerebral veins.

Physiology and function: The primary secretory product of the pineal gland is melatonin, synthesized from serotonin. Melatonin production is synchronized with the light-dark cycle through a neural pathway: retina detects light, signals the suprachiasmatic nucleus, which drives sympathetic input to the pineal gland. Melatonin levels rise in the dark and fall during the day, contributing to the regulation of circadian rhythms and sleep-wake cycles. In many vertebrates, melatonin also conveys photoperiod information that influences seasonal reproductive timing. Melatonin has additional proposed roles as an antioxidant and in modulating immune and mood processes.

Development and evolution: The pineal region is evolutionarily ancient. In many non-mammalian vertebrates, the pineal organ remains directly photosensitive, whereas in humans and most mammals its primary role is hormonal, with light entrainment mediated through neural pathways rather than intrinsic photosensitivity.

Clinical relevance: Pineal region lesions, including germ cell tumors, pineal parenchymal tumors, and cysts, can cause hydrocephalus or Parinaud syndrome due to compression of midbrain structures. Calcifications are common and often incidental on imaging. Disturbances of melatonin production have been studied in relation to sleep disorders and circadian disruption.