epifiz

The epifiz, or pineal gland, is a small endocrine gland located deep in the brain. In humans it is roughly pea-sized, situated near the center between the two hemispheres in the epithalamus, just above the third ventricle and behind the habenular nuclei. It forms part of the epithalamus and is in close anatomical relationship with nearby structures such as the habenula and the pineal recess of the third ventricle.

Anatomy and histology: The gland consists mainly of pinealocytes and supporting glial cells, with a rich network of capillaries. It is supplied by branches of the posterior cerebral and posterior choroidal arteries and drained by pineal veins into the internal cerebral veins and the great cerebral vein (vein of Galen). Sympathetic innervation from the superior cervical ganglion modulates its function.

Function: The primary role of the pineal gland is the production of melatonin, a hormone synthesized from serotonin through the action of the enzymes arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT, also known as ASMT). Melatonin secretion follows a circadian pattern, increasing in darkness and declining with light, thereby helping regulate sleep-wake cycles and other circadian rhythms. In some animal species, the pineal region can influence reproductive cycles, though in humans its primary relevance is chronobiological rather than gonadal regulation.

Clinical notes: Calcified concretions called corpora arenacea or brain sand are common and tend to increase with age. Pineal tumors are rare but can cause headaches, hydrocephalus, or Parinaud syndrome; cysts may arise and are usually benign. Disturbances in melatonin production have been investigated in relation to sleep disorders, jet lag, and seasonal mood changes, but melatonin therapy should be managed medically.

Evolution and notes: In many non-mammalian vertebrates, the pineal organ can be photosensitive, functioning as a light sensor. In humans the gland is not light-sensitive, but it remains integrated into the body’s light-entrained circadian system through retinal signaling to the brain’s clock.