Pineal gland

At a glance

English

Pineal gland

Latin (TA2)

Glandula pinealis

System

Endocrine gland

Region

Whole body

Side

Single, on the midline

About the pineal gland

The pineal gland, or pineal body, is an unpaired midline structure deep in the brain. It belongs to the epithalamus of the diencephalon and hangs backwards from the roof of the third ventricle, lying in a notch between the two superior colliculi of the midbrain. Reddish-grey and no bigger than a rice grain, five to eight millimetres long, it is named for its resemblance to a pine cone. Its base is a short stalk with two laminae: the upper one carries the habenular commissure and the lower one the posterior commissure. Between them the third ventricle sends a little pouch, the pineal recess, into the stalk.

Around it lie important neighbours. The splenium of the corpus callosum overhangs it from above, the internal cerebral veins join the great cerebral vein just above and behind it in the quadrigeminal cistern, and the cerebral aqueduct begins directly below. A thin pial capsule covers the gland and sends septa in to divide it into lobules of pinealocytes, the secretory cells, supported by astrocyte-like glial cells.

The pineal is one of the circumventricular organs, which lack a full blood-brain barrier, and its blood flow is very high for its size. Its main product, melatonin, is made from serotonin chiefly at night. The control pathway starts in the retina, whose light-sensitive ganglion cells signal the suprachiasmatic nucleus of the hypothalamus, the body's master clock. From there a chain of neurons runs through the paraventricular nucleus and the spinal cord to the superior cervical ganglion, whose sympathetic fibres reach the gland. Darkness increases secretion and light suppresses it, so the melatonin rhythm helps time sleep and other daily and seasonal cycles.

From early adulthood, and sometimes in childhood, the gland accumulates calcified granules known as corpora arenacea or brain sand. These show on skull X-rays and CT scans and once served as a midline marker; a shifted pineal suggests a mass pushing the brain aside. Tumours in this region can press on the tectum of the midbrain, causing Parinaud syndrome with loss of upward gaze, or block the aqueduct and cause hydrocephalus. Pineal cysts are a common, usually harmless finding on MRI.

Function and supply

Function

Produces melatonin, mainly at night, translating the light-dark cycle into a hormonal signal that helps regulate sleep and circadian and seasonal rhythms.

Nerve supply

Postganglionic sympathetic fibres from the superior cervical ganglion, relaying signals from the suprachiasmatic nucleus; some central fibres also enter through the stalk.

Blood supply

Choroidal twigs of the posterior cerebral artery, especially the medial posterior choroidal arteries; veins drain into the internal cerebral veins and the great cerebral vein.