Pineal body

Where is the pineal body?

It belongs to the epithalamus and sits at the back of the third ventricle, below and behind the thalamus. (OpenStax A&P 2e, 17.7 and 13.2; StatPearls, Physiology, Pineal Gland)

This unpaired gland sits between the two thalami, behind the habenular commissure, near the superior and inferior colliculi behind the third ventricle. (StatPearls, Physiology, Pineal Gland)

The thalamus and the habenular commissure lie in front of and above it, the splenium of the corpus callosum above it, and the colliculi below it. Cerebrospinal fluid reaches it through the pineal recess of the third ventricle, a small pouch that extends into its stalk. (StatPearls, Physiology, Pineal Gland and Ventricular System)

What does the pineal body do?

Light sets its output. Light on the retina sends a signal to the suprachiasmatic nucleus of the hypothalamus and on, by way of the spinal cord, to the pineal gland, which then makes less melatonin, and blood melatonin falls. (OpenStax A&P 2e, 17.7; StatPearls, Physiology, Pineal Gland and Nucleus Suprachiasmatic)

As light fades in the evening, melatonin production rises and promotes sleep. Blood melatonin is low by day and peaks at night, and secretion lasts as long as the dark does. (OpenStax A&P 2e, 17.7; StatPearls, Physiology, Pineal Gland and Melatonin)

Melatonin helps set the circadian rhythm of sleep. A pineal tumor in childhood can upset other hormones and bring on early pseudopuberty. (OpenStax A&P 2e, 17.7; StatPearls, Physiology, Pineal Gland)

The enzyme serotonin N-acetyltransferase controls the rate-limiting step from serotonin to melatonin, and its level is low in daylight. (StatPearls, Physiology, Pineal Gland)

What is the pineal body made of, and how does it form?

It is shaped like a pine cone, about 0.8 cm long and about 0.1 g in an adult, and made of pinealocytes and supporting cells. (StatPearls, Physiology, Pineal Gland)

It holds calcium deposits called corpora arenacea, or brain sand. They calcify more with age and show on X-rays, where the gland serves as a landmark. (StatPearls, Physiology, Pineal Gland)

It develops from the back of the roof of the third ventricle, where the lining thickens in the seventh week of development. (StatPearls, Physiology, Pineal Gland)

What lies around the pineal body, and what supplies it?

The cerebral aqueduct and the posterior commissure lie in front of and below it. A pineal tumor or cyst can compress the aqueduct, block the flow of cerebrospinal fluid and cause hydrocephalus. (StatPearls, Physiology, Pineal Gland)

Its blood comes from choroidal branches of the posterior cerebral artery and drains to the internal cerebral vein. (StatPearls, Physiology, Pineal Gland)