Pineal gland: where the pineal body is and how it makes melatonin

What it is

The pineal body (gland) is a small pinecone-shaped endocrine gland in the brain.

Location

It lies in the midline at the back of the third ventricle, attached by a stalk to its posterior wall, just above the superior colliculi.

Function

It secretes melatonin, which helps regulate circadian rhythms.

Body system

Endocrine · single, midline structure

FMA ID

FMA62033 (Foundational Model of Anatomy)

Key facts

• It can calcify with age and become visible on X-ray.
• Sympathetic input comes from the superior cervical ganglion.

Clinical relevance

Pineal tumours can press on the tectum (Parinaud syndrome) and the cerebral aqueduct, causing hydrocephalus.

"Pineal gland", "pineal body" and "pineal organ" are names for the same unpaired structure. The word pineal refers to its shape: it is shaped like a pine cone. It is a midline structure between the two cerebral hemispheres. (TeachMeAnatomy: The Pineal Gland.)

The gland belongs to the epithalamus, a small part of the diencephalon. The diencephalon lies deep beneath the cerebrum and forms the walls of the third ventricle. (OpenStax Anatomy and Physiology 2e, 13.2: The Central Nervous System.)

The pineal gland is a small cone-like structure that projects backwards from the posterior wall of the third ventricle, above the quadrigeminal plate (the tectum of the midbrain). It rests in the groove between the two superior colliculi and is related on each side to the back of the thalamus. (Kenhub: Pineal gland.)

Its stalk splits into a superior and an inferior lamina. The superior lamina contains the habenular commissure and the inferior lamina contains the posterior commissure. Between the two laminae, the cavity of the third ventricle extends into the stalk as the pineal recess. (Kenhub: Pineal gland.)

Above and behind the gland run the internal cerebral veins and the great cerebral vein (of Galen), under the splenium of the corpus callosum. Below and behind it is the quadrigeminal cistern, a CSF space. The posterior wall of the third ventricle is formed, from above down, by the habenular commissure, the pineal gland and the posterior commissure. (Radiopaedia: Pineal gland.)

OpenStax describes the position simply: inferior to, but somewhat posterior to, the thalamus. (OpenStax Anatomy and Physiology 2e, 17.7: The Pineal Gland.)

The gland typically measures about 7 by 6 by 3 mm, and sizes up to 14 mm have been described as normal. TeachMeAnatomy gives a length of about 6 mm. (Radiopaedia: Pineal gland.)

It contains two main cell types: pinealocytes, which secrete hormones, and glial cells, which support them. (TeachMeAnatomy: The Pineal Gland.)

The pinealocytes are modified neurons arranged in cords and clusters, with astrocyte-like glial cells between them. With age, the gland builds up deposits of calcium phosphates and carbonates called corpora arenacea (brain sand). The pineal gland is a circumventricular organ: its capillaries are fenestrated (leaky), so it has direct access to the bloodstream rather than sitting behind the usual blood-brain barrier. (Kenhub: Pineal gland.)

Arterial blood comes from fine branches of the posterior choroidal arteries, and venous blood drains through small veins into the great cerebral vein of Galen. (Radiopaedia: Pineal gland.)

TeachMeAnatomy describes the supply as profuse, second only to the kidney, with the posterior choroidal branches of the posterior cerebral artery as the main source and drainage into the internal cerebral veins. (TeachMeAnatomy: The Pineal Gland.)

The human pineal gland is controlled by a long, indirect sympathetic pathway. Darkness is detected by the retina, and the signal travels in the retinohypothalamic tract to the suprachiasmatic nucleus (SCN) of the hypothalamus, the body's master clock. The SCN signals the paraventricular nucleus, whose fibres descend to the intermediolateral column of the thoracic spinal cord. From there, preganglionic fibres ascend to the superior cervical ganglia, and postganglionic sympathetic fibres reach the pineal gland and release noradrenaline (norepinephrine) onto pinealocytes at night, which stimulates melatonin production. (Lumsden SC, Clarkson AN, Cakmak YO. Neuromodulation of the pineal gland via electrical stimulation of its sympathetic innervation pathway. Front Neurosci. 2020.)

Kenhub describes the same route: retina to suprachiasmatic nucleus, then paraventricular nucleus, the intermediolateral column of the spinal cord and postganglionic fibres of the cervical sympathetic chain. (Kenhub: Pineal gland.)

Pinealocytes secrete melatonin, an amine hormone made from serotonin. Secretion varies with the amount of light received from the environment: light stimulates the retinas, and the signal reaches the gland through a region of the hypothalamus called the suprachiasmatic nucleus. Melatonin may influence the body's circadian rhythms, which affect sleepiness, appetite and body temperature. (OpenStax Anatomy and Physiology 2e, 17.7: The Pineal Gland.)

The chemical steps take place inside the pinealocyte. Tryptophan is taken up from the blood and converted to 5-hydroxytryptophan, then to serotonin. The enzyme aralkylamine N-acetyltransferase (AANAT) turns serotonin into N-acetylserotonin, and hydroxyindole-O-methyltransferase (HIOMT) converts that into melatonin. (Lumsden SC, Clarkson AN, Cakmak YO. Neuromodulation of the pineal gland via electrical stimulation of its sympathetic innervation pathway. Front Neurosci. 2020.)

Melatonin may also affect reproductive development. Children have higher melatonin levels than adults, which may prevent the release of gonadotropins from the anterior pituitary and so delay the onset of puberty. Jet lag happens when travel across several time zones puts the light-dark cycle out of step with the body clock; some travellers take melatonin to help them sleep. (OpenStax Anatomy and Physiology 2e, 17.7: The Pineal Gland.)

The pineal gland arises in about the seventh week of gestation as a thickening of the ependyma at the posterior-most part of the third ventricle. (Radiopaedia: Pineal gland.)

Calcification is common. On lateral skull X-rays it is seen in about 50 to 70% of adults but rarely below the age of 10. On CT, it is found in about 1% of children under 6, about 39% at ages 8 to 14 and about 62% of adults, although figures vary between studies. (Radiopaedia: Pineal gland.)

Because it commonly calcifies in middle age, the gland becomes visible on radiographs and CT scans. (TeachMeAnatomy: The Pineal Gland.)

The most common pineal tumour is a germ cell tumour. Symptoms include headache, nausea and vomiting. Pressure on the superior colliculi causes Parinaud syndrome, in which the person cannot move the eyes upwards. Compression of the cerebral aqueduct, which lies just below the gland, can block CSF flow and cause hydrocephalus. In children, a tumour that destroys the gland can cause precocious (early) puberty. (TeachMeAnatomy: The Pineal Gland.)

Kenhub lists pineal tumours, craniopharyngiomas, injury to the sympathetic nerve supply and rare congenital disorders among causes of pineal dysfunction, with changes in circadian rhythm and insomnia as the main symptoms. Low melatonin is associated with precocious puberty, and high melatonin with delayed puberty. (Kenhub: Pineal gland.)

The cerebral aqueduct begins just below the posterior commissure, the lowest part of the third ventricle's posterior wall, directly beneath the pineal gland. This close relationship is why a mass in the pineal region can obstruct CSF drainage from the third ventricle. (Radiopaedia: Third ventricle.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Hair of head · skin
• Superior colliculus · nervous
• Posterior commissure · nervous
• Brachium of inferior colliculus · nervous
• Cerebral aqueduct · nervous
• Inferior colliculus · nervous
• Commissure of fornix of forebrain · nervous
• Choroid plexus of cerebral hemisphere · nervous
• Midbrain · nervous
• Medial geniculate body · nervous