Pineal gland

Glandula pinealis; corpus pineale

Also known as: epiphysis cerebri, pineal body

A pine-cone-shaped midline gland (≈5–8 mm) in the epithalamus that secretes melatonin at night, translating the light–dark cycle into a hormonal signal of darkness.

Overview

Size≈5–8 mm long

Mass≈100–180 mg

Melatonin peak≈02:00–04:00

The pineal gland projects backwards from the roof of the third ventricle, attached by a stalk containing the habenular commissure (above) and posterior commissure (below), and lies in the groove between the superior colliculi, beneath the splenium of the corpus callosum. It is outside the blood–brain barrier and has one of the highest blood flows per gram in the body.

Its parenchyma consists of pinealocytes (modified photoreceptor-lineage neuroendocrine cells) and interstitial astrocyte-like glia. Unlike in lower vertebrates, the human pineal is not directly light-sensitive; it is controlled by a multisynaptic pathway: retina → retinohypothalamic tract → suprachiasmatic nucleus → paraventricular nucleus → intermediolateral cell column of the upper thoracic cord → superior cervical ganglion → postganglionic sympathetic fibres (nervi conarii). In darkness these fibres release norepinephrine onto β1-adrenergic receptors, activating the enzyme AANAT and melatonin synthesis; light suppresses it.

From adolescence the gland accumulates calcium and magnesium phosphate concretions (corpora arenacea, "brain sand"), which make it a useful midline marker on skull radiographs and CT.

Location

Epithalamus, posterior to the third ventricle, above the superior colliculi and below the splenium of the corpus callosum.

Anatomy

Innervation

Postganglionic sympathetic fibres from the superior cervical ganglia (nervi conarii).

Arterial supply

Medial posterior choroidal arteries (branches of the posterior cerebral artery); drains to the internal cerebral veins and great cerebral vein (of Galen).

Histology

Lobules of pinealocytes (pale nuclei, long processes ending near capillaries) and interstitial glial cells; calcified concretions (corpora arenacea) increase with age.

Function

Secretes melatonin at night, signalling darkness and day length to the body.

Helps entrain circadian rhythms and sleep timing.

In seasonal mammals, regulates reproduction according to photoperiod (role in humans uncertain; may contribute to the timing of puberty).

Secretions

Melatonin (N-acetyl-5-methoxytryptamine), synthesised from tryptophan → serotonin → N-acetylserotonin (AANAT, rate-limiting) → melatonin (ASMT/HIOMT).

Clinical significance

Pineal region tumours and Parinaud syndrome

Germinomas (the commonest), pineoblastomas and pineocytomas compress the dorsal midbrain (tectum) and the cerebral aqueduct, causing Parinaud syndrome (paralysis of upward gaze, convergence–retraction nystagmus, light–near dissociation of the pupils, lid retraction) and obstructive hydrocephalus. hCG-secreting germ-cell tumours can cause precocious puberty in boys.

Pineal cysts

Small benign cysts are a common incidental MRI finding and usually need no treatment.