Epithalamus and Pineal Gland
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Direct answer
The pineal gland is the small midline appendage, five to eight millimetres across, that projects backwards from the posterior wall of the third ventricle above the superior colliculi, and is the visible hallmark of the epithalamus — which also includes the habenular nuclei, the stria medullaris thalami and the posterior commissure. Joined to the diencephalon by a stalk crossed above by the habenular commissure and below by the posterior commissure, its pinealocytes convert serotonin to melatonin under retinal–suprachiasmatic control and broadcast the circadian night signal. It calcifies with age into radiopaque "brain sand", so on a skull radiograph or CT it serves as a midline marker, while enlargement or shift flags a space-occupying lesion.
What you must remember
- Position and attachments: in the quadrigeminal cistern between the two superior colliculi, posterior to the third ventricle; the stalk's upper lamina contains the habenular commissure, the lower lamina the posterior commissure, and between them lies the pineal recess of the third ventricle.
- Structure: lobulated by septa, with pinealocytes arranged in rosettes around capillaries and interstitial glial cells; no neurons — it is an endocrine organ, not a brain "gland" in the neural sense.
- Hormone: melatonin, formed from serotonin by N-acetylation and methylation; secretion rises in darkness, entrained by the retinohypothalamic–suprachiasmatic–sympathetic pathway reaching the gland.
- Calcification: acervuli (brain sand) appear from childhood onwards and are near-universal in later life; a midline, radio-opaque spot on plain films — its displacement sideways is evidence of a hemispheric mass.
- Habenular nuclei: lie at the stalk's root, receive the stria medullaris thalami, and project through the habenulointerpeduncular tract (fasciculus retroflexus) to the midbrain interpeduncular nucleus — a limbic-to-midbrain relay.
- Parinaud syndrome: pineal region tumours compress the pretectal area and superior colliculi — vertical gaze palsy (especially upward), convergence–retraction nystagmus, light-near dissociation and eyelid retraction.
- Hydrocephalus mechanism: the tumour mass lies directly over the cerebral aqueduct; obstruction causes non-communicating hydrocephalus with headache, vomiting and papilloedema.
- Tumour types: germinoma is the commonest pineal region tumour, with a predilection for young males; teratoma, pineocytoma and pineoblastoma follow — young patients with headache and up-gaze palsy.
Reading a midline calcification
A skull radiograph of a head-injured man shows a calcified speck two millimetres off midline — the report that follows is anatomy, not luck. The pineal calcifies in most adults, and because it hangs from the midline roof of the third ventricle, displacement can only mean a mass pushing it across; a shift of more than about three millimetres is the conventional threshold for significance. The same gland's neighbourhood explains Parinaud syndrome: a germinoma enlarging in the quadrigeminal cistern compresses the pretectal nucleus and the vertical gaze centre of the superior colliculi below it, so the teenager cannot look up, his eyes converge and retract on attempted up-gaze, and pupils react to near effort but not light. Pressing forward against the aqueduct, the same tumour dams the CSF, dilating both lateral ventricles. The diagnostic reasoning never leaves this square centimetre of anatomy above the tectum.
Where students slip
Students call the pineal "part of the thalamus" — it belongs to the epithalamus with the habenulae, and its two commissures are the load-bearing detail: habenular above, posterior below, pineal recess between. The second slip is functional folklore: melatonin is secreted in darkness, not light, and its established role is circadian entrainment and reproductive rhythm modulation — claims about it as a general "anti-ageing hormone" belong to marketing, not Gray's. In viva, a pineal tumour's up-gaze palsy is explained by compression of the pretectal area and superior colliculi, never by any lesion inside the gland itself.
Frequently asked questions
Where is the pineal gland located and how is it attached?
It projects backwards from the posterior wall of the third ventricle into the quadrigeminal cistern, above the superior colliculi. Its stalk is crossed superiorly by the habenular commissure and inferiorly by the posterior commissure, with the pineal recess of the third ventricle between them.
What is the function of melatonin?
Synthesised from serotonin in darkness, melatonin transmits circadian timing to the body — sleep promotion and entrainment of reproductive rhythms. Its secretion is governed by the suprachiasmatic nucleus through a sympathetic relay to the gland.
Why is pineal calcification clinically useful?
The calcified pineal is radio-opaque and normally midline, so on plain radiographs and CT it acts as a free midline marker; lateral displacement indicates a hemispheric space-occupying lesion. Calcification itself is a normal ageing change, not pathology.
What is Parinaud syndrome and which structures are compressed?
Vertical gaze palsy (predominantly loss of up-gaze), convergence–retraction nystagmus, light-near dissociation and lid retraction from compression of the pretectal area and superior colliculi by a pineal region tumour. It is the classic dorsal midbrain (pretectal) syndrome.
What are the components of the epithalamus?
The pineal gland, the habenular nuclei with the habenular commissure, the stria medullaris thalami and the posterior commissure. The habenulae relay limbic input to the midbrain through the fasciculus retroflexus.