Third Ventricle

The third ventricle is a narrow, slit-like midline cavity in the diencephalon of the brain, situated between the two halves of the thalamus and hypothalamus. It is part of the ventricular system and contains cerebrospinal fluid (CSF), which flows through it en route from the lateral ventricles to the fourth ventricle. The third ventricle plays a crucial role in CSF circulation and is bordered by multiple neuroanatomically important structures.

Location

The third ventricle is located in the midline of the brain, within the diencephalon, between the right and left thalamus superiorly and the hypothalamus inferiorly. It lies just above the midbrain and below the corpus callosum. It is bordered:

• Laterally: By the medial surfaces of the thalami and hypothalami
• Roof: Formed by the tela choroidea and fornix
• Floor: Formed by the hypothalamic structures including the optic chiasm, infundibulum, and mammillary bodies
• Anterior wall: Formed by the lamina terminalis, anterior commissure, and columns of the fornix
• Posterior wall: Includes the pineal gland and posterior commissure

Communication with Other Ventricles

• Interventricular foramina (of Monro): Connect each lateral ventricle to the third ventricle anteriorly
• Cerebral aqueduct (of Sylvius): Connects the third ventricle to the fourth ventricle posteriorly

Features and Recesses

The third ventricle contains several small recesses that correspond to adjacent neuroanatomical structures:

• Supraoptic recess: Located above the optic chiasm
• Infundibular recess: Extends downward into the infundibulum and pituitary stalk
• Pineal recess: Extends posteriorly into the stalk of the pineal gland
• Suprapineal recess: Located above the pineal gland and posterior commissure

Interthalamic Adhesion

The two thalami may be connected across the midline by a small gray matter bridge called the interthalamic adhesion (massa intermedia), which passes through the third ventricle. It is not a true commissure and is absent in a significant percentage of individuals.

Choroid Plexus and CSF Production

The third ventricle contains part of the choroid plexus, a vascular structure lined with ependymal cells that produces cerebrospinal fluid. The choroid plexus in the third ventricle lies along its roof, within the tela choroidea. CSF produced here contributes to the overall volume circulating through the ventricular system and subarachnoid space.

Blood Supply

• Anterior choroidal artery (branch of internal carotid): Supplies anterior aspects
• Posterior choroidal arteries (branches of the posterior cerebral artery): Supply the roof and posterior regions of the third ventricle and its choroid plexus

Venous drainage is primarily through the internal cerebral veins and basal veins, draining into the vein of Galen and straight sinus.

Development

The third ventricle originates from the cavity of the diencephalon in the embryonic forebrain. As the diencephalon forms the thalamus, hypothalamus, and epithalamus, the third ventricle remains as the central cavity between these structures. Its communication with the lateral ventricles and cerebral aqueduct is established early during neural tube development.

Clinical Significance

• Colloid cyst: A benign cyst that arises near the foramen of Monro can obstruct CSF flow and cause acute hydrocephalus, potentially life-threatening if not treated.
• Hydrocephalus: Obstruction of CSF flow through the third ventricle (due to mass, congenital stenosis, or adhesions) can lead to ventricular enlargement and increased intracranial pressure.
• Tumors: Tumors in the pineal or hypothalamic region may compress or invade the third ventricle, disrupting CSF flow or endocrine function.
• Third ventriculostomy: A surgical procedure that creates an opening in the floor of the third ventricle to bypass obstruction and restore CSF circulation, typically used in non-communicating hydrocephalus.

Imaging

MRI is the preferred imaging modality for evaluating third ventricle size, shape, and associated pathology. Enlarged third ventricles may indicate obstructive hydrocephalus or brain atrophy. Sagittal and axial MRI views provide clear visualization of the third ventricle and surrounding structures. CT scans may reveal masses or hemorrhages compressing or distorting the ventricle.