In humans, the cycles of sleep and wakefulness usually mimic the rhythm of light and dark. One contributing factor to this phenomenon is the activity of a small, cone-like, flattened body called the pineal gland or the epiphysis cerebri. This endocrine gland, located deep within the brain in the posterior cranial fossa, is responsible for releasing hormones that alter the state of wakefulness and sleep. This article will cover the gross anatomy, histology, vascular supply, and some clinical implications of the gland.
The epiphysis cerebri is a reddish-grey, approximately 5 – 8 mm long, pine cone-like structure that is located in the diencephalic part of the prosencephalon (forebrain). The gland was formed as an outward growth of the roof of the third ventricle. Therefore, the gland rests between the posterior aspects of the thalami as it projects posteriorly from the wall of the third ventricle. It’s attachment to either half of the brain is by the Habenular commissure and trigone superiorly, and the posterior commissure inferiorly. The Habenular and posterior commissures are a part of the pineal stalk. The Habenular commissure is a part of the superior lamina of the stalk, while the posterior commissure is a part of the inferior lamina. The space between the laminae is known as the pineal recess. It communicates anteriorly with the hypothalamic sulcus and the third ventricle.
The pineal gland is encased by pia mater and lobulated by its connective tissue septae that projects into the gland. Within the epiphysis cerebri, there are pinealocytes and neuroglia cells. The pinealocytes account for approximately 95% of the cellular content of the gland. They are irregularly shaped with peripheral processes, and lightly staining large round nuclei. Pinealocytes are primarily concerned with the photo-regulated production of melatonin. This hormone works with the body’s circadian rhythm (which is controlled by the suprachiasmatic nucleus of the hypothalamus) to regulate the cycle of sleep and wakefulness. Additionally, some researchers believe that melatonin may alter sexual development in humans, contribute to thermoregulation, and cellular metabolism.
There are also the corpora arenacea (brain sand) bodies present within the gland. Calcification of these bodies is a common occurrence with increasing age. As a result, they appear as radiographic opacities on plain film radiography and can, therefore, be used as landmarks.
Several surrounding structures are useful in grossly identifying the epiphysis cerebri. On a coronal section of the brain (vertically through the cerebellar hemispheres and pons), the following structural boundaries can be appreciated:
- Superiorly, the splenium of the corpus callosum is observed,
- Superolaterally, the choroid plexus of the third ventricle is seen bilaterally,
- Inferiorly, the superior and inferior colliculi are seen.
In the sagittal section (along the longitudinal cerebral fissure):
- the quadrigeminal plate (in addition to the colliculi) is also readily observed inferiorly,
- the Habenular commissure and the thalamus are seen in anterosuperior relations to the gland,
- the great cerebral vein of Galen has a posterosuperior relation to the gland,
- and the posterior commissure, the cerebral peduncle and the cerebral aqueduct of Sylvius lies anteroinferiorly.
Bilaterally, the vertebral arteries arise from the first part of the subclavian artery and travels cranially, receiving anastomosing tributaries from the ascending cervical artery (a branch of the inferior thyroid artery) along the way. The left and right vertebral arteries anastomose at the level of the root of the hypoglossal nerve (CN XII), to form the basilar artery (which runs along the pons). At the level of the root of the oculomotor nerve (CN III), the basilar artery bifurcates to form the posterior cerebral artery and the superior cerebellar artery. The former vessel forms an anastomosis with the internal carotid artery by way of the posterior communicating artery. After the anastomosis, it gives off a posterior medial choroidal artery that drains to the choroid plexus of the third ventricle, which provides the pineal gland with oxygenated blood.
The internal cerebral veins drain deoxygenated blood from the pineal gland and join with the basal vein of Rosenthal and the posterior mesencephalic vein to form the great cerebral vein of Galen. After receiving tributaries from the dorsal vein of the corpus callosum and the inferior sagittal sinus, the great cerebral vein of Galen becomes the straight sinus. This sinus then terminates at the confluence of sinuses, where it is joined by the superior sagittal and left and right transverse sinuses. Eventually, these sinuses drain to the internal jugular vein, which joins the subclavian vein to become the brachiocephalic vein.