Pineal gland (epiphysis)

Location and structure

The pineal gland is a neuroendocrine organ that comprises a part of the epithalamus, one of the three divisions of the diencephalon. Other components of the epithalamus are the stria medullaris, habenular nuclei, posterior commissure and paraventricular nuclei.

The pineal gland, also called the pineal body, develops as an outward projection from the posterior wall of the third ventricle, below the splenium of corpus callosum. It sits in the groove between the two superior colliculi, and is bilaterally related to the posterior aspects of the two thalami.

The pineal gland is attached to the rest of the brain by the pineal stalk, which divides into two laminae, the inferior and superior laminae. The inferior and superior laminae contain the posterior commissure and the habenular commissure, respectively. The space between the laminae is filled with an extension of the cavity of the third ventricle known as the pineal recess of the third ventricle.

The parenchyma of the pineal gland mostly consists of cords and clusters of pinealocytes, and supporting astrocyte-like neuroglial cells. The pinealocytes are highly modified neurons that secrete the hormone melatonin into the surrounding network of fenestrated capillaries. The neuroglial cells are the main component of the pineal stalk.

Another structural component of the pineal gland are the corpora arenacea or brain sand, which are deposits of calcium, phosphates and carbonates that form multilaminar corpuscles. These accumulate with age and are mostly a byproduct of secretory activity. The corpora arenacea are clinically significant because they are sometimes used as landmarks during radiologic examinations. They also help the microscopic identification of the pineal gland, which is otherwise histologically unremarkable.

Hormones and function

The pineal gland is classified as one of the secretory circumventricular organs of the brain, meaning that it has direct access to the bloodstream via fenestrated capillaries. The most important function of the pineal gland is to gather information about the state of the day-night cycle from the outside environment and use this information to modulate its production and release of melatonin. Besides melatonin, the pineal gland also produces several other indolamine and polypeptide hormones, however their functions are poorly understood.

The hormones of the pineal gland have a highly regulatory importance in which they influence the activity of other endocrine glands, namely the pituitary gland, endocrine pancreas, adrenal gland, parathyroid gland and the gonads. The effects of the pineal gland on these glands are mostly inhibitory, by reducing the synthesis and release of the hormones produced by these glands.

To learn more about the basics of hormones and the endocrine system, check out our diagrams, videos and quizzes in the study unit below.

Melatonin: Pineal gland hormone

Melatonin is a serotonin-derived hormone that is secreted rhythmically by the pinealocytes of the pineal gland. It has two primary functions; modulation of the sleep-wake cycle by controlling the circadian rhythm, and regulation of reproductive system development.

The main factor in the production of melatonin is the amount of light that falls on the retina. When there is little to no light present, the production of melatonin is stimulated, while the presence of bright light inhibits its production. This means that its concentrations in the blood rise during the dark and fall during the day. This serves as an internal mechanism for the body to gather information about night-length, and in turn to create the circadian rhythm of the individual.