Pinealocytes are the main cells contained in the pineal gland, located behind the third ventricle and between the two hemispheres of the brain.

The primary function of the pinealocytes is the secretion of the hormone melatonin, important in the regulation of circadian rhythms.

[1] In humans, the suprachiasmatic nucleus of the hypothalamus communicates the message of darkness to the pinealocytes, and as a result, controls the day and night cycle.

These Type 1 cells have been identified through research to have a round or oval shape and a diameter ranging from 7–11 micrometers.

[6] Research has also shown that Type 1 pinealocytes contain the neurotransmitter serotonin, which later is converted to melatonin, the main hormone secreted by the pineal gland.

[3][4] Researchers have also indicated the presence of several photoreceptor proteins found in the retina in the pinealocytes in chicken and fish.

[3] Genetic evidence demonstrates that phototransduction genes expressed in the photoreceptors of the retina are also present in pinealocytes.

The main molecular control mechanism that exists for melatonin secretion in vertebrates is the enzyme AANAT (arylalkylamine N-acetyltransferase).

The SCN stimulates the release of norepinephrine from sympathetic nerve fibers from the superior cervical ganglia that synapse with the pinealocytes.

Because the release of norepinephrine from the nerve fibers occurs at night, this system of regulation maintains the body’s circadian rhythms.