The pineal gland consists mainly of pinealocytes, but four other cell types have been identified.
|pinealocytes||The pinealocytes consist of a cell body with 4-6 processes emerging. They produce and secrete melatonin. The pinealocytes can be stained by special silver impregnation methods.|
|interstitial cells||Interstitial cells are located between the pinealocytes.|
|perivascular phagocyte||Many capillaries are present in the gland, and perivascular phagocytes are located close to these blood vessels. The perivascular phagocytes are antigen presenting cells.|
|pineal neurons||In higher vertebrates neurons are located in the pineal gland. However, these are not present in rodents.|
|peptidergic neuron-like cells||In some species, neuronal-like peptidergic cells are present. These cells might have a paracrine regulatory function.|
The pineal gland receives a sympathetic innervation from the superior cervical ganglion. However, a parasympathetic innervation from the sphenopalatine and otic ganglia is also present. Further, some nerve fibers penetrate into the pineal gland via the pineal stalk (central innervation). Finally, neurons in the trigeminal ganglion innervates the gland with nerve fibers containing the neuropeptide, PACAP. Human follicles contain a variable quantity of gritty material, called corpora arenacea (or "acervuli", or "brain sand"). Chemical analysis shows that they are composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate. Recently, calcite deposits have been described as well. Calcium and phosphorous deposits in the pineal gland have been linked with aging.
In some vertebrates, exposure of the pineal to light can directly set off a chain reaction of enzymatic events which regulate circadian rhythms. In humans and other mammals, this function is served by the retinohypothalamic system that sets the rhythm within the suprachiasmatic nucleus. Cultural and social interactions produce exposures to artificial light that influence the setting of the suprachiasmatic clock.
The pineal gland is large in children, but shrinks at puberty. It appears to play a major role in sexual development, hibernation in animals, metabolism, and seasonal breeding. The abundant melatonin levels in children is believed to inhibit sexual development, and pineal tumors have been linked with precocious puberty. When puberty arrives, melatonin production is reduced. Calcification of the pineal gland is typical in adults.
Pineal cytostructure seems to have evolutionary similarities to the retinal cells of chordates. Modern birds and reptiles have been found to express the phototransducing pigment melanopsin in the pineal gland. Avian pineal glands are believed to act like the suprachiasmatic nucleus in mammals.
Studies suggest that in rodents the pineal gland may influence the actions of recreational drugs, such as cocaine, and antidepressants, such as fluoxetine (Prozac), and its hormone melatonin can protect against neurodegeneration.
The secretory activity of the pineal gland has only relatively recently become understood. Historically, its location deep in the brain suggested to philosophers that it possessed particular importance. This combination led to its being a "mystery" gland with myth, superstition and metaphysical theories surrounding its perceived function.
René Descartes, who dedicated much time to the study of the pineal gland, called it the "seat of the soul". He believed that it was the point of connection between the intellect and the body. This was in part because of his belief that it is unique in the anatomy of the human brain in being a structure not duplicated on the right and left sides. This observation is not true, however; under a microscope one finds the pineal gland is divided into two fine hemispheres. Another theory was that the pineal operated as a valve releasing fluids, thus the position taken during deep thought, with the head slightly down meeting the hand, was an allowance for the opening of these 'valves'.
Writers such as Madame Blavatsky and Alice Bailey, considered early proponents of the new age movement, use the pineal-eye as a key element in their spiritual world-view...(see Alice Bailey: "A Treatise on White Magic", Madame Blavatsky: "The Secret Doctrine")
The notion of a 'pineal-eye' is also crucial to the philosophy of the seminal French writer Georges Bataille, which is analyzed at length by literary scholar Denis Hollier in his study Against Architecture.
"But the head has a hole in it. The pineal eye, the organ of not-knowing, is the undoing of science. If science thought up man, the pineal eye unthinks him, spends him extravagantly, makes him lose the reserve in which, at the summit, from his head position, he was guarding himself.
In this work Hollier discusses how Bataille uses the concept of a 'pineal-eye' as a reference to a blind-spot in Western rationality.
"When I carefully seek out, in deepest anguish, some strange absurdity, an eye opens at the top, in the middle of my skull. This eye opening up onto the sun in all its glory, to contemplate it in its nakedness, privately, is not the work of my reason: it is a cry escaping from me. For at the moment when the flash blinds me I am the splintering brilliance of a shattered life, and this life - agony and vertigo - opening up onto an infinite void, bursts and exhausts itself all at once in this void.
In Fear and Loathing in Las Vegas by Hunter S. Thompson, Raoul Duke and his attorney amuse themselves by tricking a clueless police officer into believing that gangs of drug freaks in Los Angeles routinely decapitate suburban families to harvest their pineal glands. Later in the novel, the attorney frightens an adrenochrome-crazed Duke with various increasingly fantastical health defects that might result, should one eat extract of pineal gland as a psychoactive.