Pineal gland

The pineal gland (also called the pineal body or epiphysis) is a small endocrine gland in the brain. It is located near the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join. A recent review of the pineal and its secreted hormone, melatonin, is available.

Location

The pineal gland is a reddish-gray body about the size of a pea (8 mm in humans) located just rostro-dorsal to the superior colliculus and behind and beneath the stria medullaris, between the laterally positioned thalamic bodies. It is part of the epithalamus.

The pineal gland is a midline structure, and is often seen in plain skull X-rays, as it is often calcified.

Structure and composition

The pineal gland consists mainly of pinealocytes, but four other cell types have been identified: interstitial cells, perivascular phagocyte, pineal neurons and peptidergic neuron-like cells.

The pineal body consists in humans of a lobular parenchyma of pinealocytes surrounded by connective tissue spaces. The glands are covered on the surface by a pial capsule. The pinealocytes consist of a cell body from where 4-6 processes emerge. The pinealocytes can be stained by special silver impregnation methods. Between the pinealocytes are insterstitial cells located. Many capillaries are present in the gland, and close to these blood vessels are perivascular phagocytes located. The perivascular phagocytes are antigen presenting cells. In higher vertebrates neurons are located in the pineal gland. However, these are not present in rodents. In some species, neuronal-like peptidergic cells are present. These cells might have a paracrine regulatory function. In humans follicles containing a variable quantity of gritty material, called brain sand, acervuli, or corpora arenacea, are present. Chemical analysis of acervuli show that they are composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate. (Bocchi & Valdre, 1993). Recently, calcite deposits have been described as well (Baconnier et al., 2002).

Pinealocytes in lower vertebrate animals have a strong resemblance to the photoreceptor cells of the eye. Some evolutionary biologists believe that the vertebrate pineal gland can be evolutionarily traced to a proto-eye structure in early vertebrate organisms (Klein, 2004). In birds, the pineal gland is located on the surface of the brain, directly under the skull and contains the photoreceptors to regulate their biological clockMoore RY, Heller A, Wurtman RJ, Axelrod J. Visual pathway mediating pineal response to environmental light. Science 1967;155(759):220–3. PMID 6015532. 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. Evidence for a role for opsin-related light-sensing compounds in the skin of mammals is presently controversial.

Function

The pineal gland was originally believed to be a "vestigial remnant" of a larger organ (much as the appendix is now thought to be a vestigial digestive organ). It was only after the 1960's that scientists discovered that the pineal gland is responsible for the production of melatonin, which is regulated in a circadian rhythm. Melatonin is a derivative of the amino acid tryptophan, which also has other functions in the Central Nervous System. The production of melatonin by the pineal gland is stimulated by darkness and inhibited by light (Axelrod, 1970). The retina detects the light, and directly signals and entrains the suprachiasmatic nucleus (SCN). Fibers project from the SCN to the paraventricular nuclei (PVN), which relay the circadian signals to the spinal cord and out via the sympathetic system to superior cervical ganglia (SCG), and from there into the pineal gland.

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. When puberty arrives, melatonin production is reduced.

Pineal cytostructure seems to have evolutionary similarities to the retinal cells of chordates (Klein, 2004). 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 (Natesan et al., 2002).

Reports in rodents suggest that the pineal gland may influence the actions of drugs of abuse such as cocaine and antidepressants such as fluoxetine (Prozac)^*. In addition, it has been proposed by Rick Strassman that the pineal gland is responsible for manufacture of endogenous dimethyltryptamine (DMT) although this is only speculation. There are some who believe DMT has a role in dreaming and possibly near-death experiences and other mystical states, and, although not proven, it was hypothesized in 1988 by Jace Callaway that DMT is connected with visual dreaming.

Mythology

The pineal gland was the last endocrine gland to have its function discovered. Its location deep in the brain seemed to indicate its importance. This combination led to its being a "mystery" gland with myth, superstition and even metaphysical theories surrounding its perceived function.

Rene Descartes called the pineal gland the "seat of the soul" Descartes R. Treatise of Man. New York: Prometheus Books; 2003. ISBN 1591020905, believing 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.

The pineal gland is occasionally associated with the sixth chakra (also called Ajna or the third eye chakra in yoga). It is believed by some to be a dormant organ that can be awakened to enable "telepathic" communication.

See Discordianism for the relevance of the pineal gland in this religion.

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