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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' surface is covered by a pial capsule. The pinealocytes consist of a cell body with 4-6 processes emerging. The pinealocytes can be stained by special silver impregnation methods. Insterstitial cells are located between the pinealocytes. Many capillaries are present in the gland, and perivascular phagocytes are located close to these blood vessels. 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. Human follicles contain a variable quantity of gritty material, called brain sand, acervuli, or corpora arenacea. 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). In lower vertebrates 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 on the surface of the brain, directly under the skull and contains the photoreceptors to regulate their biological clock. 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. Research suggests that the pineal gland may serve a magnetoreceptive function in some animals (Deutschlander et al.,1999). In the tuatara the pineal body comes through a hole in the skull to the surface and has the form of a crude eye. The lamprey's pineal body is similar. Many fossil early vertebrate skulls have a pineal foramen. 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 1960s 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, 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 (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)*; and contribute to regulation of neuronal vulnerability*. Mythologies, Cultures and Philosophies 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" , 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. 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'. 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. Writers such as Alice Bailey, considered an early proponent of the new age movement, use the pineal-eye as a key element in their spiritual world-view...(see Alice Bailey: "A Treaties on White Magic") An important link from the pineal gland to modern philosophy, influences on "post-structuralism" and French thought can be found through Denis Hollier's book "Against Architecture" (English translation, Betsy Wing: 1989, MIT Press); an investigation into the work of Georges Bataille. "On five different occasions, during a rather brief period of time, Bataille wanted to 'write on' the pineal eye. He wanted to write a book that would have developed the interpretation (his word) of this fantastic and/or mythical image..." (Hollier, p121) Bataille wrote the seminal work entitled, 'The Pineal Eye' (Visions of Excess: Selected Writings, 1927-1939, MIT Press): an extreme text with surreal overtones. This article was left unpublished during his lifetime- said to have been abandoned. "The pineal eye does not let itself be put together into a concordant discourse. This failure results from the excessive, disruptive energy turned loose by the pineal eye, to be dispersed outside the book in the writing of a text." (Hollier, p118) Hollier argues that this incompleteness is a paradigmatic model for Bataille's philosophical project. Carried throughout Bataille's explorations of the acephale, delerium, rapture and transgression is a notion of incompleteness: a blind-spot in Western knowledge and thought. Hollier argues that this is a conceptual role the 'pineal-eye' occupies in his writings, embodied by its absence from published work and inability to be assimilated into discourse. Hollier, Denis (academic): Bataille (Works on), College of Sociology (Sources: editor), Bernard Tschumi (References) | ||||||||
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