|Diagram of pituitary and pineal glands in the human brain|
|Gray's||subject #276 1277|
|Artery||posterior cerebral artery|
|Precursor||Neural Ectoderm, Roof of Diencephalon|
The pineal gland (also called the pineal body, epiphysis cerebri, epiphysis, conarium or the "third eye") is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions. Its shape resembles a tiny pine cone (hence its name), and it is located near the centre of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join.
Nearly all vertebrate species possess a pineal gland. The most important exception is the hagfish, which is often thought of as the most primitive type of vertebrate. Even in the hagfish, though, there may be a "pineal equivalent" structure in the dorsal diencephalon. The lancelet amphioxus, the nearest existing relative to vertebrates, also lacks a recognizable pineal gland. The lamprey, however (considered almost as primitive as the hagfish), does possess one. A few "higher" types of vertebrates, including the alligator, lack pineal glands because they have been lost over the course of evolution.
The pineal gland is reddish-gray and about the size of a grain of rice (5–8 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.
Structure and composition
The pineal gland consists mainly of pinealocytes, but four other cell types have been identified. As it is quite cellular (in relation to the cortex and white matter) it may be mistaken for a neoplasm.
|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. Their cytoplasm is lightly basophilic. With special stains, pinealocytes exhibit lengthy, branched cytoplasmic processes which extend to the connective septa and its blood vessels.|
|Interstitial cells||Interstitial cells are located between the pinealocytes. They have elongated nuclei and a cytoplasm which is stained darker than that of 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 innervate 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. In 2002, deposits of the calcite form of calcium carbonate were described. Calcium, phosphorus deposits in the pineal gland have been linked with aging.
Pineal gland depositions
Pinealocytes in many non-mammalian vertebrates have a strong resemblance to the photoreceptor cells of the eye. Some evolutionary biologists believe that the vertebrate pineal cells share a common evolutionary ancestor with retinal cells.
In some vertebrates, exposure to light can set off a chain reaction of enzymatic events within the pineal gland that regulate circadian rhythms. Some early vertebrate fossil skulls have a pineal foramen (opening). This correlates with the physiology of the modern "living fossils," the lamprey and the tuatara, and some other vertebrates that have a parietal organ or "third eye," which, in some of them, is photosensitive. The third eye represents evolution's earlier approach to photoreception. The structures of the third eye in the tuatara are analogous to the cornea, lens and retina, though the latter resembles that of an octopus rather than a vertebrate retina. The asymmetrical whole consists of the "eye" to the left and the pineal sac to the right. "In animals that have lost the parietal eye, including mammals, the pineal sac is retained and condensed into the form of the pineal gland."
Fossils seldom preserve soft anatomy. The brain of the Russian Melovatka bird, about 90 million years old, is an exception, and it shows a larger-than-expected parietal eye and pineal gland.
The pineal gland was originally believed to be a "vestigial remnant" of a larger organ. In 1917 it was known that extract of cow pineals lightened frog skin. Dermatology professor Aaron B. Lerner and colleagues at Yale University, hoping that a substance from the pineal might be useful in treating skin diseases, isolated and named the hormone melatonin in 1958. The substance did not prove to be helpful as intended, but its discovery helped solve several mysteries such as why removing the rat's pineal accelerated ovary growth, why keeping rats in constant light decreased the weight of their pineals, and why pinealectomy and constant light affect ovary growth to an equal extent; this knowledge gave a boost to the then new field of chronobiology.
Melatonin is N-acetyl-5-methoxy-tryptamine, 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.Photosensitive cells in the retina detect light and directly signal the SCN, entraining its rhythm to the 24-hour cycle in nature. 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 human pineal gland grows in size until about 1–2 years of age, remaining stable thereafter, although its weight increases gradually from puberty onwards. The abundant melatonin levels in children are 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, and has been observed in children as young as 2. Calcification rates vary widely by country and tend to increase by age, with calcification occurring in an estimated 40% of Americans by their 17th year. Calcification of the pineal gland is largely associated with corpora arenacea (or "acervuli," or "brain sand").
Apparently the internal secretions of the pineal gland inhibit the development of the reproductive glands, because in cases where it is severely damaged in children, the result is accelerated development of the sexual organs and the skeleton. In animals, the pineal gland appears to play a major role in sexual development, hibernation, metabolism, and seasonal breeding.
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 SCN in mammals.
Studies on rodents suggest that 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.
Some recent studies show that the degree of pineal gland calcification is significantly higher in Alzheimer's disease patients vs. other types of dementia.
Pineal gland calcification may also contribute to the pathogenesis of Alzheimer's disease and may reflect an absence of crystallization inhibitors.
Possible role in dimethyltryptamine production
Dr. Rick Strassman, who conducted research on the psychedelic dimethyltryptamine (DMT) in the 1990s at the University of New Mexico, has speculated that the pineal gland plays a role in the production of DMT in the human brain. Strassman has also advanced the controversial hypothesis that a massive release of DMT from the pineal gland prior to death, or near death, can result in a near death experience (NDE). Strassman has suggested that the pineal gland is responsible for DMT production because enzymatic material needed to produce DMT is found there (see evidence in mammals) in substantially greater concentrations than in any other part of the body, Strassman (p. 69).
All tumors involving the pineal gland are rare; most (50% to 70%) arise from sequestered embryonic germ cells. They most commonly take the form of so-called germinomas, resembling testicular seminoma or ovarian dysegerminoma. Other lines of germ cell differentiation include embryonal carcinomas; choriocarcinomas; mixtures of germinom, embryonal carcinoma, and choriocarcinoma; and, uncommonly, typical teratomas (usually benign). Whether to characterize these germ cell neoplasms as pinealomas is still a subject of debate, but most pinealophiles favor restricting the terms pinealoma to neoplasms arising from the pineocytes.
A pineal tumor can compress the superior colliculi and pretectal area of the dorsal midbrain, producing Parinaud's syndrome. Pineal tumors also can cause compression of the cerebral aqueduct, resulting in a noncommunicating hydrocephalus.
These neoplasms are divided into two categories, pineoblastomas and pineocytomas, based on their level of differentiation, which in turn, correlates with their neoplastic aggressiveness. The clinical course of patients with pineocytomas is prolonged, averaging 7 years. The manifestations are the consequence of their pressure effects and consist of visual disturbances, headache, mental deterioration, and sometimes dementia-like behaviour. The lesions being located where they are, it is understandable that successful excision is at best difficult.
Mysticism, metaphysics and philosophy
The secretory activity of the pineal gland is only partially understood. Historically, its location deep in the brain suggested to philosophers that it possessed particular importance. This combination led to its being regarded as a "mystery" gland with mystical, metaphysical and occult theories surrounding its perceived functions.
René Descartes, dedicating much time to the study of the pineal (Pine-cone shaped) gland, has called it the "principal seat of the soul." He believed that it was the point of connection between the intellect and the body. Descartes attached significance to the gland because he believed it to be the only section of the brain which existed as a single part, rather than one half of a pair. He argued that because a person can never have "more than one thought at a time," external stimuli must be united within the brain before being considered by the soul, and he considered the pineal gland to be situated in "the most suitable possible place for this purpose," located centrally in the brain and surrounded by branches of the carotid arteries.
Baruch de Spinoza criticized Descartes' viewpoint for neither following from self-evident premises nor being "clearly and distinctly perceived" (Descartes having previously asserted that he could not draw conclusions of this sort), and questioned what Descartes meant by talking of "the union of the mind and the body."
The notion of a "pineal-eye" is central to the philosophy of the French writer Georges Bataille, which is analyzed at length by literary scholar Denis Hollier in his study Against Architecture. In this work Hollier discusses how Bataille uses the concept of a "pineal-eye" as a reference to a blind-spot in Western rationality, and an organ of excess and delirium. This conceptual device is explicit in his surrealist texts, The Jesuve and The Pineal Eye.
Numerous spiritual philosophies contain the notion of an inner Third Eye that is related to the ajna chakra and also the pineal gland, and to which is attributed significance in mystical awakening or enlightenment, clairvoyant perception and higher states of consciousness. This idea occurs historically in ancient, central and east Asia; and also in contemporary metaphysical theories relating to yoga, Theosophy, Pagan religions, and New Age spiritual philosophies.
The pineal body is labeled in these images.
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- Descartes and the Pineal Gland (Stanford Encyclopedia of Philosophy)
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