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Pineal gland
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The pineal gland (also called the pineal body, epiphysis cerebri, epiphysis or the "third eye") is a small endocrine gland in the vertebrate brain. It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and photoperiodic (seasonal) functions. It is shaped like a tiny pine cone (hence its name), and is located near to the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join.
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The pineal gland (also called the pineal body, epiphysis cerebri, epiphysis or the "third eye") is a small endocrine gland in the vertebrate brain. It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and photoperiodic (seasonal) functions. It is shaped like a tiny pine cone (hence its name), and is located near to the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join. Unlike much of the rest of the brain, the pineal gland is not isolated from the body by the blood-brain barrier system.
Location
The pineal gland is reddish-gray and 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. Calcification is typically due to intake of the fluoride found in water and toothpaste.
Structure and composition
The pineal body consists in humans of a lobular parenchyma of pinealocytes surrounded by connective tissue spaces. The gland's surface is covered by a pial capsule.
The pineal gland consists mainly of pinealocytes, but four other cell types have been identified.
| Cell type | Description
>-
| 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.
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| interstitial cells | -
| perivascular phagocyte | -
| pineal neurons | -
| 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 phosphorus deposits in the pineal gland have been linked with aging.
Miscellaneous anatomy
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 of the pineal to light can directly set off a chain reaction of enzymatic events which regulate circadian rhythms. Some early vertebrate fossil skulls have a pineal foramen (opening). This corroborates with the physiology of the modern "living fossils", the lamprey and the tuatara, and some other vertebrates which 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 homologous 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.
In humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through the retinohypothalamic system to the suprachiasmatic nuclei (SCN) and the pineal.
Function
The pineal gland was originally believed to be a "vestigial remnant" of a larger organ (much as the appendix was thought to be a vestigial digestive organ). Aaron Lerner and colleagues at Yale University discovered that melatonin, the most potent compound then known to lighten frog skin, was present in the highest concentrations in the pineal. 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. Photosensitive cells in the retina detect light and directly signal the suprachiasmatic nucleus (SCN), entraining it to the 24 hour clock. 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 function(s) of melatonin in humans is not clear; it is commonly prescribed for the treatment of circadian rhythm sleep disorders.
The compound pinoline is also produced in the pineal gland; it is one of the beta-carbolines.
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 . 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.
Cultures, philosophies and mythologies
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.
The notion of a 'pineal-eye' is central 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. In this work Hollier discusses how Bataille uses the concept of a 'pineal-eye' as a reference to a blind-spot in Western rationality.
Additional images
External links
- "Endocrine System: pineal gland "
- Images of Pineal region http://rad.usuhs.edu/medpix/medpix.html?mode=image_finder&action=search&srchstr=pineal&srch_type=all#top
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