The
pineal gland is a small
endocrineIn physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...
gland in the
vertebrateVertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
brainThe brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. It produces the
serotoninSerotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...
derivative
melatoninMelatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
, a
hormoneA hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
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
hemispheresA cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
, tucked in a groove where the two rounded
thalamic bodiesThe thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
join.
Location
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 colliculusThe optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
and behind and beneath the
stria medullarisThe stria medullaris, also known as stria medullaris thalami, is a fiber bundle containing efferent fibers from the septal nuclei, lateral preoptico-hypothalamic region, and anterior thalamic nuclei to the habenula...
, between the laterally positioned thalamic bodies. It is part of the
epithalamusThe epithalamus is a dorsal posterior segment of the diencephalon which includes the habenula, the stria medullaris and the pineal body...
.
The pineal gland is a midline structure shaped like a pine cone , and is often seen in plain
skullThe human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, as it is often calcified.
Structure and composition
The pineal body consists in humans of a lobular
parenchymaParenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
of pinealocytes surrounded by
connective tissue"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
spaces. The gland's surface is covered by a pial capsule.
The pineal gland consists mainly of pinealocytes, but four other
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
types have been identified. As it is quite cellular (in relation to the cortex and white matter) it may be mistaken for a neoplasm.
| Cell type |
Description
>-
| PinealocytePinealocytes are the main cells of the pineal gland. They produce and secrete melatonin. Pinealocytes have an organelle called the synaptic ribbon; this is considered to be a specific marker for pinealocytes...
s |
melatoninMelatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
. 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 cellInterstitial cell refers to any one of a number of different types of cells characterized by their interstitial nature Examples include:...
s |
|
>-
| Perivascular phagocytePhagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are...
|
>-
| pineal neuronA neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s
|
>-
| peptidePeptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
rgic 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
sympatheticThe sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
innervation from the
superior cervical ganglionThe superior cervical ganglion , the largest of the cervical ganglia, is placed opposite the second and third cervical vertebræ. It contains neurons that supply sympathetic innervation to the face....
. However, a parasympathetic innervation from the
sphenopalatineSphenopalatine may refer to:* sphenopalatine artery, an artery of the head, commonly known as the artery of epistaxis* sphenopalatine ganglion * sphenopalatine nerves...
and
otic gangliaThe otic ganglion is a small, oval shaped, flattened parasympathetic ganglion of a reddish-gray color, located immediately below the foramen ovale in the infratemporal fossa. It gives innervation to the parotid gland for salivation....
is also present. Further, some nerve fibers penetrate into the pineal gland via the pineal stalk (central innervation). Finally, neurons in the
trigeminal ganglionThe trigeminal ganglion is a sensory ganglion of the trigeminal nerve that occupies a cavity in the dura mater, covering the trigeminal impression near the apex of the petrous part of the temporal bone.-Relations:It is somewhat crescentic in shape, with its convexity...
innervate the gland with nerve fibers containing the
neuropeptideNeuropeptides are small protein-like molecules used by neurons to communicate with each other. They are neuronal signaling molecules, influence the activity of the brain in specific ways and are thus involved in particular brain functions, like analgesia, reward, food intake, learning and...
, PACAP. Human follicles contain a variable quantity of gritty material, called
corpora arenaceaCorpora arenacea are calcified structures in the pineal gland and other areas of the brain such as the choroid plexus. Older organisms have numerous corpora arenacea, whose function, if any, is unknown. Concentrations of "brain sand" increase with age, so the pineal gland becomes increasingly...
(or "acervuli," or "brain sand"). Chemical analysis shows that they are composed of
calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
,
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
,
magnesium phosphateMagnesium phosphate is a general term for salts of magnesium and phosphate appearing in three forms:* Magnesium phosphate monobasic * Magnesium phosphate dibasic * Magnesium phosphate tribasic...
, and
ammonium phosphateAmmonium phosphate is the salt of ammonia and phosphoric acid. It has the formula 3PO4 and consists of ammonium cations and phosphate anion. It is obtained as a crystalline powder upon mixing concentrated solutions of ammonia and phosphoric acid, or on the addition of excess of ammonia to the...
. In 2002, deposits of the
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
form of calcium carbonate were described. Calcium, phosphorus and
fluorideFluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
deposits in the pineal gland have been linked with aging.
Miscellaneous anatomy
PinealocytePinealocytes are the main cells of the pineal gland. They produce and secrete melatonin. Pinealocytes have an organelle called the synaptic ribbon; this is considered to be a specific marker for pinealocytes...
s in many non-mammalian
vertebrateVertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s have a strong resemblance to the photoreceptor cells of the
eyeEyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
. Some evolutionary biologists believe that the vertebrate pineal cells share a common
evolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary 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 rhythmA circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
s. Some early vertebrate fossil skulls have a pineal foramen (opening). This correlates with the physiology of the modern "
living fossilLiving fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
s," the
lampreyLampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...
and the
tuataraThe tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...
, and some other vertebrates that have a
parietal organA parietal eye, also known as a parietal organ or third-eye or pineal eye, is a part of the epithalamus present in some animal species...
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."
Unlike much of the rest of the mammalian brain, the pineal gland is not isolated from the body by the
blood-brain barrierThe blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion...
system; it has profuse blood flow, second only to the kidney.
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
retinohypothalamicThe retinohypothalamic tract is a photic input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells , which contain the photopigment melanopsin...
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. In 1917 it was known that extract of cow pineals lightened frog skin. Dermatology professor
Aaron B. LernerAaron Bunsen Lerner was an American physician, researcher and professor. Born in 1920 in Minneapolis, he received his medical degree and a PhD in chemistry from the University of Minnesota in 1945. After teaching at the universities of Michigan and Oregon, he joined the Yale University School of...
and colleagues at
Yale UniversityYale University is a private, Ivy League university located in New Haven, Connecticut, United States. Founded in 1701 in the Colony of Connecticut, the university is the third-oldest institution of higher education in the United States...
, hoping that a substance from the pineal might be useful in treating skin diseases, isolated and named the hormone
melatoninMelatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
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
chronobiologyChronobiology is a field of biology that examines periodic phenomena in living organisms and their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobiology comes from the ancient Greek χρόνος , and biology, which pertains to the study, or science,...
.
MelatoninMelatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
is N-acetyl-5-methoxy-tryptamine, a derivative of the
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
tryptophanTryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
, which also has other functions in the
central nervous systemThe central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
. The production of melatonin by the pineal gland is stimulated by darkness and inhibited by light.
Photosensitive cellsPhotosensitive ganglion cells, also called photosensitive Retinal Ganglion Cells , intrinsically photosensitive Retinal Ganglion Cells or melanopsin-containing ganglion cells, are a type of neuron in the retina of the mammalian eye.They were discovered in the early 1990sand are, unlike other...
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 cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
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 disorderCircadian rhythm sleep disorders are a family of sleep disorders affecting, among other things, the timing of sleep. People with circadian rhythm sleep disorders are unable to sleep and wake at the times required for normal work, school, and social needs. They are generally able to get enough sleep...
s.
The compound
pinolinePinoline is a methoxylated tryptoline that occurs in the pineal gland during the metabolism of melatonin. Its chemical names are 5-methoxytryptoline and 6-methoxy-1,2,3,4-tetrahydro-β-carboline . The biological activity of this molecule is of interest as a potential free radical scavenger ,...
is also produced in the pineal gland; it is one of the
beta-carbolineβ-Carboline is an organic amine that is the prototype of a class of compounds known as β-carbolines.-Pharmacology:...
s.
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 pubertyAs a medical term, precocious puberty describes puberty occurring at an unusually early age. In most of these children, the process is normal in every respect except the unusually early age, and simply represents a variation of normal development. In a minority of children, the early development is...
. When puberty arrives, melatonin production is reduced.
CalcificationCalcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...
of the pineal gland is typical in adults.
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,
hibernationHibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
, metabolism, and seasonal breeding.
Pineal cytostructure seems to have evolutionary similarities to the retinal cells of chordates. Modern
birdBirds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s and
reptileReptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s have been found to express the phototransducing
pigmentA pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
melanopsinMelanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...
in the pineal gland. Avian pineal glands are believed to act like the
SCNThe suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
in
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s.
Studies on rodents suggest that the pineal gland may influence the actions of recreational drugs, such as cocaine, and
antidepressantAn antidepressant is a psychiatric medication used to alleviate mood disorders, such as major depression and dysthymia and anxiety disorders such as social anxiety disorder. According to Gelder, Mayou &*Geddes people with a depressive illness will experience a therapeutic effect to their mood;...
s, such as fluoxetine (Prozac), and its hormone melatonin can protect against
neurodegenerationNeurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative processes. As research progresses, many...
.
Pathology
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; choriocariconmas; mixtures of germinom, embryonal carcinoma, and choriocarcinoma; and , uncommonly, typical teratomas (usually benign). Whether to charcterize 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.
Pinelomas
These neoplasms are divided into two catergories, 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,
headacheA headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
, mental deterioration, and sometimes dementia-like behaviour. The lesions being located where they are, it is understandable that successful excision is at best difficult.
Metaphysics and philosophy
The secretory activity of the pineal gland is only relatively 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
occultThe word occult comes from the Latin word occultus , referring to "knowledge of the hidden". In the medical sense it is used to refer to a structure or process that is hidden, e.g...
theories surrounding its perceived functions.
René DescartesRené Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...
, 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. 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 arteriesCarotid artery can refer to:* Common carotid artery* External carotid artery* Internal carotid artery...
.
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 BatailleGeorges Bataille was a French writer. His multifaceted work is linked to the domains of literature, anthropology, philosophy, economy, sociology and history of art...
, 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.
The notion of an inner
third eyeThe third eye is a mystical and esoteric concept referring in part to the ajna chakra in certain spiritual traditions. It is also spoken of as the gate that leads within to inner realms and spaces of higher consciousness...
(attributed mystical significance) also occurs in ancient, oriental, and new age philosophies.
External links