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The pineal gland, conarium, or epiphysis cerebri, is a small
endocrine gland Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones, directly into the blood. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thy ...
in the
brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...
of most
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s. The pineal gland produces
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
, a
serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and va ...
-derived
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
which modulates sleep patterns in both
circadian A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...
and seasonal cycles. The shape of the gland resembles a
pine cone A conifer cone (in formal botanical usage: strobilus, plural strobili) is a seed-bearing organ on gymnosperm plants. It is usually woody, ovoid to globular, including scales and bracts arranged around a central axis, especially in conifers an ...
, which gives it its name. The pineal gland is located in the
epithalamus The epithalamus is a posterior (dorsal) segment of the diencephalon. The epithalamus includes the habenular nuclei and their interconnecting fibers, the habenular commissure, the stria medullaris and the pineal gland. Functions The function ...
, near the center of the brain, between the two hemispheres, tucked in a groove where the two halves of the
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
join. The pineal gland is one of the
neuroendocrine Neuroendocrine cells are cells that receive neuronal input (through neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release messenger molecules (hormones) into the blood. In this way they bri ...
secretory circumventricular organs in which capillaries are mostly
permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...
to
solutes In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solven ...
in the blood. Nearly all vertebrate species possess a pineal gland. The most important exception is a primitive vertebrate, the
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
. Even in the hagfish, however, there may be a "pineal equivalent" structure in the dorsal
diencephalon The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in o ...
. The
lancelet The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochord ...
''
Branchiostoma lanceolatum ''Branchiostoma lanceolatum'', the European lancelet or Mediterranean amphioxus, is a lancelet in the subphylum Cephalochordata. It is a marine invertebrate with a notochord but no backbone and is used as a model organism to study the evoluti ...
'', an early chordate which is a close relative to vertebrates, also lacks a recognizable pineal gland. The
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
(another primitive vertebrate), however, does possess one. A few more complex vertebrates have lost pineal glands over the course of their evolution. The results of various scientific research in evolutionary biology, comparative neuroanatomy and neurophysiology have explained the evolutionary history (
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological s ...
) of the pineal gland in different vertebrate species. From the point of view of biological evolution, the pineal gland is a kind of atrophied photoreceptor. In the
epithalamus The epithalamus is a posterior (dorsal) segment of the diencephalon. The epithalamus includes the habenular nuclei and their interconnecting fibers, the habenular commissure, the stria medullaris and the pineal gland. Functions The function ...
of some species of amphibians and reptiles, it is linked to a light-sensing organ, known as the
parietal eye A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...
, which is also called the pineal eye or ''third eye.''
René Descartes René Descartes ( or ; ; Latinized: Renatus Cartesius; 31 March 1596 – 11 February 1650) was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Ma ...
believed the human pineal gland to be the "principal seat of the soul." Academic philosophy among his contemporaries considered the pineal gland as a neuroanatomical structure without special metaphysical qualities; science studied it as one endocrine gland among many.


Etymology

The word ''pineal'', from Latin ''pinea'' ( pine-cone), was first used in the late 17th century to refer to the cone shape of the brain gland.


Structure

The pineal gland is a midline brain structure that is unpaired. It takes its name from its pine-cone shape. The gland is reddish-gray and about the size of a grain of rice (5–8 mm) in humans. The pineal gland, also called the pineal body, is part of the
epithalamus The epithalamus is a posterior (dorsal) segment of the diencephalon. The epithalamus includes the habenular nuclei and their interconnecting fibers, the habenular commissure, the stria medullaris and the pineal gland. Functions The function ...
, and lies between the laterally positioned thalamic bodies and behind the habenular commissure. It is located in the quadrigeminal cistern near to the
corpora quadrigemina Corpus is Latin for "body". It may refer to: Linguistics * Text corpus, in linguistics, a large and structured set of texts * Speech corpus, in linguistics, a large set of speech audio files * Corpus linguistics, a branch of linguistics Music * ...
. It is also located behind the
third ventricle The third ventricle is one of the four connected ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ...
and is bathed in
cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates. CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the ...
supplied through a small pineal recess of the third ventricle which projects into the stalk of the gland.


Blood supply

Unlike most of the mammalian brain, the pineal gland is not isolated from the body by the
blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from ''non-selectively'' crossing into the extracellular fluid of the central nervous system where ne ...
system; it has profuse blood flow, second only to the
kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...
, supplied from the choroidal branches of the posterior cerebral artery.


Nerve supply

The pineal gland receives a sympathetic innervation from the superior cervical ganglion. A
parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...
innervation from the pterygopalatine and otic ganglia is also present. Further, some nerve fibers penetrate into the pineal gland via the pineal stalk (central innervation). Also, neurons in the
trigeminal ganglion A trigeminal ganglion (or Gasserian ganglion, or semilunar ganglion, or Gasser's ganglion) is the sensory ganglion at the base of each of the two trigeminal nerves (CN V), occupying a cavity ( Meckel's cave) in the dura mater, covering the tri ...
innervate the gland with nerve fibers containing the
neuropeptide Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like t ...
PACAP Pituitary adenylate cyclase-activating polypeptide also known as PACAP is a protein that in humans is encoded by the ''ADCYAP1'' gene. pituitary adenylate cyclase-activating polypeptide is similar to vasoactive intestinal peptide. One of its eff ...
.


Microanatomy

The pineal body in humans consists of a lobular
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word � ...
of pinealocytes surrounded by
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
spaces. The gland's surface is covered by a pial capsule. The pineal gland consists mainly of pinealocytes, but four other
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
types have been identified. As it is quite cellular (in relation to the cortex and white matter), it may be mistaken for a
neoplasm A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
.


Development

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 In medicine, precocious puberty is puberty occurring at an unusually early age. In most cases, the process is normal in every aspect except the unusually early age and simply represents a variation of normal development. In a minority of childr ...
. When puberty arrives, melatonin production is reduced.


Symmetry

In the
zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
the pineal gland does not straddle the midline, but shows a left-sided bias. In
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s, functional cerebral dominance is accompanied by subtle anatomical asymmetry.


Function

One function of the pineal gland is to produce
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
. Melatonin has various functions in the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
, the most important of which is to help modulate sleep patterns. Melatonin production is stimulated by darkness and inhibited by light. Light sensitive nerve cells in the retina detect light and send this signal to the
suprachiasmatic nucleus The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regu ...
(SCN), synchronizing the SCN to the day-night cycle. Nerve fibers then relay the daylight information from the SCN to the
paraventricular nuclei The paraventricular nucleus (PVN, PVA, or PVH) is a nucleus in the hypothalamus. Anatomically, it is adjacent to the third ventricle and many of its neurons project to the posterior pituitary. These projecting neurons secrete oxytocin and a smaller ...
(PVN), then to the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
and via the sympathetic system to superior cervical ganglia (SCG), and from there into the pineal gland. The compound
pinoline Pinoline is a methoxylated tryptoline (5-methoxytryptoline) long claimed to be produced in the pineal gland during the metabolism of melatonin, however its pineal occurrence remains controversial. Its IUPAC name is 6-methoxy-1,2,3,4-tetrahydro-� ...
is also claimed to be produced in the pineal gland; it is one of the
beta-carboline β-Carboline (9''H''- pyrido ,4-''b'' ndole) represents the basic chemical structure for more than one hundred alkaloids and synthetic compounds. The effects of these substances depend on their respective substituent. Natural β-carbolines prima ...
s. This claim is subject to some controversy.


Regulation of the pituitary gland

Studies on rodents suggest that the pineal gland influences the
pituitary gland In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The h ...
's secretion of the sex hormones,
follicle-stimulating hormone Follicle-stimulating hormone (FSH) is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the gonadotropic cells of the anterior pituitary gland and regulates the development, growth, pubertal maturation, ...
(FSH), and
luteinizing hormone Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (GnRH) ...
(LH).
Pinealectomy Pinealectomy is a surgical procedure in which the pineal gland is removed. It is performed only in rare cases, where a pineocytoma or a pineal gland cyst has become life-threatening. See also * Hypophysectomy * Thyroidectomy * Adrenalectomy * Li ...
performed on rodents produced no change in pituitary weight, but caused an increase in the concentration of FSH and LH within the gland. Administration of
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
did not return the concentrations of FSH to normal levels, suggesting that the pineal gland influences pituitary gland secretion of FSH and LH through an undescribed transmitting molecule. The pineal gland contains
receptor Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
s for the regulatory
neuropeptide Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like t ...
,
endothelin-1 Endothelin 1 (ET-1), also known as preproendothelin-1 (PPET1), is a potent vasoconstrictor peptide produced by vascular endothelial cells. The protein encoded by this gene ''EDN1'' is proteolytically processed to release endothelin 1. Endotheli ...
, which, when injected in
picomolar Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...
quantities into the lateral cerebral ventricle, causes a
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
-mediated increase in pineal
glucose metabolism Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms. Carbohydrates are central to many essential metabolic pathways. P ...
.


Regulation of bone metabolism

Studies in mice suggest that the pineal-derived melatonin regulates new bone deposition. Pineal-derived melatonin mediates its action on the bone cells through MT2 receptors. This pathway could be a potential new target for osteoporosis treatment as the study shows the curative effect of oral melatonin treatment in a postmenopausal osteoporosis mouse model.


Clinical significance


Calcification

Calcification Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature M ...
of the pineal gland is typical in young adults, and has been observed in children as young as two years of age. The internal secretions of the pineal gland are known to inhibit the development of the reproductive glands because when it is severely damaged in children, development of the sexual organs and the skeleton are accelerated. Pineal gland calcification is detrimental to its ability to synthesize melatonin and scientific literature presents inconclusive findings on whether it causes sleep problems. The calcified gland is often seen in skull
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s. Calcification rates vary widely by country and correlate with an increase in age, with calcification occurring in an estimated 40% of Americans by age seventeen. Calcification of the pineal gland is associated with corpora arenacea, also known as "brain sand".


Tumors

Tumor A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s of the pineal gland are called pinealomas. These tumors are rare and 50% to 70% are germinomas that arise from sequestered embryonic
germ cells Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embryo ...
.
Histologically Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
they are similar to testicular
seminoma A seminoma is a germ cell tumor of the testicle or, more rarely, the mediastinum or other extra-gonadal locations. It is a malignant neoplasm and is one of the most treatable and curable cancers, with a survival rate above 95% if discovered in ...
s and ovarian
dysgerminoma A dysgerminoma is a type of germ cell tumor; it usually is malignant and usually occurs in the ovary. A tumor of the identical histology but not occurring in the ovary may be described by an alternate name: seminoma in the testis or germinoma in ...
s. A pineal tumor can compress the
superior colliculi In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form '' tectal'' is common ...
and
pretectal area In neuroanatomy, the pretectal area, or pretectum, is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Through reciprocal bilateral projections from the retina, it is involved primarily in mediati ...
of the dorsal
midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', ...
, producing
Parinaud's syndrome Parinaud's syndrome is an inability to move the eyes up and down. It is caused by compression of the vertical gaze center at the rostral interstitial nucleus of medial longitudinal fasciculus (riMLF). The eyes lose the ability to move upward and ...
. Pineal tumors also can cause compression of the
cerebral aqueduct The cerebral aqueduct (aqueductus mesencephali, mesencephalic duct, sylvian aqueduct or aqueduct of Sylvius) is a conduit for cerebrospinal fluid (CSF) that connects the third ventricle to the fourth ventricle of the ventricular system of the br ...
, resulting in a noncommunicating
hydrocephalus Hydrocephalus is a condition in which an accumulation of cerebrospinal fluid (CSF) occurs within the brain. This typically causes increased pressure inside the skull. Older people may have headaches, double vision, poor balance, urinary i ...
. Other manifestations are the consequence of their pressure effects and consist of visual disturbances,
headache Headache is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of depression in those with severe headaches. Headaches can occur as a result ...
, mental deterioration, and sometimes dementia-like behaviour. These
neoplasms A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
are divided into three categories: pineoblastomas,
pineocytomas Pineocytoma, is a benign, slowly growing tumor of the pineal gland. Unlike the similar condition pineal gland cyst, it is uncommon. Diagnosis Pineocytomas are diagnosed from tissue, i.e. a brain biopsy.They consist of: * cytologically benign cel ...
, and mixed tumors, based on their level of differentiation, which, in turn, correlates with their neoplastic aggressiveness. The clinical course of patients with pineocytomas is prolonged, averaging up to several years. The position of these tumors makes them difficult to remove surgically.


Other conditions

The morphology of the pineal gland differs markedly in different pathological conditions. For instance, it is known that its volume is reduced both in
obese Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's we ...
patients as well as patients with primary
insomnia Insomnia, also known as sleeplessness, is a sleep disorder in which people have trouble sleeping. They may have difficulty falling asleep, or staying asleep as long as desired. Insomnia is typically followed by daytime sleepiness, low energy, ...
.


Other animals

Most living vertebrates have pineal glands. It is likely that the
common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
of all
vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
had a pair of photosensory organs on the top of its head, similar to the arrangement in modern
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s. Some extinct
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
fishes have two parietal foramina in their skulls, suggesting an ancestral bilaterality of parietal eyes. The parietal eye and the pineal gland of living
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s are probably the descendants of the left and right parts of this organ, respectively. During
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
nic development, the
parietal eye A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...
and the pineal organ of modern
lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...
s and
tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...
s form together from a pocket formed in the brain
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
. The loss of parietal eyes in many living tetrapods is supported by developmental formation of a paired structure that subsequently fuses into a single pineal gland in developing embryos of turtles, snakes, birds, and mammals. The pineal organs of mammals fall into one of three categories based on shape. Rodents have more structurally complex pineal glands than other mammals.
Crocodilians Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...
and some tropical lineages of mammals (some
xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. ...
ns (
sloth Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...
s),
pangolin Pangolins, sometimes known as scaly anteaters, are mammals of the order Pholidota (, from Ancient Greek ϕολιδωτός – "clad in scales"). The one extant family, the Manidae, has three genera: '' Manis'', ''Phataginus'', and '' Smuts ...
s,
sirenian The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...
s (
manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living speci ...
s and
dugong The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest m ...
s), and some
marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...
s (
sugar gliders The sugar glider (''Petaurus breviceps'') is a small, omnivorous, arboreal, and nocturnal gliding possum belonging to the marsupial infraclass. The common name refers to its predilection for sugary foods such as sap and nectar and its ability ...
)) have lost both their parietal eye and their pineal organ. Polar mammals, such as walruses and some seals, possess unusually large pineal glands. All amphibians have a pineal organ, but some frogs and toads also have what is called a "frontal organ", which is essentially a parietal eye.
Pinealocyte Pinealocytes are the main cells contained in the pineal gland, located behind the third ventricle and between the two hemispheres of the brain. The primary function of the pinealocytes is the secretion of the hormone melatonin, important in th ...
s in many non-mammalian
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s have a strong resemblance to the
photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiat ...
s of the eye. Evidence from morphology and developmental biology suggests that pineal cells possess a common
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary ancestor with retinal cells. Pineal cytostructure seems to have evolutionary similarities to the retinal cells of the lateral eyes. Modern
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s and
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
s express the
phototransducing Visual phototransduction is the sensory transduction process of the visual system by which light is detected to yield nerve impulses in the rod cells and cone cells in the retina of the eye in humans and other vertebrates. It relies on the visu ...
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
melanopsin Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene ''Opn4''. In the mammalian retina, there are two additional categories of opsins, both involved in the f ...
in the pineal gland. Avian pineal glands are thought to act like the
suprachiasmatic nucleus The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regu ...
in
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s. The structure of the pineal eye in modern lizards and tuatara is analogous to the cornea, lens, and retina of the lateral eyes of vertebrates. In most vertebrates, exposure to light sets off a chain reaction of enzymatic events within the pineal gland that regulates
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...
s. 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 gland. The fossilized skulls of many extinct vertebrates have a pineal
foramen In anatomy and osteology, a foramen (;Entry "foramen"
in
(opening), which in some cases is larger than that of any living vertebrate. Although fossils seldom preserve deep-brain soft anatomy, the brain of the Russian fossil bird ''Cerebavis cenomanica'' from Melovatka, about 90 million years old, shows a relatively large parietal eye and pineal gland.


Society and culture

Seventeenth-century philosopher and scientist
René Descartes René Descartes ( or ; ; Latinized: Renatus Cartesius; 31 March 1596 – 11 February 1650) was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Ma ...
was highly interested in anatomy and physiology. He discussed the pineal gland both in his first book, the ''Treatise of Man'' (written before 1637, but only published posthumously 1662/1664), and in his last book, ''The Passions of the Soul'' (1649) and he regarded it as "the principal seat of the soul and the place in which all our thoughts are formed." In the ''Treatise of Man'', Descartes described conceptual models of man, namely creatures created by God, which consist of two ingredients, a body and a soul. In the ''Passions'', Descartes split man up into a body and a soul and emphasized that the soul is joined to the whole body by "a certain very small gland situated in the middle of the brain's substance and suspended above the passage through which the spirits in the brain's anterior cavities communicate with those in its posterior cavities". Descartes attached significance to the gland because he believed it to be the only section of the brain to exist as a single part rather than one-half of a pair. Some of Descartes's basic anatomical and physiological assumptions were totally mistaken, not only by modern standards, but also in light of what was already known in his time. The notion of a "pineal-eye" is central to the philosophy of the French writer
Georges Bataille Georges Albert Maurice Victor Bataille (; ; 10 September 1897 – 9 July 1962) was a French philosopher and intellectual working in philosophy, literature, sociology, anthropology, and history of art. His writing, which included essays, novels ...
, 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''.Bataille, G, ''Visions of Excess: Selected Writings, 1927–1939 (Theory and History of Literature, Vol 14)'', trans. Allan Stoekl et al., Manchester University Press, 1985 In the late 19th century
Madame Blavatsky Helena Petrovna Blavatsky, uk, Олена Петрівна Блаватська, Olena Petrivna Blavatska (; – 8 May 1891), often known as Madame Blavatsky, was a Russian mystic and author who co-founded the Theosophical Society in 1875 ...
(who founded
theosophy Theosophy is a religion established in the United States during the late 19th century. It was founded primarily by the Russian Helena Blavatsky and draws its teachings predominantly from Blavatsky's writings. Categorized by scholars of religion a ...
) identified the pineal gland with the Hindu concept of the
third eye The third eye (also called the mind's eye or inner eye) is a mystical invisible eye, usually depicted as located on the forehead, which provides perception beyond ordinary sight. In Hinduism, the third eye refers to the ajna (or brow) chakra. In ...
, or the Ajna chakra. This association is still popular today. In the short story " From Beyond" by H. P. Lovecraft, a scientist creates an electronic device that emits a resonance wave, which stimulates an affected person's pineal gland, thereby allowing them to perceive planes of existence outside the scope of accepted reality, a translucent, alien environment that overlaps our own recognized reality. It was adapted as a film of the same name in 1986. The 2013 horror film ''
Banshee Chapter ''Banshee Chapter'' (sometimes referred to as ''The Banshee Chapter'') is a 2013 American horror film and the directorial debut of Blair Erickson. The film had its first screening at the Fantasy Filmfest on August 22, 2013, and released on video ...
'' is heavily influenced by this short story.


History

The secretory activity of the pineal gland is only partially understood. Its location deep in the brain suggested to philosophers throughout history that it possesses particular importance. This combination led to its being regarded as a "mystery" gland with
mystical Mysticism is popularly known as becoming one with God or the Absolute, but may refer to any kind of ecstasy or altered state of consciousness which is given a religious or spiritual meaning. It may also refer to the attainment of insight in u ...
,
metaphysical Metaphysics is the branch of philosophy that studies the fundamental nature of reality, the first principles of being, identity and change, space and time, causality, necessity, and possibility. It includes questions about the nature of conscio ...
, and
occult The occult, in the broadest sense, is a category of esoteric supernatural beliefs and practices which generally fall outside the scope of religion and science, encompassing phenomena involving otherworldly agency, such as magic and mysticism a ...
theories surrounding its perceived functions. The pineal gland was originally believed to be a "
vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
remnant" of a larger organ. In 1917, it was known that extract of cow pineals lightened frog skin. Dermatology professor
Aaron B. Lerner Aaron Bunsen Lerner (September 21, 1920 – February 3, 2007), also known in scientific articles as Aaron B. Lerner, was an American physician, researcher and professor. Life and career Born in 1920 in Minneapolis, Lerner received his medical degr ...
and colleagues at
Yale University Yale University is a private research university in New Haven, Connecticut. Established in 1701 as the Collegiate School, it is the third-oldest institution of higher education in the United States and among the most prestigious in the w ...
, hoping that a substance from the pineal might be useful in treating skin diseases, isolated and named the hormone
melatonin Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
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 Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronob ...
. Of the endocrine organs, the function of the pineal gland was the last discovered.


Additional images

The pineal body is labeled in these images. File:Gray715.png, Mesal aspect of a brain sectioned in the median sagittal plane File:Gray716.png, Dissection showing the ventricles of the brain File:Gray719.png, Hind- and mid-brains; antero-lateral view File:Gray720.png, Median sagittal section of brain File:Slide4ff.JPG, Pineal gland File:Slide2BRA.JPG, Brainstem; posterior view


See also

* Pineal gland cyst


References


External links

{{DEFAULTSORT:Pineal Gland Circadian rhythm Endocrine system Epithalamus Glands Human head and neck Sleep physiology