pineal gland

 

(pĭn'eəl) , small organ (about the size of a pea) situated in the brain. Long considered vestigial in humans, the structure, which is also called the pineal body or the epiphysis, is present in most vertebrates. It is sensitive to different levels of light and is essential to the functioning of an animal's biological clock. In many animals, including humans, the pineal gland synthesizes a hormone called melatonin in periods of darkness. Melatonin synthesis is halted when light hits the retina of the eye, sending impulses to the gland via the optic nerve. Besides influencing daily, or circadian, rhythms such those of as sleep and temperature, the pineal gland and melatonin appear to direct annual rhythms and seasonal changes in animals. The pineal gland and melatonin are now being studied for their roles in sleep, reproduction, aging, and seasonal affective disorder. In humans the pineal gland begins to produce melatonin at age 3 months; production falls steadily from puberty on.

Pineal gland
Endocrine system
Diagram of pituitary and pineal glands.
Latin	glandula pinealis
Gray's	subject #276 1277
Artery	superior cerebellar artery
MeSH	Pineal+gland
Dorlands/Elsevier	g_06/12392585

The pineal gland (also called the pineal body, epiphysis cerebri, or epiphysis) is a small endocrine gland in the brain. It is shaped like a tiny pine cone, and is located near the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join. It produces melatonin, a hormone that may weakly modulate wake/sleep patterns. ^[1]

Location

The pineal gland is a reddish-gray body about the size of a pea (8 mm in humans), located just rostro-dorsal to the superior colliculus and behind and beneath the stria medullaris, between the laterally positioned thalamic bodies. It is part of the epithalamus.

The pineal gland is a midline structure, and is often seen in plain skull X-rays, as it is often calcified.

Structure and composition

The pineal body consists in humans of a lobular parenchyma of pinealocytes surrounded by connective tissue spaces. The glands' surface is covered by a pial capsule.

The 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.
interstitial cells	Interstitial cells are located between 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 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. ^[2]. Recently, calcite deposits have been described as well ^[3].

In lower vertebrates

Pinealocytes in lower vertebrate animals have a strong resemblance to the photoreceptor cells of the eye. Some evolutionary biologists believe that the vertebrate pineal cells share a common evolutionary ancestor with retinal cells^[4].

In some vertebrates, exposure to light can set off a chain reaction of enzymes, hormones, and neuroreceptors, which may help regulate the animal's circadian rhythm^[5].

In humans and other mammals, this function is served by the retinohypothalamic system that sets the rhythm within the suprachiasmatic nucleus. Cultural and social interactions produce exposures to artificial light that influence the setting of the suprachiasmatic clock. Evidence for a role for opsin-related light-sensing compounds in the skin of mammals is presently controversial. Research suggests that the pineal gland may serve a magnetoreceptive function in some animals. ^[6]

Some early vertebrate fossil skulls have a pineal foramen. This corroborates with the physiology of the modern lamprey, tuatara, and some other vertebrates.

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 ^[7]. 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. ^[8] The retina detects the light, and directly signals and entrains the suprachiasmatic nucleus (SCN). Fibers project from the SCN to the paraventricular nuclei (PVN), which relay the circadian signals to the spinal cord and out via the sympathetic system to superior cervical ganglia (SCG), and from there into the pineal gland. The function(s) of melatonin in humans is not clear.

In his book DMT: The Spirit Molecule, Dr. Rick Strassman theorized that the pineal gland may produce the psychedelic chemical DMT[1], but this has not been proven[2].

The pineal gland is large in children, but shrinks at puberty. It appears to play a major role in sexual development, hibernation in animals, metabolism, and seasonal breeding. The abundant melatonin levels in children is believed to inhibit sexual development, and pineal tumors have been linked with precocious puberty. When puberty arrives, melatonin production is reduced. Calcification of the pineal gland is typical in adults.

Pineal cytostructure seems to have evolutionary similarities to the retinal cells of chordates. ^[9] 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. ^[10]

Reports in rodents suggest that the pineal gland may influence the actions of drugs of abuse such as cocaine ^[11] and antidepressants such as fluoxetine (Prozac)^[12]; and contribute to regulation of neuronal vulnerability.^[13]

Mythologies, cultures and philosophies

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,^[14] called it the "seat of the soul" ^[15], believing it is unique in the anatomy of the human brain in being a structure not duplicated on the right and left sides. This observation is not true, however; under a microscope one finds the pineal gland is divided into two fine hemispheres. Another theory was that the pineal operated as a valve releasing fluids, thus the position taken during deep thought, with the head slightly down meeting the hand, was an allowance for the opening of these 'valves'.

The pineal gland is occasionally associated with the sixth chakra (also called Ajna or the third eye chakra in yoga) or sometimes the Seventh (Crown) chakra. It is believed by some to be a dormant organ that can be awakened to enable telepathic communication.

The relevance of the pineal gland to Discordianism, an idiosyncratic religion with roots in California psychedelic culture whose doctrines display great fondness for paradox, is great if not well understood.

Writers such as Alice Bailey, considered an early proponent of the new age movement, use the pineal-eye as a key element in their spiritual world-view...(see Alice Bailey: "A Treatise on White Magic")

The notion of a 'pineal-eye' is also crucial to the philosophy of the seminal French writer Georges Bataille, which is analysed at length by literary scholar Denis Hollier in 'Against Architecture' (1990, trans. Betsy Wing). 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

Mesal aspect of a brain sectioned in the median sagittal plane.

Dissection showing the ventricles of the brain.

Hind- and mid-brains; postero-lateral view.

Median sagittal section of brain.

References

External links

• BrainMaps at UCDavis pineal%20gland
• NeuroNames hier-280
• Histology at BU 14401loa - "Endocrine System: pineal gland "
• Anatomy Atlases - Microscopic Anatomy, plate 15.296
• Images of Pineal region http://rad.usuhs.edu/medpix/medpix.html?mode=image_finder&action=search&srchstr=pineal&srch_type=all#top

Brain: diencephalon
Epithalamus	Pineal body • Habenula (Habenular nuclei)
Hypothalamus	anterior: Anterior hypothalamic nucleus • Paraventricular nucleus • Preoptic area • Supraoptic nucleus • Suprachiasmatic nucleus intermediate/middle/tuberal/pituitary: infundibulum • median eminence • arcuate nucleus • Ventromedial nucleus • Dorsomedial hypothalamic nucleus • Tuber cinereum, Pituitary gland (Anterior pituitary, Posterior pituitary) posterior/lateral: posterior nucleus • Mammillary body • Lateral nucleus other: Medial forebrain bundle
Subthalamus	Subthalamic nucleus • Zona incerta • Thalamic fasciculus • Lenticular fasciculus
Thalamus/nuclei	Ventral nuclear group (VA/VL, VP/VPM/VPL) • MD • AN • LNG (Pulvinar) • Intralaminar nucleus (Centromedian nucleus) • Midline nuclear group • Thalamic reticular nucleus • Metathalamus (MG, LG) • Interthalamic adhesion
Third ventricle	recesses: (Optic recess, Infundibular recess, Suprapineal recess, Pineal recess) • Hypothalamic sulcus
Other	Interventricular foramina • Optic chiasm • Subfornical organ • Mammillothalamic tract

• [[3]]

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