pineal gland

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gland

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Introduction

Anatomy of the pineal gland

Endocrine function of the pineal gland

pineal gland, endocrine gland found in vertebrates that is the

source of melatonin, a hormone derived from tryptophan that

regulates circadian rhythm (sleep cycle). The pineal gland develops

from the roof of the diencephalon, a section of thebrain. In some

lower vertebrates the gland has a well -developed eyelike structure;

in others, though not organized as an eye, it functions as a

l ight receptor.

The pineal gland, the most enigmatic of endocrine organs, has long

been of interest to anatomists. Several mil lennia ago it was thought

to control the f low of memories into consciousness. The 17th-

century French philosopher-mathematician René

Descartes concluded that the pineal gland was the seat of the soul.

A corollary notion was that calci f ication of the pineal caused

psychiatric disease, but modern imaging techniques revealed that

the pineal gland becomes more or less calcif ied in most people.

Anatomy of the pineal gland

The pineal gland is located behind the third cerebral ventricle in

the midline (between the two cerebral hemispheres) of the brain.

Its name is derived from its shape, which is l ike that of a pine cone

(Latin pinea). In adult humans it is about 0.8 cm (0.3 inch) long and

weighs approximately 0.1 gram (0.004 ounce). The gland is

relatively large in children and begins to shrink with the onset

ofpuberty. It has a rich supply of adrenergic nerves that greatly

inf luence its function. Microscopically, the gland is composed of

pinealocytes (rather typical endocrine cells except for extensions

that mingle with those of adjacent cells) and supporting cells that

are similar to the astrocytes of the brain. In adults, small deposits

of calcium often make the pineal body visible on X-rays.

Endocrine function of the pineal gland

In humans and other animals, the pineal gland produces hormones

that have important endocrine functions. For example, in

several vertebrate species, pineal hormones influence sexual

development, hibernation, and seasonal breeding. The pineal gland

contains several neuropeptides and neurotransmitters, such

as somatostatin, norepinephrine, and serotonin. The major pineal

hormone, however, is melatonin, a derivative of the amino

acid tryptophan. Melatonin was f irst discovered because it

l ightens amphibian skin, an effect opposite to that

of adrenocorticotropic hormone and melanocyte-stimulating

hormone of the anterior pituitary gland.

The secretion of melatonin is increased by sympathetic nervous

system stimulation. In humans, melatonin secretion increases soon

after a person is placed in the dark and decreases soon after

exposure to l ight. A major action of melatonin that has been well

documented in animals is to block the secretion of gonadotropin-

releasing hormone by the hypothalamus. This results in decreased

secretion of gonadotropins (e.g., luteinizing hormone and foll icle-

stimulating hormone) by thepituitary gland. In humans, however,

the function of melatonin is less well understood. Its production is

high in infancy and childhood and declines with age, and

abnormally high levels of melatonin in children are associated with

delayed sexual development.