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Pineal gland

Synonyms: epiphysis cerebri, pineal body, conarium
German: Glandula pinealis

1 Definition

The pineal gland is an endocrine gland – located in the epithalamus – which produces melatonin, depending on light conditions.

2 Anatomy

As a part of the epithalamus, the pineal gland is located on the posterior wall of the brain's third ventricle above the corpora quadrigemina. It is surrounded by a capsule of connective tissue.

3 Histology

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The neuroendocrine cells of the pineal gland, called pinealocytes, are separated into lobules by septa of connective tissue. The pineal gland can contain corpora arenacea (brain sand) which are calcified inclusions that can be visible in AP x-rays.

4 Physiology

The pineal gland receives afferent fibers from the cervical sympathetic trunk which in turn indirectly connect to the retina by way of the intermediolateral nucleus and the suprachiasmatic nucleus of the hypothalamus. This way the pineal gland is regulated according to the light situation: In times of low light, the pinealocytes use tryptophan to synthesize the hormone melatonin which regulates the circadian rhythm. The pineal gland is also thought to delay puberty which is supported by the fact that children with faulty pineal glands sometimes experience precocious puberty.

5 Phylogenetic development

In lower vertebrates the pineal gland is exposed to and stimulated by direct light through a parietal eye. The thicker skull in higher vertebrates makes it impossible for light to reach the pineal gland directly. As a result, the pineal gland is stimulated by nerve tracts that connect it indirectly to the eyes.

6 History

French mathematician and philosopher René Descartes regarded the pineal gland as the location of the soul because of its photosensitivity in lower animals. Back then the pineal gland was the only known unpaired part of the brain which led Descartes to believe that it was the point where the contents of both hemispheres converged and became conscious thoughts that reached the soul (a theory that was backed up by the fact that the pineal gland is funnel-shaped).

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