Corpus: Pineal gland

The pineal gland is a small endocrine gland located in the epithalamus that secretes light-dependent melatonin.

The pineal gland is located as part of the epithalamus on the posterior wall of the third cerebral ventricle above the four-humped plate. It is encased in a connective tissue capsule. Its size is normally 7 x 6 x 3 mm, sometimes up to 14 mm.

Topography[Bearbeiten]

The epiphysis is surrounded by the following structures:

• anterior: pineal recess of the 3rd ventricle
• anterosuperior: commissura habenularum
• superior: Deep cerebral veins incl. Vena cerebri magna (Galen vein), Striae medullares, Splenium corporis callosi, Velum interpositum
• Posteroinferior: Cisterna cerebelli superior
• Inferior: superior colliculus of the mesencephalon
• anteroinferior: Commissura posterior

Vascular supply[Bearbeiten]

The pineal gland is supplied with blood by small vascular branches of the posterior choroidal artery. Venous drainage takes place via several branches into the great cerebral vein.

Innervation[Bearbeiten]

The pineal gland receives sympathetic nerve fibres from the superior cervical ganglion and parasympathetic fibres from the pterygopalatine ganglion and the otic ganglion. Central nervous afferents come via PACAP-containing neurones in the habenula and the trigeminal ganglion.

Embryology[Bearbeiten]

The pineal gland develops in the 7th week of pregnancy by thickening of the ependyma of the posterior part of the 3rd ventricle.

The neuroendocrine cells of the pineal gland, the pinealocytes, are divided into lobules by connective tissue septa. Calcifications, so-called brain sand (acervulus), can be recognised as inclusions; they appear in the a.p. X-ray image of the skull as bright spots in the median line.

The pineal gland has no blood-brain barrier. Accordingly, it belongs to the circumventricular organs.

The pineal gland receives afferent fibres from the sympathetic cervical border cord, which in turn are indirectly connected to the retina via the nucleus intermediolateralis and the nucleus suprachiasmaticus of the hypothalamus. In this way, the function of the pineal gland is regulated depending on light: In the absence of light stimuli, the pinealocytes synthesise the hormone melatonin from tryptophan, which regulates the sleep-wake rhythm.

The pineal gland is also thought to have a puberty-inhibiting function. Pubertas praecox can occasionally be observed in children with functional defects of the gland.

In lower vertebrates, the pineal gland is exposed to direct light via a parietal eye and is therefore light-sensitive. In higher vertebrates, the thickness of the skull cap prevents the passage of light. The function of the pineal gland is thus regulated via fibre tracts indirectly connected to the eyes.

Pathology[Bearbeiten]

Space-occupying lesions in the area of the pineal gland are summarised as pinealomas. The most common are the benign, non-neoplastic pineal cysts. Germinomas are the most common primary neoplasia.

A haemorrhagic infarction of the pineal gland is referred to as pineal apoplexy.

Radiology[Bearbeiten]

In about 70% of adults, calcifications of the pineal gland are recognisable in lateral radiographs and especially in computer tomography (CT). The commissura habenularum can also calcify and be visible as a small C-shaped shadow above and in front of the epiphysis.

In magnetic resonance imaging (MRI), the calcifications appear hypointense in the SWI sequence. The gland shows a similar signal intensity as the grey matter. The lack of a blood-brain barrier leads to a clear accumulation of contrast medium.

For the French mathematician and philosopher René Descartes, the pineal gland was the seat of the soul due to its sensitivity to light in lower animals. As the pineal gland was the only known unpaired part of the brain, Descartes surmised that it was the point at which the contents of both hemispheres of the brain were brought together and the soul became conscious. (cf. funnel shape of the pineal gland)

Central nervous system