The epiphysis is a small endocrine gland located in the center of the skull, where it makes up a large part of the epitalamus. Also known as the pineal gland (due to the shape that broadly follows that of a pine cone), the epiphysis is responsible for the synthesis and secretion of a hormone, called melatonin.
From the anatomical point of view, the epiphysis is covered by a connective covering capsule, made up of the same connective tissue as the pia mater. Inside the gland a parenchyma is recognized consisting of two main cell types: a dense web of interstitial cells that support endocrine cells called pinealocytes (or main cells), which synthesize melatonin.
Despite the small size (diameter of about 8mm) and the negligible weight (0.1g), the epiphysis is far from being a superfluous structure, as it was described until a few decades ago; in fact, melatonin is a key hormone in the regulation of the sleep-wake cycle.
The pineal gland or epiphysis also has an inhibitory effect on the pituitary-gonadal axis; not surprisingly, if its removal or surgical ablation is carried out in the pre-pubertal period there is an early appearance of puberty, whereas when it is performed in adulthood it is accompanied, especially in the male, by hypergonadism. This effect is more pronounced in animals that have a reproductive season in the period in which the day is longer (therefore, as we shall see, the secretion of melatonin is minimal).
Melatonin also seems able to influence the levels of leptin, GH and probably those of many other hormones, since in addition to regulating circadian rhythms (daily), it also contributes to modulating the seasonal rhythms. As if this were not enough, the pineal gland is richly vascularized, with a relative blood flow second only to the renal one.
Melatonin also has an important stimulatory effect on the immune system.
This hormone should not be confused with melanin, a skin pigment that gives dark tones to skin, hair and eyes; in reality, even if only in amphibians, melatonin has opposite effects on the skin compared to melanin.
In mammals, including humans, melatonin is produced by pinealocytes (epiphyseal cells responsible for this synthesis) starting from the amino acid tryptophan, which is converted into serotonin, then into acetylserotonin and finally into melatonin. The activity of this enzyme increases at night and decreases during daytime; consequently, melatonin secretion is stimulated by darkness and inhibited by light. Recent studies indicate that the production of melatonin by the epiphysis also changes in relation to changes in the earth's magnetic field.
Ample and still evolving is the therapeutic use of melatonin, given its hypnotic properties (induces sleep), antidepressants (improves mood disorders), neuroprotective and antioxidants (both melatonin and its metabolites are able to neutralize reactive oxygen and nitrogen species).
The epiphysis was once considered useless, due to the numerous calcification points found within it. Today we know that the process of calcification of the gland begins during puberty and continues into adulthood and old age, gradually undermining its effectiveness.