Cells of pineal gland

Five types of cells that differ in their cellular structures are found during pineal gland activation. Pinealocytes are the most abundant cells of the third eye. They are made up of a cell body with 4-5 emerging processes and can be stained by special silver impregnation methods. They secrete melatonin. The cytoplasm is lightly basophilic and shows lengthy and branched cytoplasmic processes that extend to the connective tissue septa. Next to pinealocytes are the interstitial cells that are located in between them. They have elongated nuclei and their cytoplasm takes darker stain as compared to the pinealocytes. The gland is traversed by many capillaries which contain perivascular phagocytes. These perivascular phagocytes are antigen presenting cells. In higher vertebrates neurons are also present in the gland but they are absent in rodents. In some species neuron-like peptidergic cells are also present and it is believed that they have a paracrine regulatory role.

Nerve supply

Superior cervical ganglion sends sympathetic nerve supply to the pineal gland but sphenopalatine and otic ganglia also supply parasympathetic nerves to it. Neurons of the trigeminal ganglia finally innervate the gland with nerve fibers containing a neuropeptide, PACAP. Human follicles have a variable amount of a gritty material known as copora arenacea which is chemically made up of calcium phosphate, calcium carbonate, magnesium phosphate and ammonium phosphate. Studies carried out in 2002 reveal that calcium carbonate is present in the form of calcite. Calcium, phosphorous and fluoride deposits present in the pineal gland play some role in ageing.

Anatomy

Pinealocytes of many non-mammalian vertebrates resemble photoreceptor cells of eye. Some evolutionary biologists believe that vertebrate pineal cells share an ancestral relationship with the retinal cells. In vertebrates, an exposure to light can trigger a series of enzymatic events in the pineal gland that regulate circadian rhythms. Early vertebrate fossil skulls have shown the presence of pineal foramen which is considered as a photosensitive structure in the living fossils namely lamprey and tuatara. Third eye represents evolution’s approach towards photoreception. The structures associated with the third eye in tuatara are homologous to cornea, lens and retina. However, in animals like humans who have lost the third eye or the parietal eye a pineal sac is retained which is known as the pineal gland. This gland is not isolated from the blood brain barrier and has a rich supply of blood vessels next to the kidneys. A 90 million years old fossil of brain of the Russian Melovatka bird shows exceptionally large parietal and pineal gland. In humans and other mammals light signals are sent from the retina of the eye to the suprachiasmatic nucleus (SCN) through the retinohypothalamic tract which finally enters pineal gland.

Function

Earlier pineal gland was believed as a vestigial remnant of a larger organ. In 1917 it was found that the frog’s skin color was lightened when a dose of extract of pineal gland of cows was given. Dermatology professor Aaron B. Lerner and his colleagues at the Yales University isolated the hormone melatonin in 1958. Removal of pineal gland induced ovarian growth in rats while keeping the rats in constant light decreased the weight of their pineal gland and other related findings resulted in the development of a new branch of science known as chronobiology. Melatonin is N-acetyl-5-hydroxy-tryptamine which is a derivative of the amino acid tryptophan and has some functions coupled with the central nervous system. Melatonin production by the pineal gland is stimulated in dark but inhibited in light. Photosensitive cells present in the retina of eye detect light and send signal to the SCN which in turn entrain the 24-hour cycle in nature. Fibers arising from the SCN enter the paraventricular nucleus (PVN) which passes the circadian signal to the spinal cord via sympathetic system. The signal then reaches the superior cervical ganglia (SCG) and finally enters the pineal gland. The exact function of melatonin in humans is still not clear but it is used in the treatment of circadian rhythm sleep disorders. A compound known as pinoline has been isolated from the pineal gland which is chemically a beta-carboline.

The human pineal gland grows in size till 1-2 years of age and then becomes stable but its weight increases until the individual enters puberty. The elevated levels of melatonin in children are believed to inhibit sexual development and pineal tumors have been found to be associated with precocious puberty. As an individual enters puberty melatonin secretion is reduced. Calicification of the pineal gland is a typical feature in adult vertebrates. Internal secretions of the third eye inhibit development of reproductive organs. In animals pineal gland is known to play a major role in sexual development, hibernation, metabolism and seasonal breeding. Cytostructure of pineal gland bears evolutionary resemblance with the retinal cells of chordates.

The pineal gland activation of modern birds and reptiles is known to produce a photo-transducing pigment known as melanopsin. Avian third eye is believed to behave like the SCN of mammals. Studies have shown that the pineal gland contains magnetic material in birds and other animals where it acts as center for navigation. Researchers believe that the gland acts as a magnetoreceptor and thus, helps the body to align in space. Electromagnetic fields (EMF) are known to suppress its activity and thus, reduce melatonin production. EMF activity is also known to affect the circadian rhythmicity. In lower vertebrates the pineal gland has a structure resembling the eye so it functions as a light receptor. Studies carried out on rodents suggest that pineal gland activation may influence the actions of recreational drugs like cocaine and antidepressants like Prozac. Melatonin is known to protect against neuro-degeneration.

Melatonin

The production of melatonin is greatly affected by the amount of light received by the third eye. It acts as a circadian clock due to its sensitivity to light and role in regulation of sleep-wake cycle of animals. While sleeping in the night the levels of melatonin in the body increase reach a peak between 11 PM and 2 AM and decline as the day begins. Melatonin production is age dependent. Its production increases at the age of three months, reaches its peak at the age of six and then declines at the beginning of puberty. The chemical structure of melatonin is very simple but it plays an important role in regulation of hormone production by different glands of human body. It also controls the over-stimulation of the sympathetic nerves thus, lowers blood pressure and slows heart rate. It also lessens mental stress, improves sleep, adjusts the circadian clock, strengthens immunity, relives jet lag, increases body’s resistance to germs as well as prevents cancer and senile dementia.

The production of melatonin is inversely proportional to that of serotonin which acts as a neurotransmitter and constricts the blood vessels. Brain is highly active during the day so the production of serotonin increases during the day. However, during night when mind is less active the production of serotonin decreases while that of melatonin increases. When eyes perceive light, the production of melatonin decreases that is why the individuals who sleep with lights on in the room have lesser immune power and are more prone to cancer and other infections. Two studies carried out in the United States show that bright light reduces melatonin production and increases estrogen secretion in women which in turn increases the incidence of breast cancer in night shift workers. Research carried out on the infants who died from sudden infant death syndrome (SIDS) showed that the infants had underdeveloped pineal glands due to which melatonin levels got lowered and the immune power of the body to deal with the free radicals got diminished.

Recent studies have indicated that melatonin also acts as an antioxidant and neuroprotector that may be of importance in ageing and Alzheimer’s disease. Circadian disorders such as sleep-wake cycle disturbances are linked with ageing and are more pronounced in Alzheimer’s disease (AD). A dysfunction of the sympathetic regulation of melatonin synthesis and the SCN is responsible for the melatonin changes during the early stages of AD. However, reactivation of the circadian rhythm by using light therapy and melatonin supplementation has given positive results. Melatonin supplements are available in the United States since 1996. Tumors of pineal gland are rare and most of them arise due to abnormal embryonic germ cells. They generally take the form of germinomas which resemble the testicular seminoma or ovarian dysegerminoma. The secretory activity of pineal gland is only partially understood. Small amounts of melatonin have been isolated from plant species also for example, oats, sweet corn, ginger, tomato, bananas and barley. Intake of other food items like soybeans, kelp, pumpkin and water melon seeds, almonds, peanuts, yeast, malt and milk also increases melatonin production in the body. You can also increase your melatonin levels by listening to pineal gland activation music.

Navodita Maurice