Second Eye

TITLE
PINEAL GLAND: “THE HIDDEN EYE”

INTRODUCTION

The third eye, also referred as parietal eye or pineal eye, is a photosensory organ that triggers hormone production and thermoregulation. This eye cannot see in quite the same way as the main eyes, instead only detecting shapes and shadows rather than full pictures. They are also highly sensitive to changes in light and dark – producing markedly different hormones based on time of day. It produces melatonin, a hormone that affects the modulation of sleep/wake patterns and photoperiodic (seasonal) functions. It also contributes to the sexual development of a person. Its location on the near or center of the two hemispheres of the brain takes into a lead that it’s the link of physical and
spiritual worlds. Mysticism and superstition were linked to the pineal gland over a long time because it’s hidden. Development of psychic talents has been closely associated with this organ of higher vision. Since it is pseudoscience, it will not be the main focus of the study.

OBJECTIVES
In this study, the embryonic, morphologic, anatomic, and phylogenetic aspects of pineal gland development were reviewed. The relationship between pineal gland and retina were examined based on their morphological, functional and structural similarities and differences. Additionally, the comparative evolutionary study of the pineal gland across vertebrate classes was put with emphasis. Furthermore, its potential to become an actual eye was also examined.

DISCUSSION
The pineal gland is a photoreceptive neuroendocrine gland in many vertebrate species.
This pine cone-shaped gland is derived from the evagination of the diencephalon roof. Topographically, the pineal gland occupies the impression between the superior colliculi of the mesencephalon and is burrowed between the two hemispheres of the brain. This gland can produce hormones that are focused for neurological regulation such as the serotonin and its derivative melatonin, which encodes and disseminates information on circadian rhythm, thus coupling the outside world to the biochemical and physiological internal demands of the
body.
As aforementioned, the pineal gland has photoreceptive capabilities which link it with that of the retinal photoreceptor cells. The evolutionary relationship between these two organs can be examined through comparative studies on their morphological and functional similarities or differences.
Several relationships exist between the pineal gland and the retina. Their similarities in development and morphology led to the further understanding on their functional similarities. A study showed the notable feature of both the retina and the pineal gland is the production of the indolamine hormone, melatonin. (Wiechmann, 1986)
Due to the discovery of the similarities between these two organs, scientists separately studied these two and unexpectedly concluded that the pineal gland was the evolutionary precursor to the modern eye.
(Miller, 1985) The pineal gland was then considered as the “mind’s eye” because of its involvement in the activity of the eye in making the person feel sleepiness and the ability of photoreception even though it does not receive direct light to form images (occurs especially when a person is dreaming due to synthesis of DMT hormone in-charge of it). By this, the pineal gland has then the biological potential of being an actual
eye. “Under the skin in the skull of a lizard lies a light-responsive “third eye” which is the evolutionary equivalent of the bone-encased, hormone-secreting pineal gland in the human brain. The human pineal is denied access to light directly, but like the lizard’s “third eye,” it shows enhanced release of its hormone, melatonin, during the night. The challenge is to understand the mechanisms which regulate the synthesis and release of melatonin. The pineal gland is the ‘mind’s eye.’ Dissected, the reptile’s pineal looks much like an eye, with the same shape and tissue.” – Dr. Cheryl Craft, Ph.D., Chair of the Department of Cell and Neurobiology, University of Southern California. (Bancarz)
Through dissections, the pineal gland and eyes of vertebrates were examined and compared with the human eye and pineal gland. Like the eye, the pineal gland has retinal tissue that is composed of rods and cones (photoreceptors) inside its interior lining and wired into the visual cortex of the brain. The gland also contains vitreous fluid. (Bancarz)
According to some series of genetic studies, the transcription of factor Pax6 is responsible for the master control gene of eye morphogenesis and eye evolution across animal species. (Gehring et al 1999). From these findings, a common evolutionary origin can be observed through molecular and phenotypic evidences, in spite of their anatomical differences. The pineal gland and the neural retina of the eye are related particularly within their photoreceptor cell lines.
The development of the pineal gland resulted from the evagination of the roof of the diencephalon. In an adult brain, this gland exhibits varying shapes among vertebrate species. Additionally, different accessory organs are often associated with it, such as parapineal organ in lampreys and fishes, the frontal organ in amphibians, and the parietal eye in lacertilian reptiles. (Oksche, 1965)
In some vertebrate classes, such as lampreys, fishes, amphibians, reptiles and birds, the pineal gland serves as photosensory organ having photosensitivity capabilities. While in mammals, the pineal gland lost such photosensitivity capabilities and instead, serves as a neuroendocrine organ which regulates melatonin synthesis. This gland in several species also contains an intrinsic circadian oscillator, which is entrained to environmental light–dark cycles due to the endogenous photic input pathway. (Falcón, 1999)
The evolutionary changes of the pineal gland from the photosensory to neuroendocrine gland are in lieu with the changes in its cell composition and morphology. In lampreys, fishes and amphibians, its pineal photoreceptor cells closely resemble the retinal photoreceptor cells possessing well-developed lamellar outer segments. The light signals that the pineal photoreceptor cells capture are transmitted to secondary neurons, which innervate the central brain areas and are similar to retinal ganglion cells. In reptiles and birds, the pineal photoreceptor cells are also called as modified photoreceptor cells endowing regressed outer segments with lamellar structures variously degenerated. There is less abundancy with the secondary neurons in the pineal gland among these classes. Lastly, in mammals, the pinealocytes in mature pineal gland are already non photosensory and light insensitive lacking now the lamellar outer segments.
During the evolution of vertebrates, the phylogenetic changes of the pineal gland photoreceptor cells were observed with the gradual transformation in a single cell lineage. (Figure 1.a) However, recent studies proposed new hypothesis stating that the pineal gland of vertebrates originated in cell-fate restriction from a common progenitor cells during their evolution (Ekström et al, 2003). (Figure 1.b)

Figure 1. Evolutionary changes of vertebrate pineal cells
Further evidence for a common origin of the pinealocyte and retinal photoreceptor comes from studies showing that numerous gene products otherwise involved in retinal phototransduction are also expressed in the adult mammalian pineal gland. Similarly, the retina expresses genes dedicated to melatonin synthesis, although at distinctly lower levels than in the pineal gland; the capacity to synthesize melatonin in small quantities is seen in all vertebrates. (Rath et al, 2013)

CONCLUSION
The human glandula pinealis is more but a phylogenetic rudiment of the so-called ‘third eye’, or the parietal eye of lower vertebrates. Despite it being rudimentary, its functions, in general, in all vertebrates are still as a neuroendocrine transducer. It is currently known that depending on the species the pineal gland can either directly or indirectly convert environmental lighting conditions into a neurohormonal message, the nocturnally elevated synthesis of melatonin.
Due to the discovery of the similarities between the pineal gland and the retina, scientists separately studied these two and unexpectedly concluded that the pineal gland was the evolutionary precursor to the modern eye. The pineal gland was then considered as the “mind’s eye” because of its involvement in the activity of the eye in making the person feel sleepiness and the ability of photoreception even though it does not receive direct light to form images. By this, the pineal gland has then the biological potential of being an actual eye.

REFERENCES
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Gehring, W. J. and K. Ikeo (1999) Pax 6: Mastering eye morphogenesis and eye evolution. Trends Genet. 15, 371–377.
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Falcón, J. (1999) Cellular circadian clocks in the pineal. Prog. Neurobiol. 58, 121–162.
Ekström, P. and H. Meissl (2003) Evolution of photosensory pineal organs in new light: The fate of neuroendocrine photoreceptors.Phil. Trans. R. Soc. Lond. B 358, 1679–1700.
Rath, M. F., Rohde, K., Klein, D. C., & Møller, M. (2013). Homeobox genes in the rodent pineal gland: roles in development and phenotype maintenance. Neurochemical Research, 38(6), 1100–1112. doi:10.1007/s11064-012-0906-y
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Miller, Julie Ann (1985) Eye to (Third) Eye; Scientists are Taking Advantage of Unexpected Similarities between the Eye’s Retina and the Brain’s Pineal Gland Retrieved December 6, 2014 from www.highbeam.com/publications/science-news-p5686/november-1985

Wiechmann, AF. (1986). Melatonin: Parallels in Pineal Gland and Retina. Retrieved December 6, 2014 from www.ncbi.nlm.nih.gov/pubmed/3013666

Bailey, Regina. (n.d.). Pineal gland. Retrieved December 6, 2014 from www.biology.about.com/ od/anatomy/p/pineal-gland.htm