Parietal eye

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The parietal eye (very small grey oval between the regular eyes) of a juvenile bullfrog (Lithobates catesbeianus) Frog parietal eye.JPG
The parietal eye (very small grey oval between the regular eyes) of a juvenile bullfrog ( Lithobates catesbeianus )
Adult green anole (Anolis carolinensis) clearly showing the parietal eye (small grey/clear oval) at the top of its head. Anolis carolinensis parietal eye.JPG
Adult green anole (Anolis carolinensis) clearly showing the parietal eye (small grey/clear oval) at the top of its head.
Parietal eye of the Merrem's Madagascar swift (Oplurus cyclurus) is surrounded by a black-and-white spot on the skin, giving it the "three-eyed" appearance Madagascar spiny tailed iguana cropped.jpg
Parietal eye of the Merrem's Madagascar swift ( Oplurus cyclurus ) is surrounded by a black-and-white spot on the skin, giving it the "three-eyed" appearance

A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation. [1] The hole in the head which contains the eye is known as a pineal foramen or parietal foramen, since it is often enclosed by the parietal bones.

Contents

The parietal eye was discovered by Franz Leydig in 1872 from lizards. [2]

Discovery

Franz Leydig, professor of zoology at the University of Tübingen, dissected four species of European lizards – the slow worm ( Anguis fragilis ) and three species of Lacerta – [2] in 1872, [3] and found cup-like protrusions under the middle portion of their brains. He believed these to be glandular in nature, and gave them the name frontal organ (German Stirnorgan). [2]

In 1886, University of Oxford anatomist Walter Baldwin Spencer reported his dissection results of 29 species of lizards, and noted the presence of the same structure Leydig had described. He called it the pineal eye or parietal eye, as he saw it was associated with the parietal foramen and the pineal stalk. [4] In 1918, Swedish zoologist Nils Holmgren found the pineal eye in frogs and dogfish. [5] He noted that the structure contained sensory cells that looked like the cone cells of the retina, [6] and hypothesised that this pineal eye could be a primitive light-sensing organ (photoreceptor). The organ has become popularly known as the "third eye". [5]

Presence in various animals

The parietal eye is found in the tuatara, most lizards, frogs, salamanders, certain bony fish, sharks, and lampreys. [7] [8] [9] It is absent in mammals, but was present in their closest extinct relatives, the therapsids, suggesting it was lost during the course of the mammalian evolution due to it being useless in endothermic animals. [10] It is also absent in the ancestrally endothermic ("warm-blooded") archosaurs such as birds. The parietal eye is also lost in ectothermic ("cold-blooded") archosaurs like crocodilians, and in turtles, which may be grouped with archosaurs in Archelosauria. [11] Despite being lepidosaurs, as lizards and tuatara are, snakes lack a parietal eye. [12] [13]

Anatomy

The third eye, where present, is always much smaller than the main paired eyes, and, in living species, it is always covered by skin, and is usually not readily visible externally. [14]

The parietal eye is a part of the epithalamus, which can be divided into two major parts; the epiphysis (the pineal organ, or pineal gland if mostly endocrine) and the parapineal organ (often called the parietal eye, or if it is photoreceptive, the third eye). These structures arise as a single anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon, but during development it divides into two more or less bilaterally symmetric organs which rotate their location to become a caudal pineal organ and a parapineal organ. In some species, it protrudes through the skull. [15] [16] The parietal eye uses a different biochemical method [ further explanation needed ] of detecting light from that of rod cells or cone cells in a normal vertebrate eye. [17]

Many of the oldest fossil vertebrates, including ostracoderms, placoderms, crossopterygians, and even early tetrapods, had a socket in the skull that appears to have held a functional third eye. This socket remains as a foramen between the parietal bones even in many living amphibians and reptiles, although it has vanished in birds and mammals.

Lampreys have two parietal eyes, one that developed from the parapineal organ and the other from the pineal organ. These are one behind the other in the centre of the upper surface of the braincase. Because lampreys are among the most primitive of all living vertebrates, it is possible that this was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above. [14]

Saniwa , an extinct varanid lizard, probably had two parietal eyes, one that developed from the pineal organ and the other from the parapineal organ. Saniwa is the only known jawed vertebrate to have both a pineal and a parapineal eye. In most vertebrates, the pineal organ forms the parietal eye, however, in lepidosaurs, it is formed from the parapineal organ, which suggests that Saniwa re-evolved the pineal eye. [18]

Comparative anatomy

The parietal eye of amphibians and reptiles appears relatively far forward in the skull; thus it may be surprising that the human pineal gland appears far away from this position, tucked away between the corpus callosum and cerebellum. Also the parietal bones, in humans, make up a portion of the rear of the skull, far from the eyes. To understand this, note that the parietal bones formed a part of the skull lying between the eyes in sarcopterygians and basal amphibians, but have moved further back in higher vertebrates. [19] Likewise, in the brain of the frog, the diencephalon, from which the pineal stalk arises, appears relatively further forward, as the cerebral hemispheres are smaller but the optic lobes are far more prominent than the human mesencephalon, which is part of the brain stem. [20] In humans the optic tract, commissure, and optic nerve bridge the substantial distance between eyes and diencephalon. Likewise the pineal stalk of Petromyzon elongates very considerably during metamorphosis. [21]

Analogs in other species

Crustaceans at the nauplius stage (first-stage larva) have a single eye atop the head. This eye has a lens and senses the direction of light but cannot resolve details. More sophisticated segmented eyes develop later on the sides of their heads, but the initial eye also stays for some time. So, it is possible to say that, at some stage of development, crustaceans also have a "third eye". Some species, like the brine shrimp, retain this primary eye through all stages of their life. Most arthropods have one or more simple eyes, called ocelli, between their main, compound eyes. [22]

See also

Related Research Articles

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<i>Thliptosaurus</i> Extinct genus of dicynodonts

Thliptosaurus is an extinct genus of small kingoriid dicynodont from the latest Permian period of the Karoo Basin in KwaZulu-Natal, South Africa. It contains the type and only known species T. imperforatus. Thliptosaurus is from the upper Daptocephalus Assemblage Zone, making it one of the youngest Permian dicynodonts known, living just prior to the Permian mass extinction. It also represents one of the few small bodied dicynodonts to exist at this time, when most other dicynodonts had large body sizes and many small dicynodonts had gone extinct. The unexpected discovery of Thliptosaurus in a region of the Karoo outside of the historically sampled localities suggests that it may have been part of an endemic local fauna not found in these historic sites. Such under-sampled localities may contain 'hidden diversities' of Permian faunas that are unknown from traditional samples. Thliptosaurus is also unusual for dicynodonts as it lacks a pineal foramen, suggesting that it played a much less important role in thermoregulation than it did for other dicynodonts.

The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region, usually lying medial (inwards) relative to the squamosal and lateral (outwards) relative to the parietal and/or postparietal. It may also contact the postorbital or intertemporal, or tabular, when those bones are present.

The postfrontal is a paired cranial bone found in many tetrapods. It occupies an area of the skull roof between and behind the orbits, lateral to the frontal and parietal bones, and anterior to the postorbital bone.

References

  1. ↑ Eakin, R. M (1973). The Third Eye. Berkeley: University of California Press.
  2. 1 2 3 Eakin, Richard M. (1973), "3 Structure", The Third Eye, University of California Press, pp. 32–84, doi:10.1525/9780520326323-004, ISBN   978-0-520-32632-3 , retrieved 2023-03-28
  3. ↑ Flemming, A.F. (1991). "A third eye". Culna (40): 26–27 – via Sabinet.
  4. ↑ Spencer, Sir Baldwin (1885). "On the Presence and Structure of the Pineal Eye in Lacertilia". Quarterly Journal of Microscopy. London. pp. 1–76.{{cite book}}: CS1 maint: location missing publisher (link)
  5. 1 2 Wurtman, R. J.; Axelrod, J. (1965). "The pineal gland". Scientific American. 213 (1): 50–60. Bibcode:1965SciAm.213a..50W. doi:10.1038/scientificamerican0765-50. ISSN   0036-8733. PMID   14298722.
  6. ↑ Pearce, J.M.S. (2022). "The pineal: seat of the soul". Hektoen International. ISSN   2155-3017 . Retrieved 2023-03-28.
  7. ↑ Dodt, Eberhard (1973). "The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates". Visual Centers in the Brain. Handbook of Sensory Physiology. Vol. 7 / 3 / 3 B. Springer, Berlin, Heidelberg. pp. 113–140. doi:10.1007/978-3-642-65495-4_4. ISBN   9783642654978.
  8. ↑ Uetz, Peter (2003-10-07). "Sphenodontidae". The EMBL reptile database. European Molecular Biology Laboratory, heidelberg. Archived from the original on 2007-02-21. Retrieved 2007-02-22.
  9. ↑ Gundy, GC; Wurst, GZ (1976). "The occurrence of parietal eyes in recent Lacertilia (Reptilia)". Journal of Herpetology. 10 (2): 113–121. doi:10.2307/1562791. JSTOR   1562791.
  10. ↑ Benoit, Julien; Abdala, Fernando; Manger, Paul R.; Rubidge, Bruce S. (2016-03-17). "The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids". Acta Palaeontologica Polonica. 61 (4): 777–789. doi: 10.4202/app.00219.2015 . ISSN   0567-7920.
  11. ↑ Emerling, Christopher A. (2017-03-01). "Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck". Molecular Biology and Evolution. 34 (3): 666–676. doi: 10.1093/molbev/msw265 . ISSN   1537-1719. PMID   27940498.
  12. ↑ Infectious diseases and pathology of reptiles : color atlas and text. Elliott R. Jacobson. Boca Raton. 2007. p. 21. ISBN   978-1-4200-0403-8. OCLC   317753687.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
  13. ↑ Tosini, Gianluca (1997-10-01). "The pineal complex of reptiles: Physiological and behavioral roles". Ethology Ecology & Evolution - ETHOL ECOL EVOL. 9 (4): 314. Bibcode:1997EtEcE...9..313T. doi:10.1080/08927014.1997.9522875.
  14. 1 2 Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 471–473. ISBN   978-0-03-910284-5.
  15. ↑ Light-sensitive organs that evaginate from the diencephalon - NCBI
  16. ↑ Zug, George; Vitt, Laurie Vitt; Caldwell, Janalee (2002). Herpetology: An introductory biology of amphibians and reptiles (Second ed.). San Diego, CA: Academic Press. p. 75. ISBN   978-0-12-782622-6.
  17. ↑ Xiong, Wei-Hong; Solessio, Eduardo C.; Yau, King-Wai (1998). "An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor". Nature Neuroscience. 1 (5): 359–365. doi:10.1038/1570. PMID   10196524 . Retrieved 2007-02-22.
  18. ↑ Smith, Krister T.; Bhullar, Bhart-Anjan S.; Köhler, Gunther; Habersetzer, Jörg (2 April 2018). "The only known jawed vertebrate with four eyes and the bauplan of the pineal complex". Current Biology . 28 (7): 1101–1107.e2. doi: 10.1016/j.cub.2018.02.021 . ISSN   0960-9822. PMID   29614279.
  19. ↑ "FRONTAL AND PARIETAL BONES=" . Retrieved 2011-09-08.
  20. ↑ "Edible Frog Brain Clipart". Etc.usf.edu. Retrieved 2011-09-08.
  21. ↑ Journal of morphology - Google Books. 1887. Retrieved 2011-09-08.
  22. ↑ Mayer, Georg (2006-12-01). "Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?". Arthropod Structure & Development. 35 (4): 231–245. Bibcode:2006ArtSD..35..231M. doi:10.1016/j.asd.2006.06.003. ISSN   1467-8039. PMID   18089073.
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