Parietal eye

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Lithobates catesbeianus
)
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

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.

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

Discovery

Franz Leydig, professor of zoology at the

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

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

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

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

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

Petromyzon elongates very considerably during metamorphosis.[21]

Analogs in other species

ocelli, between their main, compound eyes.[22]

See also

References

  1. ^ Eakin, R. M (1973). The Third Eye. Berkeley: University of California Press.
  2. ^
    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. ^
    PMID 14298722
    .
  6. ISSN 2155-3017
    . Retrieved 2023-03-28.
  7. 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. JSTOR 1562791
    .
  10. ISSN 0567-7920
    .
  11. PMID 27940498
    .
  12. OCLC 317753687.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link
    )
  13. doi:10.1080/08927014.1997.9522875
    .
  14. ^ a b 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.
    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".
    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. PMID 18089073
    .
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