Parietal eye

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Tuataras have a well-developed (but only seen in hatchlings) parietal eye.
Tuataras have a well-developed (but only seen in hatchlings) parietal eye.

A parietal eye, also known as a parietal organ or third-eye is a part of the epithalamus present in some animal species. The eye may be photoreceptive and is usually associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation.[1]

[edit] Function

The lizard-like reptile tuatara has a "well-developed parietal eye, with small lens and retina".[2][3] Parietal eyes are also found in lizards, frogs and lampreys, as well as some species of fish, such as tuna and pelagic sharks, where it is visible as a light-sensitive spot on top of their head. A poorly developed version, often called the parapineal gland, occurs in salamanders. In birds and mammals the parietal organ (but not the pineal gland) is absent.

[edit] Physiology

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 parietal organ (often called the parietal eye, or third eye if it is photoreceptive). It arises as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon. In some species, it protrudes through the skull.[4] The parietal eye uses a different biochemical method of detecting light than rod cells or cone cells in a normal vertebrate eye.[5]

[edit] References

  1. ^ Eakin, R. M (1973). The Third Eye. Berkeley: University of California Press. 
  2. ^ Uetz, Peter (2003-10-07). Sphenodontidae. The EMBL reptile database. European Molecular Biology Laboratory, heidelberg. Retrieved on 2007-02-22.
  3. ^ Parietal eye. Tuatara Glossary. School of Biological Sciences, Victoria University of Wellington (2007-09-11). Retrieved on 2008-05-28.
  4. ^ Zug, George; Vitt, Laurie Vitt; and Caldwell, Janalee (2002). Herpetology: An Introductory Biology of Amphibians and Reptiles, Second Edition. San Diego: Academic Press, 75. 0-12-782622-X. 
  5. ^ Xiong, Wei-Hong; Solessio,Eduardo C.; and Yau, King-Wai (1998). "An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor". Nature Neuroscience 1: 359–65.