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. [cite book
last =Eakin
first =R. M
authorlink =
coauthors =
title =The Third Eye
publisher =University of California Press
date =1973
location =Berkeley
pages =
url =
doi =
id = ]

Function

The lizard-like reptile tuatara has a "well-developed parietal eye, with small lens and retina". [cite web
last =Uetz
first =Peter
authorlink =
coauthors =
title =Sphenodontidae
work =The EMBL reptile database
publisher =European Molecular Biology Laboratory, heidelberg
date =2003-10-07
url =http://www.embl-heidelberg.de/~uetz/families/Sphenodontidae.html
format =
doi =
accessdate = 2007-02-22 ] [cite web
last =
first =
authorlink =
coauthors =
title =Parietal eye
work =Tuatara Glossary
publisher =School of Biological Sciences, Victoria University of Wellington
date = 2007-09-11
url =http://www.victoria.ac.nz/sbs/tuatara/glossary.aspx#_N-Q
format =
doi =
accessdate = 2008-05-28] 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.

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. [cite book
last =Zug
first =George
authorlink =
coauthors =Vitt, Laurie Vitt; and Caldwell, Janalee
title =Herpetology: An Introductory Biology of Amphibians and Reptiles, Second Edition
publisher = Academic Press
date =2002
location =San Diego
pages =75
url =
doi =
id = 0-12-782622-X ] The parietal eye uses a different biochemical method of detecting light than rod cells or cone cells in a normal vertebrate eye. [cite journal
last =Xiong
first =Wei-Hong
authorlink =
coauthors =Solessio,Eduardo C.; and Yau, King-Wai
title =An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor
journal =Nature Neuroscience
volume =1
issue =
pages =359–65
publisher =
date =1998
url =http://www.nature.com/neuro/journal/v1/n5/full/nn0998_359.html
doi =
id =
accessdate = 2007-02-22 ]

References