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]

Contents

Function

Physiology

References

Function

The lizard-like reptile tuatara has a "well-developed parietal eye, with small lens and retina".^[2] 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 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.^[3] The parietal eye uses a different biochemical method of detecting light than rod cells or cone cells in a normal eye.^[4]

References

1. The Third Eye, , R. M, Eakin, University of California Press, ,
2. Sphenodontidae
3. Herpetology: An Introductory Biology of Amphibians and Reptiles, Second Edition, , George, Zug, Academic Press, , 0-12-782622-X
4. An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor, , Wei-Hong, Xiong, Nature Neuroscience,