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Katie Irwin, Ashley Margret Rasys, Tathiana Roldan, Douglas B Menke, James D Lauderdale; The lizard third eye. Invest. Ophthalmol. Vis. Sci. 2018;59(9):577.
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© ARVO (1962-2015); The Authors (2016-present)
The third eye, commonly known as the parietal eye, is a non-visual, photosensitive parapineal organ found in most lizards, frogs, the tuatara, and some species of fish. The parietal eye of the lizard is remarkably well developed, possessing a lens, cornea, and retina, but little else is currently known about this organ. This project aims to compare the parietal to the lateral eye in the Anolis sagrei lizard, hypothesizing that parietal and lateral eye development employ similar gene networks. Because of the parietal eye’s fascinating eyelike structures, understanding its development in comparison to the lateral eye will provide a unique system, not present in humans or many other vertebrates, for gaining insight into mechanisms underlying formation of vertebrate eye structures.
A. sagrei eggs were collected from adult lizards, and embryos were removed from their shells and assigned a developmental stage based on morphological criteria. Tissue was then dissected, fixed, and processed for paraffin wax sectioning. After sectioning, tissue was stained with hematoxylin and eosin and imaged in order to construct a timeline of morphological development encompassing each embryonic stage, the hatchling, and the adult. Indirect immunofluorescence microscopy was used to assess the expression of genes associated with eye development, including Pax6.
The parietal and pineal vesicle form from an evagination of the dorsal diencephalon during the period of embryo development concurrent with formation of the lens vesicle in the eye. Shortly after initial formation, the parietal (anterior) and pineal (posterior) vesicles separate. The dorsal aspect of the parietal vesicle gives rise to lens-like cells, and the ventral aspect of the vesicle gives rise to retinal-like cells. Cells in the surface ectoderm immediately adjacent to the vesicle develop into a corneal-like tissue. Pax6 is robustly expressed in the parietal lens epithelium and cornea; low levels of Pax6 are expressed in presumptive glial cells in the parietal retina.
The development of the parietal eye occurs in a series of morphogenic events similar to those expected of eye development. Differences in the histogenesis of the parietal eye compared to the lateral eye suggest a new pathway for lens induction. Further elucidating the specific roles of parietal eye regulatory networks could complement lateral eye studies to lead to better understanding of vertebrate eye formation.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
A. sagrei parietal eye
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