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N Katagiri, Y Shimatani, Y Katagiri; Morphogenesis of Dorsal Eye which Contains Ciliary- and Rhabdomeric-type Photoreceptor Cells in the Marine Gastropod, Onchidium sp . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2708.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Onchidium has two kinds of eye: stalk eyes (SE) and dorsal eyes (DE). The DE is known as the vertebrate-type, because it resembles the vertebrate eye in structure. It has two types of photoreceptor cells, ciliary-type cells in the retina and rhabdomeric-type cells in the lens, and differs from the SE which has the typical structure of gastropod eyes and develops earlier at the veliger stage. We examined the morphogenesis of the DE by light microscopy. Methods: Adult Onchidium were collected at Boso Peninsula and were allowed to lay eggs in tanks in the laboratory. The veliger larvae metamorphosed into miniature adults up to hatching out in the egg capsules. Serial semi-thin sections of juvenile dorsal mantles were prepared with standard techniques for TEM observation. Results: At hatching, Onchidium has paired SEs but no DEs. The DEs are first recognized as dark spots in the central region of the mantle of 1.5 mm juveniles at 2 to 4 weeks after hatching. A group including 3 to 4 DEs develops in an ocellous papilla that is added onto the mantle every 4 to 5 days. The morphogenesis of the DE was divided into five stages. (1) Epidermal cells of the prospective ocellous papilla thickened and a nerve bundle grew towards it. (2) Undifferentiated cells and pigment cells clustered and this area began to protrude. (3) Large cells appeared in the cluster of cells that were surrounded by pigment cells. (4) The immature DE developed into a cup of loosely arranged pigment cells, a large lens and a cluster of immature ciliary cells. Between stages 3 and 4, two types of photoreceptor cells differentiated: ciliary cells which are characterized by deformed ciliary membranes; and rhabdomeric cells which have microvilli. (5) The DE developed into a miniature version of the adult form: the retina consisted of a layer of ciliary cells and the lens cells had massive microvilli. Conclusion: The SE has been known to develop from invagination of the epidermis adjacent to the CNS, but the DE is formed by different process and its unique morphogenesis is noteworthy.
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