Publication: Electron microscopic study of the occlusible tapetum lucidum of the southern fiddler ray (Trygonorhina fasciata)
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Date
1991
Authors
Braekevelt, Charlie R.
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Publisher
Murcia : F. Hernández
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DOI
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info:eu-repo/semantics/article
Description
Abstract
The choroidally located tapetum lucidum of
the southern fiddler ray (Trygonorhina fasciata) has
been examined by light and electron microscopy in both
light- and dark-adaptation. In this species. the tapetum
consists of a single layer of overlapping cells oriented at
an angle of about 30" to the incoming light. These are
situated immediately external to the choriocapillaris.
These tapetal cells alternate with and are separated
from one another by melanocytes which have an inner
extension that curves and intervenes between the tapetal
cells and the choriocapillaris. The tapetal cells and
the melanocytes are flattened cells with their widest
dimension facing the retina. Internally the tapetal cells
display a peripherally-located, vesicular nucleus with
most of the cell organelles in a paranuclear location. The
bulk of the cell is packed with regularly-spaced crystals
reported to be guanine. The size and spacing of these
reflective crystals is commensurate with constructive
interference. In light-adaptation the small melanosomes
of the melanocytes are widely dispersed and fill the
portion of the cell intervening between the tapetal cells
and the incoming light. This effectively occludes the
tapetum as light is unable to reach the reflective material.
In dark-adaptation the melanosomes withdraw from this
location, exposing the tapetum to light and allowing it to
act as a reflective layer. The retinal epithelium overlying
the tapetal area is totally unpigmented so as not to
interfere with the passage of light.
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