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Abstract |
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Retinal Photoreceptor Fine Structure in some reptiles Fairoze Khattab; (Department of Zoology, Faculty of Science, Abbassia, Abstract
The structure of the photoreceptors of four different reptiles: the
homed viper Cerastes cerastes
(diurnal and nocturnal), the European Chameleo
chameleon (diurnal), the gold skink Eumeces
schneidrii (diurnal) and the Egyptian gecko, Tarentola annularis (nocturnal) has been investigated by light
and electron microscopy.
The photoreceptors of diurnal reptiles were mainly of the cone type
and those of nocturnal were mainly rods. The ellipsoid
region of both double rods in the nocturnals and large single cones in the species having
both nocturnals and diurnal activity, consist of several mitochondria
arranged in a remarkable radially gradient architecture which accommodates with the
specific function of this region as a focusing device helping to condense
light onto the outer segments. Moreover the principle cone of
double cone and single cone of diurnal reptiles possessed a large oil droplet
in the region between the inner segment and outer segment. This droplet is
thought to play a role in filtering light and so doing enhanced contrast
reduce glare and lessen chromatic aberration. It is worth to mention that the
outer segment of rods in nocturnal reptiles approaches a length of
approximately four folds the length of the inner segments of the same
photoreceptors cells. This character is of a particular interest, since the outer segment is
the site of photopigments and the increase in its length magnifies its
ability of light and consequently accommodate with the night vision. |
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