EFFECT OF LIGHT AND DARK ON THE DEVELOPING RETINA OF THE EGYPTIAN TOAD, BUFO REGULARIS
M.I. Michael D.A.El-Mekkawy T.A.Rizk (Dept. of Zoology, Faculty of Science, University of Alexandria, Egypt)
The effect of light and dark on the retina of the Egyptian toad, Bufo regularis Reuss, was studied during its development from early larval stages till the end of metamorphosis. The observed changes in response to light and dark were restricted to the pigment epithelium and the photo-receptor cells. The animals examined were divided into four groups: 1-controls fixed in light (CL); 2-controls fixed in dark (CD); 3-animals maintained under continuous darkness (DD): and 4-animals exposed to continuous lighting (LL). In CD-and DD-animals, the location of melanin granules in the pigment epithelial processes (PEP) was restricted to the peripheral area between the apical portions of photoreceptor cells (scleral position). In CL-and LL-animals, on the contrary, abundant melanin granules were found in the PEP where they were dispersed more centripetally between the outer segments and ellipsoids of the visual cells (vitreal position). Photomechanical movements of the epithelial pigments were indicated in this species in response to light and dark before reaching the limb-bud stage. In DD-eye of the newly metamorphosed toadlets, the pigment epithelium appeared considerably thick and containd large lipid droplets when compared to that of the other three groups. Only in DD-animals some rod outer segments showed degeneration during the later stages of development. In addition, there was a remarkable reduction in the number of cones in the eyes of this group as a result of continuous light deprivation. Some differences in the position of cone nuclei were also observed in the retinae of the four groups.