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Retinal Photoreceptor Fine Structure in Some Reptiles

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Retinal Photoreceptor Fine Structure in Some Reptiles The Egyptian Journal of Hospital Medicine Vol., 17 :167 – 186 Dec.2004 I.S.S.N: 12084 1687 -2002 Retinal Photoreceptor Fine Structure in some reptiles Fairoze Khattab; Fahmy I. Khattab; Nagui Fares and Aman Zaki (Department of Zoology, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt.) 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. Introduction The structure of retinal photoreceptors found in most scleral parts of the inner has been investigated in a variety of segment, mainly in cones, in amphibians, vertebrate species (Cohen, 1963 a & b, reptiles and birds ( Hailman, 1976 & Mac Braekevelt, 1972, 1975, 1989, 1992, 1994). Nichol et .al 1978).Synapses of rods and While some variation is noted between cones were first described by Dickson & species, the typical photoreceptor consists Hollenberg,1971,Borwein & Hollenberg, of an outer segment, connecting cilium, 1973). inner segment nuclear region with synaptic Rods and cones are two distinct types process and paired or double cones occur of photoreceptors of the retina were first widely in all groups below the placental described by Schultze (1867 & 1873) , he mammals, including the mammalian studied both nocturnal and diurnal marsupials (Braekevelt,1972). vertebrate species and formulated his The outer segment of the photore- duplicity theory. ceptor lies close to and intimately asso- Vertebrate retinae are duplex, ciated with the pigment epithelium ( Hogan, containing both rods operating maximally et. al , 1974). at low light intensities and cones ,operating The outer and inner segments were maximally at high insensitive and in colour connected by an eccentrically positioned vision (Borwein,1981). Some retinae had stalk (cilium) ( Fawcett, 1966 ; Borwein & been reported to contain rods only, e.g. in Hollenberg, 1973). rats, but a few cones have been shown to be Coloured or colourless oil droplets are present by Hughes (1977). permanent inclusions of the adult visual cell Rods and cones appear together in in some non mammlian species ,They are primates. The cone inner segment is much 167 Retinal Photoreceptor Fine Structure……… bulky than the rod inner segment(Miller & four studied reptiles were shown in first Snyder,1973) The difference in diameter paper in figs. (1 a, b, c & d) between the cone and rod outer segments, however is smaller in cones(Young,1971 I.Cerastes cerastes and Borwein et.al,1980). The retina of Cerastes cerastes is As part of a comparative morphol- formed of two types of cones ,large single ogical study of vertebrate photoreceptors, cones and small single cones ,The later are this report describes the fine structure of in association with the rod type segments these rods and cones in the retinae of some (Figs. 1a & 1 ).The large and small single reptiles and new observations are added to cones are composed of an outer and inner existing studies. segments. The outer segments are closely associated with the pigment epithelial cells Materials And Methods and their processes (Fig.1).The inner For this study the eyes of four healthy segments have densly packed mitochondria adult Egyptian reptiles, the homed viper in their ellipsoid region (Figs.2 & 5).The (Cerastes cerastes),European Chameleon outer segments of large single cones are (Chameleo chameleon), gold skink short and wide (Figs.2&3).They are (Eumeces schneiderii) and Egyptian gecko occupied by double membrane disks, or (Tarentola annularis) were used. flat tened sacs (Figs.3&4). The myoid All animals under investigation were region of the large single cones displays a collected from Abou-Rawash district, Giza barrel shape and is approximately close to Governorate in Egypt. Animals were the outer limiting membrane (Figs.2&6) sacrificed and the whole eye of each animal and also contains numerous Golgi cisternae was quickly removed, opened at the equator with vacoules (Fig.6).Fine microvillar and fixed for 5 hours in 5% gluteraldehyde processes extend from the inner segment of buffered at pH 7.3 with 0.1 micron the large single cones which are specifically Sorensen's phosphate buffer at 4oC. The found in this species (Fig.6).The second posterior half of the eye ball was then type of cones are the small single cones removed washed in 5% sucrose in 0.1 m which appear smaller than the large single phosphate buffer (ph 7.3) (Hayat, 1970) and cones (Figs 1a,1&2).The small single cones cut into pieces less than 1mm2. The tissue have short outer segment ,which is not as was then post-fixed for 2 hours in 1% wide as the corresponding segment of the osmium tetroxide in the same phosphate large cone (Figs.1& 2).This region is filled buffer they were washed twice for 15 with flattened double membrane disks minutes in a phosphate buffer pH 7.3.then (Fig.7). dehydrated through graded ethanols to There is an accumulation of propylene oxide and embedded in aralldite. condensed mitochondria(ellipsoid) at the Semithin sections of 0.7 m thickness were apex of the inner segment of small single cut with glass knives on the 6000 MT RMC cones (Figs. 8 & 9) The ellipsoid region of ultratome. then stained with 0.25% the small single cones differ from the toluidine blue (Davis,1971) and examined ellipsoid region of the large single cones in by light microscopy. Thin sections (600- the lack of association of mitochondria and 700A0)were then cut and collected on microdroplet (Figs.7&9).The myoid region copper grids. These sections were stained in of the small single cones contains numerous aqueous uranyl acetate and lead citrate and Golgi cisternae, with numerous vacuoles examined and photographed in a JEOL (Fig.8).Each small single cone has a 1200 EXIL transmission electron cytoplasmic process that extends from its microscope. inner segment and is called the claycal process (Fig.7). Results The rods have long cylindrical outer Light microscopic examination of the segments that reach to the pigments the semi-thin sections of the retinae of all the epithelial cells. The lamellar sacs or disks are more closely packed than the 168 Fairoze Khattab et al corresponding compartment of the other The membrane surrounding the oil photoreceptor (Figs.1,5 & 7). A prominent droplets has an irregular outline with ellipsoid region is located at the apex of the several reinforcements, or dense foci at inner segment of the rod, but no oil droplets definitive points of adhesion with the or microdroplets are present (Figs.1 & external membranes of the mitochondria 7).Unlike the cones ellipsoid the that surround the oil droplet (Fig.13).These mitochondria of the rods ellipsoid are less mitochondria encircle the entire perimeter packed and are mostly elongated to oval in of the oil droplet. In the internal portion of shape (Figs.1& 7). the cone inner segments, the mitochondria A structure similar to ellipsoid are spherical, while in the external region of containing closely packed mitochondria is the inner segments they are oval, or infrequently located underneath the outer elongated in shape (Fig.12).As the limiting membrane(Fig.6). mitochondria approach the scleral portions The connecting cilium is the only of the ellipsoid, they acquire a transverse connection between the outer segment and disposition, with the cristae parallel to the the inner segment of both large cone and major axis of the mitochondria (Figs 12 & small cone (Figs.3&10).This cilium is an 15).At this level ,adhesions between the eccentrically positioned stalk containing mitochondrial membranes are frequently nine pairs of microtubules arranged observed. A greater electron density of the circumferentially. mitochondrial matrix is also noticeable in the scleral portions of the inner segments II.Chameleo chameleon (Figs.12&13) The photoreceptor layer of Chameleo The outer and inner segments are chameleon contains only the cone-type of connected by a well developed connecting photoreceptors. It comprises single and cilium (Fig.14). double cone (Figs.1b & 11). In the inner segments, glycogen The double cone consists of a granules are usually accumulated in a principal cone and an accessory cone. Both compact area,(the parabloid).In this region, the single and double cones are composed the glycogen granules are associated with of an outer and an inner segments. The several profiles of both types of inner segment posses a large oil droplet ,an endoplasmic reticulum (Fig.16). The ellipsoid, parabloid and myoid . principal cone does not have a parabloid However, the accessory cone lacks the region(Fig.11).The parabloid of the oil droplet, ellipsoid , parabloid and myoid.
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