Robert F. Dunn

Studies on the retina of the gecko Coleonyx variegatus: I. The visual cell classification

Light microscopy studies have established the retina of Coleonyx as containing only rod-type visual cells of three classes, Single rods, and two classes of Double rods. Three additional classes of rods were found in this electron microscopic study which included a Twin rod and two classes of Triplet rods. A new visual cell class nomenclature was presented and defined, and was based on the general morphology of the receptor cell and the relationship of the receptor members to each other. The terminology presented was: Single, Doublets (D 1 and D 2 ), Twin, Triplet 1 and Triplet 2 . A Quintuplet rod was described, but only one was ever found, and this class may not be of physiological significance. The receptor outer segment disks had a five-layered membrane structure when the tissue was fixed in osmium tetroxide and the sections were stained with saturated uranyl acetate at 60°C. The ellipsoid body was comprised of tightly packed mitochondria which fell into two size groups: small mitochondria (3.0–5.5 m μ ) and large mitochondria (11–18 m μ , with a few up to 35 m μ ). The distribution of the small mitochondria formed a hemisphere about the base and sides of the ellipsoids with the large mitochondria in the ellipsoid center. Two types of radially symmetrical paraboloids were described, a granular type which had a central granular mass surrounded by a system of membrane lamellae, and a membranous type which lacked the central granular mass and consisted only of radially symmetrically arranged paired membrane lamellae between which granules were located. In cross sections of the vitread paraboloid and myoid, radial plasma membrane extensions from the visual cells, the lateral fins, were seen to interdigitate freely with Muller cell cytoplasmic extentions. A function of the inner segment-paraboloid-myoid structures in relation to a possible metabolic pathway was presented and discussed.

Quantitative analysis of micrographs by computer graphics

A computer‐aided graphical analysis system is presented which can be used to quantify several types of information from micrographs of cross‐sectioned peripheral nerves. A simple input consisting of eight points, corresponding to two diameters of each axon and two for each axon plus myelin, is used to compute statistical and numerical estimates of these diameters, the myelin thickness being expressed as a function of axon diameter, and both the spatial position and distribution of nerve fibres within the bundle. The computer programs presented and summarized here describe the procedure for digitizing the input data and the subsequent computations for editing and analysis. The general applicability of this approach for automated analyses of other anatomical areas is also presented.

Studies on the retina of the gecko Coleonyx variegatus: II. The rectilinear visual cell mosaic

The regular rectilinear visual cell mosaic of the central retina was described and found to be present at all levels of the receptor cells. This pattern consisted of primary rows comprised entirely of D 1 rods alternating with alternate rows comprised of Single, Twin, Triplet 1 , and Triplet 2 rods. The exclusive presence of D 1 rods in the primary rows readily accounted for the 51–52% D 1 rods found.

Graphic three‐dimensional representations from serial sections

A method is described whereby steric configurations and surface contours may be produced in schematic form from serial light or electron micrographs. The resultant illustration, similar to those used by cartographers, is easily prepared and obviates the necessity of publishing large numbers of half‐tone micrographs to substantiate the analysis.

The dimensions of rod outer segments related to light absorption in the gecko retina

Abstract Measurements of the percentages of retinal area occupied by visual cell outer segments in cross section were completed for two geckos, Gekko gekko and Eublipharis macularis , and found to be 63 and 57 per cent respectively. From these measurements, the visual pigment density at λ max per individual rod was calculated to be 0.58 and 0.71 respectively, and the corrected densities for the entire central retina 0.48 and 0.52 respectively. These latter values correspond to mean values of 67 and 70 per cent, for the percentage of light absorbed at λ max by the entire retina. Errors associated with these measurements are discussed.

On-line computerized entry and display of nerve fiber cross-sections using single or segmented histological records.

Abstract A computer program was developed to measure rapidly the diameters of large numbers of single nerve fibers, to retain their spatial position within the nerve bundle, and to accept photomicrographs whose areas were larger than that of the active input tablet area. The diameters were calculated as circles of best fit based upon the mean of two measured diameters. The position of each fiber within its parent bundle was stored as XY center coordinates which were computed from the mean input diameters. The XY offset for each segment of an oversized photomicrograph was computed from the difference between two pairs of specific translation points. These values were added to the XY values in subsequent segments with the result that fiber measurements from the entire bundle were dimensionally correct within a single logical file.

Studies on the retina of the gecko Coleonyx variegatus: III. Photoreceptor cross-sectional area relationships1

The percentage of the retina occupied by outer segments in cross section was found to be 53%. Based on these measurements, the percentage of light absorption for the total retina was calculated to be 40% which agreed better with the value of 39% given for the cat allowing for the effect of the tapetum than with the value of 68% for a nocturnal gekkonidean. This would indicate the increased outer segment length gives Coleonyx at least the same advantage as the tapetum of the cat. Errors such as tissue shrinkage, convergent lens effect of the inner segment, and measuring errors, which would tend to increase the value of 40%, were discussed and approximated.

Reproducibility of electron microscope magnifications with digital display of individual lens currents

Quantitative measurements on electron micrographs with less than 10% error require precise calibration of all lenses contributing to the final image magnification. Calibration of the magnification to approximately 1.5% is easily attained by using simple statistical methods and a digital display milliampere meter capable of measuring the objective, intermediate and projector lens currents independently. The method detailed, readily adaptable to other electron microscopes, requires the projector lens current be held constant, with the magnification determined by varying the intermediate lens current and corrections made on the basis of calibrated objective lens current effect. The errors of magnification, approximately 1.5%, are the same range as the test grid variation which appears to be the limiting factor. Even with this limitation, magnifications have been determined reproducible to within less than 0.2% in daily use over a 6 month period.