A systematic electron microscopic study was made of the structure of foveal cones of Macaca spp. Transverse sections of inner (IS) and outer segments (OS) were made in sequence, from the pigment epithelial zone (PEZ) to the outer limiting membrane (OLM). The smallest diameters of hundreds of cone sections were measured from electron micrographs with a Zeiss particle-size analyzer, and analyzed statistically. Some details are also included about Cebus photoreceptors. It is claimed in the literature that foveal cones are rod-like (cylindrical) and untapered. Our study shows the foveolar cone to be a tapered structure. There has been some confusion between the foveola, which is rod-free, and the fovea, which has a high concentration of cones, but is not rod-free. Within the fovea, as the ratio of cones to rods falls from infinity to 1, with distance from the central bouquet of cones, the cone center-to-center distances increase, the inner segment diameters increase, and the number of cones/sq mm decreases. The tapered calycal processes are more massive in M. irus than M. mulatta, and the lateral fins are better developed. Lateral fins are not present in the foveola. The cones are arranged in straight lines.

SUMMARY In a morphological survey of 73 human retinae spanning 9 decades, and including 20 retinae which were obtained from eyes enucleated for malignant melanomas, nodular excrescences were noted in the outer segments of rods with an incidence which increased with age. These structures were sectioned in both their vertical and horizontal axes and on electron microscopical examination were seen to result from the localised convolution of affected outer segments. The topographic morphology of such convolutions is described and their modes of formation are discussed.The process of ageing and its manifestations in the cells of the human retina has for some time been a major research interest in our laboratory. These studies include the electron microscopical examination of 73 human retinae obtained from both enucleations and cadaver eyes. This series contains specimens from each of 9 decades, the youngest eye being 19 years and the oldest 90 years. The detailed morphology of the photoreceptor cells, the pigment epithelium, and Briich's membrane have been investigated in order to establish control reference material for a future study of senile macular degenerations and inherited retinal dystrophies. We were interested therefore by a recent report purporting to show the fusion of the outer segments of juxtapositioned rod photoreceptor cells in the retina of a 47-year-old woman whose eye was enucleated because of the presence of a choroidal melanoma (Borwein et al., 1977). In the present paper morphological evidence shows that this suggested fusion of appositioned cells is an age-related process occurring within the outer segments of individual rods. MethodsOne of the commonest conditions that leads to enucleation of human eyes is malignant melanoma of the uveal tract. The retinae of 20 such eyes have been studied in the present survey, with an age Address for reprints: Dr John Marshall, Department of Visual Science, Institute of Ophthalmology, Judd Street, London WC1H 9QS range spanning 7 decades. The immediate postoperative procedures varied with the individual demands of participating centres. In most centres corneal discs were trephined for subsequent use in graft procedures. One centre required the dissection of the globe and the hemisection of the unfixed tumour so that samples could be processed for tissue culture as well as histopathology. In some cases intact globes were presented immediately after enucleation, and here transcorneal incisions were made in the sagittal plain before immersion in fixative.All eyes were fixed within minutes of enucleation by immersion in 100 ml of 0 3 M glutaraldehyde buffered in 0-1 M sodium cacodylate containing 10 mg/ml of calcium chloride with a final pH 7-4. Intact globes were progressively dissected in this solution, the cornea being removed at the sulcus sclerae after 5 minutes, the iris after a further 5 minutes, and the lens, ciliary body, and vitreous 20 minutes after the initial immersion.After a total of 1 hour in fixative all posterior eye cups were washed in 0-1 M ...

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The photoreceptors of the adult Anableps anableps have been investigated by light microscopy, scanning and transmission electron microscopy. The fish is a surface swimmer and the eye is divided by the water meniscus. In general, the photoreceptors (rods, single cones, and double equal cones) resemble those of other vertebrates, but there are several unusual features: (1) The outer segment discs of the double cones differ in the two members. (2) All cones have a prominent accessory outer segment derived from the single connecting cilium, and there is no second centriole. (3) The exterior of the inner segments is ridged and grooved longitudinally, most markedly so in the cones. (4) A membranebounded oil droplet is present in the distal cone inner segment, formed from mitochondria which enlarge, fuse and transform in a vitreal‐scleral gradient. (5) There are knob‐like invaginations of rod cytoplasm into the cones immediately scleral to the external limiting membrane. (6) Subsurface cisterns underlie apposed plasma membranes of double cone inner segments and direct rod‐cone inner segment contacts. (7) Fine “fins” on the cones interdigitate, with Müller cell cytoplasm between, just scleral to the external limiting membrane. (8) In the rod spherule there is a greater density of vesicles and the cytoplasm is darker than in the cone pedicle. The well‐defined cone mosaic has a linear pattern peripherally and a square pattern centrally. The photoreceptors undergo photomechanical movements. Photoreceptor ultrastructure is alike in both dorsal and ventral parts of the retina, but the ventral retina contains more cells and is thicker than the dorsal retina. The adjustments necessary for simultaneous air and water vision are found mainly in lens shape, corneal thickness and curvature, and the greater number of cells in the ventral retina.

Rhesus monkey retina and especially the foveal photoreceptors (PR) were investigated by scanning electron microscopy (SEM). There are a few scattered SEM photomicrographs of the primate retina in the literature but this is the first detailed and comprehensive view by SEM of a primate retina. Some new aspects of surface morphology are displayed and the study also highlights and emphasizes some aspects of photoreceptor structure that have either been overlooked or not clearly displayed in studies using transmission electron microscopy only. For examination by SEM retinas were fixed in glutaraldehyde-formaldehyde, osmicated, immersed in thiocarbohydrazide, dehydrated in alcohols, and gold-coated. The fovea appears as a sharply defined pit with steep slopes, and its vitreal surface looks different from that of the rest of the retina. It appears to have a matted surface. The rest of the vitreal surface is relatively smooth and displays distinct lines which diverge in a radiating pattern from the foveal slopes. The choroid has a spongelike appearance; the sclera appears fibrous with the fibers running parallel to the vitreal surface. Photoreceptor nuclei are sometimes lost during tissue processing. They leave a discrete "nuclear nest" formed from Müller cell processes. Henle fibers turn at a sharp angle from the cones to run parallel to the vitreal surface. The external limiting membrane is seen as a clear line. Immediately vitreal to it, the Müller cell microvilli surround the proximal inner segments. The cone inner segment (CIS) narrows toward the cilium where the cell is markedly constricted. The ciliary connectives are aligned and appear as a shadowy, slightly wavy zone when the retina is viewed in vertical section. The freestanding, tapering calycal processes (CP) arise from and are continuous with longitudinal CIS ridges. CP surround the proximal parts of the outer segments (OS), but there are no CP around the ciliary backbone. Some CP bear small protrusions. OS break off and remain embedded among the pigment epithelium microvilli (PEM) more often than PEM remain attached to OS distal ends. The foveal OS tapers slightly from its proximal to its distal end. The OS may bear knoblike swellings and convolutions in their more distal regions but not at their tips.