The chick eye is able to change its refractive state by as much as 7 D by pushing the retina forward or pulling it back; this is effected by changes in the thickness of the choroid, the vascular tissue behind the retina and pigment epithelium. Chick eyes first made myopic by wearing diffusers and then permitted unrestricted vision developed choroids several times thicker than normal within days, thereby speeding recovery from deprivation myopia. Choroidal expansion does not occur when visual cues are reduced by dim illumination during the period of unrestricted vision. Furthermore, in chick eyes presented with myopic or hyperopic defocus by means of spectacle lenses, the choroid expands or thins, respectively, in compensation for the specific defocus imposed. Consequently, when the lenses are removed, the eye finds its refractive error suddenly of opposite sign, and the choroidal thickness again compensates by changing in the opposite direction. If a local region of the eye is made myopic by a partial diffuser and then given unrestricted vision, the choroid expands only in the myopic region. Although the mechanism of choroidal expansion is unknown, it might involve either a increased routing of aqueous humor into the uveoscleral outflow or osmotically generated water movement into the choroid. The latter is compatible with the increased choroidal proteoglycan synthesis either when eyes wear positive lenses or after diffuser removal.

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Electron micrograph composites of tangenital sections of the fovea centralis of three cynomolgus monkeys (Macaca irus) and one baboon (Papio anubis) were used to determine the spatial density of the principal retinal cells. In the center of the foveola, the density of cones ranged from 113,000 to 230,000/mm2, and pigment epithelial cells from 4,900 to 7,000/mm2. At a distance of 500 microns from the foveolar center the density of the cone cell pedicles ranged from 29,000 to 36,300/mm2, and the density of horizontal cells ranged from 19,000 to 25,100/mm2. Densities of bipolar, Müller, and amacrine cells were determined in only two monkeys and in the baboon. The fact that the cone cell pedicles have a larger diameter than the foveolar cones explains the geometry of the fovea. The morphology of the junction between foveolar cone outer segments and the pigment epithelium reflects the complex metabolism of this functional unit. The comparison with the peripheral primate retina suggests that the densities of horizontal and bipolar cells, but not of amacrine and Müller cells, are correlated with the density of cone cell pedicles.

Isolated rods enzymatically removed from normal adult rat retina have been transplanted to the subretinal space of adult rats with a retinal dystrophy winich has destroyed almost all the photoreceptors. These transplanted rods survive for months after transplantation during which time they form synapses with other retinal cells. Rod spherules with large amounts of synaptic vesicles and synaptic ribbons are found forming discreet contacts with pre- and postsynaptic densities in arrangements closely resembling those seen in the normal retina.

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By using electron microscopy to study the quantitative morphology of the retina, it was possible to determine the spatial density of all principal retinal cells at a defined retinal location. In two retinas of cynomolgus monkeys at a position of 30 degrees nasal of the fovea centralis, the following cell densities were determined from composite electron micrographs: retinal pigment epithelium: 3,400 cell/mm2; rod cells: 115,000 and 168,000 cells/mm2; cone cells: 8,200/mm2; horizontal cells: 7,000/mm2; bipolar cells: 50,000/mm2; amacrine cells: 11,500/mm2; Müller cells: 16,000/mm2; and ganglion cells: 5,350 and 6,750/mm2.

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