Fine structure of the photoreceptor cells of the ground squirrel (Citellus tridecemlineatus tridecemlineatus)

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Abstract
The structure of the photoreceptor cells of the ground squirrel retina has been studied by light and electron microscopy. The ground squirrel photoreceptor cells are of particular morphologic interest because this mammal is believed to possess a pure cone retina. Light micrographs show that the ground squirrel retina possesses an unusually narrow outer nuclear layer, a characteristic feature of pure cone or predominantly cone retinae. By light microscopy all the photoreceptor outer segments appear uniform in size, shape and position and resemble cones more closely than rods. Electron micrographs of ground squirrel photoreceptor cells also reveal a uniform structure typical of cones of other vertebrates. Each of the photoreceptor outer segments is formed by flattened saccules lying one on top of the other and many of the saccules, as is characteristic of cones, are continuous with the overlying plasma membrane. The basal processes of the photoreceptor cells also are cone‐like in nature since each contains several synaptic lamellae and makes contact with a large number of dendritic processes arising from cells in the inner nuclear layer. It is concluded that the ground squirrel retina, from a purely morphologic standpoint, is exclusively or almost exclusively pure cone in type.