Anatomical evidence for cone and rod‐like receptors in the gray squirrel, ground squirrel, and prairie dog retinas

In the gray squirrel (Sciurus carolinensis), the prairie dog (Cynomys ludovicianus), and the Mexican and 13-line ground squirrels (Citellus mexicanus and C. tridecemlineatus) there exist two distinct classes of photo-receptors that have cone-like and rod-like anatomical features respectively. These two receptor classes were previously known to exist in the gray squirrel, but only the cone-like (C) receptor had been observed in the other species. We have now found small numbers of rod-like (R) receptors in the other species as well. R-receptors comprise about 40% of the receptors in the gray squirrel, 10% of the receptors in the prairie dog, and 4-5% of the receptors in the two species of ground squirrel. This paper describes certain light and electron microscopic features of these two receptor classes including their synaptic connections with second-order cells and with each other. We find that the C-receptor has a morphology and synaptic organization characteristic of other mammalian cones. However, the R-receptor differs from other mammalian rods in certain morphological respects, and its synaptic organization has both cone and rod characteristics as well as some unusual features.