Identification of a subtype of cone photoreceptor, likely to be blue sensitive, in the human retina.

A minor population of cone photoreceptors (called B-cones) can be distinguished from the major population (called R-cones) on morphological criteria as seen by light microscopy in foveal and peripheral human retina. The B-cones are characterized by a longer inner segment projecting into subretinal space, a larger-diameter inner segment, an increased staining intensity of the inner segment, and a different distribution relative to the R-cones in the cone mosaic. B-cones occur even in the foveolar center (3-5%) and rise to a maximum (15%) in the foveolar slope. They can also be identified in peripheral retina where they form 7-10% of the total cone population. The B-cone population follows the distribution profile postulated for the blue-sensitive system from histochemical studies on monkeys and from psychophysical studies on humans. The B-cones also share many of the same morphological features of the putative blue cones of the ground squirrel and monkey retinas. For these reasons we suggest that our B-cone group is the blue cone population of the human retina.