Cryptochrome 1b: a possible inducer of visual lateralization in pigeons?

The visual system of adult pigeons shows a lateralization of object discrimination with a left hemispheric dominance on the behavioural, physiological and anatomical levels. The crucial trigger for the establishment of this asymmetry is the position of the embryo inside the egg, which exposes the right eye to light falling through the egg shell. As a result, the right-sided retina is more strongly stimulated with light during embryonic development. However, it is unknown how this embryonic light stimulation is transduced to the brain as rods and cones are not yet functional. A possible solution could be the blue-light-sensitive molecule cryptochrome 1 (Cry1), which is expressed in retinal ganglion cells (RGCs) of several mammalian and avian species. RGCs have been shown to be functional during the time of induction of asymmetry and possess projections to primary visual areas. Therefore, Cry1-containing RGCs could be responsible for induction of asymmetry. The aim of this study was to identify the expression pattern of the Cry1 subtype Cry1b in the retina of embryonic, post-hatch and adult pigeons by immunohistochemical staining and to show whether Cry1b-containing RGCs project to the optic tectum. Cry1b-positive cells were indeed mainly found in the RGC layer and to lesser extent in the inner nuclear layer at all ages, including the embryonic stage. Tracing in adult animals revealed that at least a subset of Cry1b-containing RGCs project to the optic tectum. Thus, Cry1b-containing RGCs within the embryonic retina could be involved in the induction of asymmetries in the visual system of pigeons.