Purpose We examined the generation of rod and cone photoreceptors form retinal progenitors in the developing outer nuclear layer (ONL) of the pig retina.
Methods Retinas were removed from pigs at various development ages, and retinal development was followed by H&E staining of retinal sections.  Retinal progenitors and rod and cone photoreceptors progenitors were followed by immunostaining of retina sections. 
Results We identify a population of cone progenitors which are specified and in place in the outer two rows of the developing swine retina before midgestation.  These cells are marked by Islet1 and recoverin,and they appear before NRL⁺ rods are specified from PAX6⁺ retinal progenitor cells. Further we demonstrate that L/M opsin cones are
generated from this population before S opsin cones. 
Conclusions The results provide the first evidence of a distinct population of cone progenitors, which are specified before midgestation.  These cone progenitors are distinct from the later-appearing rod progenitors.  These
cone progenitors do not differentiate and express opsins for 35 days. As they differentiate, they give rise sequentially to L/M opsin cones and the S opsin cone. The positioning of the cone progenitors in the outer retina leads to complete segregation of cones and rods in the adult with cones comprising the outer two rows of the ONL and rods all confined to the interior rows.  Taken together, the results suggest differences in pathways leading to rod and cone specification in the mouse and pig.