LIGANDS OF STEROID THYROID RECEPTORS INDUCE CONE PHOTORECEPTORS IN VERTEBRATE RETINA/

The mechanisms by which multipotent progenitor cells are directed to a lternative cell identities during the histogenesis of the vertebrate c entral nervous system are likely to involve several different types of signaling systems. Recent evidence indicates that 9-cis retinoic acid , which acts through members of the steroid/thyroid superfamily of rec eptors, directs progenitor cells to the rod photoreceptor cell fate. W e now report that another effector of this family of receptors, thyroi d hormone, induces an increase in the number of cone photoreceptors th at develop in embryonic rat retinal cultures, and that combinations of 9-cis retinoic acid and triiodothyronine cause isolated progenitor ce lls to differentiate as either rods or cones, depending on the relativ e concentrations of the ligands. These results implicate thyroid hormo ne in CNS cell fate determination, and suggest that different photorec eptor phenotypes may be modulated through the formation of thyroid/ret inoid receptor heterodimers.