Molecular organization of the ferret visual thalamus.

The visual system encodes and deciphers information using parallel, anatomically segregated, processing streams. To reveal patterns of gene expression in the visual thalamus correlated with physiological processing streams, we designed a custom ferret cDNA microarray. By isolating specific subregions and layers of the thalamus, we identified a set of transcription factors, including Zic2, Islet1, and Six3, the unique distribution profiles of which differentiated the lateral geniculate nucleus (LGN) from the associated perigeniculate nucleus. Within the LGN, odd homeobox1 differentiated the A layers, which contain X cells and Y cells, from the C layers. One neuron-specific protein, Purkinje cell protein 4 (PCP4), was strongly expressed in Y cells in the ferret LGN and in the magnocellular layers of the primate LGN. In the ferret LGN, PCP4 expression began as early as postnatal day 7 (P7), suggesting that Y cells are already specified by P7. These results reveal a rich molecular repertoire that correlates with functional divisions of the LGN.