We propose here a novel interpretation of the embryonic origin of cells of diencephalic sensory relay nuclei in teleosts based on our recent studies of gene expression patterns in the medaka ( Oryzias latipes) embryonic brain and comparative hodological studies. It has been proposed that the diencephalic sensory relay system in teleosts is unique among vertebrates. Teleost relay nuclei, the preglomerular complex ( PG), have been assumed to originate from the basal plate ( the posterior tuberculum) of the diencephalon, whereas relay nuclei in mammals are derived from the alar plate ( dorsal thalamus) of the diencephalon. Our results using in situ hybridization show, however, that many pax6- or dlx2-positive cells migrate laterally and ventrocaudally from the diencephalic alar plate to the basal plate during development. Massive clusters of the migrated alar cells become localized in the mantle layer lateral to the posterior tubercular neuroepithelium, from which main nuclei of the PG appear to differentiate. We therefore consider most if not all neurons in least in part, can be regarded as migrated alar nuclei. Developmental and hodological data strongly suggest that the teleost PG is homologous to a part of the mammalian dorsal thalamus. The organization and origin of the diencephalic sensory relay system might have been conserved across Copyright (c) 2007 S. Karger AG, Basel.