PAX2 IS EXPRESSED IN MULTIPLE SPINAL-CORD INTERNEURONS, INCLUDING A POPULATION OF EN1(+) INTERNEURONS THAT REQUIRE PAX6 FOR THEIR DEVELOPMENT

Members of the PAX family of transcription factors are candidates for controlling cell identity in the spinal cord, We have morphologically analyzed cells that express one of these transcription factors, PAX2, demonstrating multiple interneuron cell types express PAX2. Two ventra l populations of PAX2-expressing interneurons in the spinal cord are m arked by coexpression of the transcription factors, EN1 and EVX1. Inte restingly, the expression domains of PAX2, EN1 and EVX1 in postmitotic neurons correlate closely with those of Pax6 and Pax7 in the ventricu lar zone, implicating these patterning genes in the regulation of PAX2 , EN1 and EVX1. We show that one of these patterning genes, Pax6, is r equired for the correct specification of ventral PAX2(+) interneurons that coexpress EN1. These results demonstrate that the early activity of patterning genes in the ventricular zone determines interneuron ide ntity in the spinal cord.