Pax2/5 and Pax6 subdivide the early neural tube into three domains

The nested expression patterns of the paired-box containing transcription f actors Pax2/5 and Pax6 demarcate the midbrain and forebrain primordium at t he neural plate stage. We demonstrate that, in Pax2/5 deficient mice, the m esencephalon/metencephalon primordium is completely missing, resulting in a fusion of the forebrain to the hindbrain. Morphologically, in the alar pla te the deletion is characterized by the substitution of the tectum (dorsal midbrain) and cerebellum (dorsal metencephalon) by the caudal diencephalon and in the basal plate by the replacement of the midbrain tegmentum by the ventral metencephalon (pons). Molecularly, the loss of the tectum is demons trated by an expanded expression of Pax6, (the molecular determinant of pos terior commissure), and a rostral shift of the territory of expression of G bx2 and Otp (markers for the pens), towards the caudal diencephalon. Our re sults suggest that an intact territory of expression of Pax2/5 in the neura l plate, nested between the rostral and caudal territories of expression of Pax6, is necessary for defining the midbrain vesicle. (C) 1999 Elsevier Sc ience Ireland Ltd. All rights reserved.