DORSAL-VENTRAL PATTERNING DURING NEURAL INDUCTION IN XENOPUS - ASSESSMENT OF SPINAL-CORD REGIONALIZATION WITH XHB9, A MARKER FOR THE MOTOR-NEURON REGION

While the role of the notochord and floor plate in patterning the dors al-ventral (D/V) axis of the neural tube is clearly established, relat ively little is known about the earliest stages of D/V regionalization . In an effort to examine more closely the initial, preneural plate st ages of regionalization along the prospective D/V neural axis, we have performed a series of explant experiments employing xHB9, a novel mar ker of the motor neuron region in Xenopus. Using tissue recombinants a nd Keller explants we show that direct mesodermal contact is both nece ssary and sufficient for the initial induction of xHB9 in the motor ne uron region. We also show that presumptive neural plate explants remov ed as early as midgastrulation and cultured in isolation are already s pecified to express xHB9 but do so in an inappropriate spatial pattern while identical explants are specified to express the floor plate mar ker vhh-1 with correct spatial patterning. Our data suggest that, in a ddition to floor plate signaling, continued interactions with the unde rlying mesoderm through neural tube stages are essential for proper sp atial patterning of the motor neuron region. (C) 1997 Academic Press.