PATTERNING OF THE NEURAL PRIMORDIUM IN THE AVIAN EMBRYO

The quail-chick chimera system has been used to study two problems rel ated to the patterning of the neural primordium. We first report our a nalysis of the secondary neurulation as it proceeds in the avian tail bud caudally to the posterior neuropore. We show that the territory lo cated caudally to the primary neural tube and joining it to the caudal end of the notochord, designated as the cordoneural hinge (CNH), can be assimilated to the remainder of the Hensen's node. The CNH undergoe s a craniocaudal progression during tail bud development and lays down the notocord and floor plate which thus originate from closely relate d progenitor cells. We show that the floor plate material becomes inse rted within the neurectoderm thus forming the neural plate. The second part of this article is devoted to the hindbrain development. We show that transposition of the presumptive territory of anterior rhombomer es (r1 to r6) at the level of r7-r8 results in their phenotypic poster iorization which is preceded by their expression of posterior Hox gene s characteristic to the AP level at which they are transplanted and he nce that there are inductive signals within the neural epithelium itse lf. (C) 1996 Academic Press Ltd