Cellular dynamics and molecular control of the development of organizer-derived cells in quail-chick chimeras.

Malformations affecting the nervous system in humans are numerous and various in etiology. Many are due to genetic deficiencies or mechanical accidents occurring at early stages of development. It is thus of interest to reproduce such human malformations in animal models. The avian embryo is particularly suitable for researching the role of morphogenetic movements and genetic signaling during early neurogenesis. The last ten years of research with Nicole Le Douarin in the Nogent Institut have brought answers to questions formulated by Etienne Wolff at the beginning of his career, by showing that Hensen's node, the avian organizer, is at the source of all the midline cells of the embryo and ensures cell survival, growth and differentiation of neural and mesodermal tissues.

Inhibition of neuroepithelial patched-induced apoptosis by sonic hedgehog.

During early development in vertebrates, Sonic hedgehog (Shh) is produced by the notochord and the floor plate. A ventrodorsal gradient of Shh directs ventrodorsal patterning of the neural tube. However, Shh is also required for the survival of neuroepithelial cells. We show that Patched (Ptc) induces apoptotic cell death unless its ligand Shh is present to block the signal. Moreover, the blockade of Ptc-induced cell death partly rescues the chick spinal cord defect provoked by Shh deprivation. Thus, the proapoptotic activity of unbound Ptc and the positive effect of Shh-bound Ptc on cell differentiation probably cooperate to achieve the appropriate spinal cord development.

Dual origin of the floor plate in the avian embryo.

Molecular analysis carried out on quail-chick chimeras, in which quail Hensen's node was substituted for its chick counterpart at the five- to six-somite stage (ss), showed that the floor plate of the avian neural tube is composed of distinct areas: (1) a median one (medial floor plate or MFP) derived from Hensen's node and characterised by the same gene expression pattern as the node cells (i.e. expression of HNF3beta and Shh to the exclusion of genes early expressed in the neural ectoderm such as CSox1); and (2) lateral regions that are differentiated from the neuralised ectoderm (CSox1 positive) and form the lateral floor plate (LFP). LFP cells are induced by the MFP to express HNF3beta transiently, Shh continuously and other floor-plate characteristic genes such as NETRIN: In contrast to MFP cells, LFP cells also express neural markers such as Nkx2.2 and Sim1. This pattern of avian floor-plate development presents some similarities to floor-plate formation in zebrafish embryos. We also demonstrate that, although MFP and LFP have different embryonic origins in normal development, one can experimentally obtain a complete floor plate in the neural epithelium by the inductive action of either a notochord or a MFP. The competence of the neuroepithelium to respond to notochord or MFP signals is restricted to a short time window, as only the posterior-most region of the neural plate of embryos younger than 15 ss is able to differentiate a complete floor plate comprising MFP and LFP. Moreover, MFP differentiation requires between 4 and 5 days of exposure to the inducing tissues. Under the same conditions LFP and SHH-producing cells only induce LFP-type cells. These results show that the capacity to induce a complete floor plate is restricted to node-derived tissues and probably involves a still unknown factor that is not SHH, the latter being able to induce only LFP characteristics in neuralised epithelium.