Induction of the isthmic organizer and specification of the neural plate border

Abstract: The vertebrate nervous system is extremely complex and contains a wide diversity of cell types. The formation of a functional nervous system requires the differential specification of progenitor cells at the right time and place.The generation of many different types of neurons along the rostro-caudal axis of the CNS begins with the initial specification of a few progenitor domains. This initial coarse pattern is refined by so-called secondary organizers arising at boundaries between these domains. The Isthmic Organizer (IsO) is a secondary organizer located at the boundary between the midbrain and the hindbrain. Although the function and maintenance of the IsO are well understood, the processes underlying its initial specification have remained elusive. In the present work we provide evidence that convergent Wnt and FGF signals initiate the specification of the IsO during late gastrulation as part of the neural caudalization process.The initial step in the generation of the nervous system is the division of the embryonic ectoderm into three cell populations: neural cells giving rise to the CNS, neural plate border cells giving rise to the peripheral nervous system, and epidermal cells giving rise to the outer layer of the skin. While the choice between neural and epidermal fate has been well studied, the mechanism by which neural plate border cells are generated is less well understood. At rostral levels of the neuraxis, the neural plate border gives rise to the olfactory and lens placodes, thickenings of the surface ectoderm from which sensory organs are derived. More caudally, the neural plate border generates neural crest cells, a transient population that migrates extensively and contributes to neurons and glia of the peripheral nervous system. How the early patterning of the central and peripheral nervous systems are coordinated has remained poorly understood. Here we show that the generation of neural plate border cells is initiated at the late blastula stage and involves two phases. During the first phase, neural plate border cells are exposed to Wnt signals in the absence of BMP signals. Simultaneous exposure to Wnt and BMP signals at this early stage leads to epidermal induction. Wnt signals induce expression of Bmp4, thereby regulating the sequential exposure of cells to Wnt and BMP signals. During the second phase, at the late gastrula stage, BMP signals play an instructive role to specify neural plate border cells of either placodal or neural crest character depending on the status of Wnt signaling. At this stage, Wnt signals promote caudal character simultaneously in the neural plate border and in the neural ectoderm. Thus, the choice between epidermal and neural plate border specification is mediated by an interplay of Wnt and BMP signals that represents a novel mechanism involving temporal control of BMP activity by Wnt signals. Moreover, the early development of the central and peripheral nervous systems are coordinated by simultaneous caudalization by Wnt signals.