The Vertebrate Organizer

pp 299-313

Molecular Patterning of the Embryonic Brain

  • Esther Bell
  • , Ali H. BrivanlouAffiliated withThe Laboratory of Vertebrate Embryology, The Rockefeller University

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The development of the nervous system is a very complex process, of which the underlying mechanisms are slowly beginning to be elucidated. This chapter will focus on the early molecular patterning of the different regions within the embryonic brain: how development occurs from a single fertilized egg to a fully functional and differentiated nervous system. The nervous system is comprised of three axes, the anterior-posterior (AP), dorsal-ventral (DV) and left-right (LR). Each of these axes is patterned by a different combination of signals. Patterning along the AP axis subdivides the nervous system into four main regions, most rostral the forebrain (prosencephalon; subdivided later into telencephalon and diencephalon), the midbrain (mesencephalon; tectum and tegmentum), hindbrain (rhombencephalon; rhombomeres) and, finally, most caudal the spinal cord. One proposal of how the AP axis of the nervous system is initially established is the Nieuwkoop model which proposes that patterning occurs by two signals, an “activation” signal which initially induces neural tissue with anterior character (forebrain and midbrain) followed by a “transformation” signal, which posteriorizes the neural tissue (hindbrain and spinal cord; Nieuwkoop et al. 1952). This model seems to prevail based on current molecular knowledge of CNS development. The DV axis is established by a combination of signals. From the underlying axial mesoderm factors such as sonic hedgehog (Shh) induce and maintain the ventral fate of the neural tube. From the epidermis (or non-neural ectoderm) BMP/GDF family members induce and maintain the dorsal fate. Finally, while less is understood about how the LR axis is specified, signaling mediated by members of the TGFβ family, such as nodal, have been suggested to be involved. This chapter will discuss what is known to date about early neural patterning. The data discussed originate from a variety of model systems and hence provide us with a comparative molecular approach to understanding these issues.