The acquisition of neural fate in the chickIntroductionStructural components of early chick embryosFate and specification of the neuroectodermInductive activities of transplanted tissuesThe posterior marginal zone and Koller’s sickleHypoblastPrimitive streakHensen’s nodeEndodermPrechordal mesendoderm/Head processApplication of purified signalling moleculesFibroblast growth factors (FGFs)WntcVg1BMP4 and BMP7Chordin and NogginNodalSteps towards the acquisition neural fateReferencesReviewThe acquisition of neural fate in the chickLars Wittler,*,1, Michael KesselDepartment of Molecular Cell Biology, Max-Planck-Institut fu¨r biophysikalische Chemie, Am Fassberg 11, 37077 Go¨ttingen, GermanyReceived 5 April 2004; received in revised form 9 May 2004; accepted 9 May 2004AbstractNeural development in the chick embryo is now understood in great detail on a cellular and a molecular level. It begins already beforegastrulation, when a separation of neural and epidermal cell fates occurs under the contol of FGF and BMP/Wnt signalling, respectively. Thisearly specification becomes further refined around the tip of the primitive streak, until finally the anterior–posterior level of theneuroectoderm becomes established through progressive caudalization. In this review we focus on processes in the chick embryo and putclassical and more recent molecular data into a coherent scenario.q 2004 Elsevier Ireland Ltd. All rights reserved.Keywords: Neural fate; Chick; Neural development; Neural induction; Hensen’s node1. IntroductionThe acquisition of neural fate in the chick is a complexprocess, which begins while the egg is still in the uterus andoccupies much of the first day of extrauterine development.It involves a tightly regulated series of inductions betweenthe different tissues and morphological structures of theearly embryo, before and during gastrulation. Today, we donot only know the fine anatomy of the early chick embryo,but also obtained an understanding of the dynamics ofdevelopment, thanks to the mapping efforts in manylaboratories. The establishment of specification mapsrevealed the intrinsic states of commitment for many keytissues at different embryonic stages. In addition, a wealth ofmolecular information was collected in the recent years,defining the major molecular players of the neural inductionprocess. In particular, the major signalling molecules seemto be identified and a coherent picture of their interwoven,sometimes multiple activities is emerging. It has become acommon practice to evaluate findings for avian embryos inthe light of models derived from other, mostly amphibianembryos. These include the organizer, or head–trunkorganizer concepts of H. Spemann and O. Mangold, aswell as the activation–transformation hypothesis ofP. Nieuwkoop (Mangold, 1933; Nieuwkoop, 1952;Spemann and Mangold, 1924). Here, we will largely refrainfrom such attempts, which certainly have their benefits, butalso appear difficult taking into account the differentembryologies and, consequently, specific technologies. Inthis review, we will not summarize the large amount ofinformation available on neural induction in amphibia, butwill rather put together the extensive knowledge focussingon the chick. After reviewing briefly the early morphology,fate maps and specification status, we will summarizeexperimental evidence from tissue transplantation experi-ments and from applications of purified factors. Finally, wewill try to generate a coherent scenario, and describe thecurrent views on the sequential inductions and molecularevents during the acquisition of neural fate in the chick.2. Structural components of early chick embryosThe first cleavages of the unicellular chick embryo do notdivide the complete yolk, but occur only superficially, sothat a syncytium develops on top of the yolk sphere, with allearly cells open to the yolk (Eyal-Giladi and Kochav, 1976).After about 10 h of intrauterine development, a disc-shapedblastoderm is recognizable for the first time. Three hourslater a differentiation of the blastoderm is apparent, with0925-4773/$ - see front matter q 2004 Elsevier Ireland Ltd. All rights reserved.doi:10.1016/j.mod.2004.05.004Mechanisms of Development 121 (2004) 1031–1042www.elsevier.com/locate/modo1Present address: Department of Developmental Genetics, Max-PlanckInstitute for Molecular Genetics, Ihnestrasse 63-73, 14195 Berlin,Germany.*Corresponding author. Tel.: þ 493084131261; fax: þ 493084131385.E-mail address: [email protected] (L. Wittler).only the peripheral cells open to the yolk, and the centralcells separated by transverse membranes. Thus, a yolk-rich‘area opaca‘ surrounds a translucent ‘area pellucida’,the latter by now consisting of a single cell layer. By thetime of egg laying, after about nineteen hours in the uterus,dispersed cell aggregates indicate the beginning formationof a two layered embryo consisting of an upper ‘epiblast’and a lower ‘hypoblast’. In such embryos, the intermediatezone between the area opaca and pellucida can berecognized as the ‘marginal zone’ (Fig. 1, top row).Further incubation leads to the completion of the lowerlayer, the hypoblast, within the first five hours. The hypoblastspreads from the prospective posterior pole of the embryo,where a sickle shaped structure, ‘Koller’s sickle’, is more orless obvious at the anterior border of the posterior marginalzone (‘PMZ’ Bachvarova et al., 1998; Callebaut and VanNueten, 1994; Eyal-Giladi and Kochav, 1976; Izpisua-Bel-monte et al., 1993). After about 6 h a triangular thickeningindicates the generation of the ‘primitive streak’, i.e. the siteof ingression for endo- and mesodermal cells. The hypoblastnow becomes displaced by another tissue of the extraem-bryonic, primitive endoderm, the ‘endoblast’ (Bachvarovaet al., 1998; Callebaut et al., 1998; Foley et al., 2000;Vakaet, 1970; Fig. 1, top row). Within the next twelvehours, the primitive streak elongates significantly, until itreaches a length of about 1.8 mm at the definitive streakstage. The first cells to ingress through the tip of theprimitive streak become definitive endoderm, and extra-embryonal mesoderm ingresses through its posterior por-tion. With the ongoing of gastrulation, future head and heartmesoderm is generated in the anterior streak (Garcia-Marti-nez and Schoenwolf, 1993; Psychoyos and Stern, 1996a;Fig. 1, third row). After the mid-streak stage, the tip of thestreak thickens to form