INTRODUCTION The organization of the vertebrate brain and its differentiation into functionally and anatomically distinct areas is based on early patterning and regional specification of the neural plate

The organization of the vertebrate brain and its differentiation into functionally and anatomically distinct areas is based on early patterning and regional specification of the neural plate during embryonic development. Both vertical signals that emanate from the mesendoderm and planar signals travelling within the plain of the neuroectoderm itself are thought to be involved in neural plate patterning (Ruiz i Altaba, 1994; Gurdon et al., 1995; Kelly and Melton, 1995; Lumsden and Krumlauf, 1996; Wilson et al., 2002). Embryonic development of the midbrain and the anterior hindbrain in particular depend on an ectodermal population of cells located at the midbrainhindbrain junction [the mid-hindbrain boundary (MHB) or isthmic organizer]. The organizer potential was initially demonstrated by transplantation experiments in chicken embryos, where isthmic tissue grafts induced midbrain and cerebellum ectopically. In its normal location, the MHB organizer was then proposed to regulate polarized morphological differentiation of the adjacent tectum and elaboration of the cerebellar anlage (Martinez and AlvaradoMallart, 1990; Marin and Puelles, 1994; Garda et al., 2001; Liu and Joyner, 2001; Rhinn and Brand, 2001; Wurst and BallyCuif, 2001). Evidence for this concept has come from functional studies of the molecules involved. Several transcription factors of the Pax, Otx, Gbx and Lim class play pivotal roles during development and function of the MHB organizer. In zebrafish, null mutations for the pax2.1 gene (pax2a – Zebrafish Information Network) (no isthmus or noi) or inactivation of the eng2 and eng3 genes causes absence of the midbrain, MHB and cerebellum (Brand et al., 1996; Lun and Brand, 1998; Pfeffer et al., 1998; Scholpp and Brand, 2001), similar to the cognate mouse phenotypes (Millen et al., 1994; Wurst et al., 1994; Favor et al., 1996; Urbanek et al., 1997; Schwarz et al., 1997). The secreted signaling molecules Wnt1 (McMahon et al., 1992) and Fgf8 are thought to mediate organizer function. Fgf8 in particular is expressed in the MHB organizer and when 917 Development 129, 917-933 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV1755