249dose-dependent fashion to promote ventrolateral mesendoderm and epidermis, and
can prevent neural development of dissociated animal caps and inhibit dorsal
mesendoderm formation in explant assays (Jones et al. 1992 ; Dale et al. 1992 ;
Fainsod et al. 1994 ; Suzuki et al. 1994 ; Graff et al. 1994 ; Wilson and Hemmati-
Brivanlou 1995 ; Hammerschmidt et al. 1996b; Streit et al. 1998 ).
The first insights into the molecular biology of the organizer came from the isola-
tion of conserved homeobox and forkhead transcription factors expressed in the
organizer: goosecoid (gsc), lim homeobox 1 (lhx1), and forkhead box a4 (foxa4)
(Cho et al. 1991 ; Taira et al. 1992 ; Dirksen and Jamrich 1992 ). Importantly, ectopic
expression of gsc in ventral cells caused second axes with notochordal tissue, indi-
cating that gsc was not merely a marker of, but a critical functional component of the
organizer (Cho et al. 1991 ). Studies in other organisms subsequently found a deeply
conserved role for these proteins in regulating axial development in both proto-
stomes and deuterostomes and even to some extent in diploblasts (Martindale 2005 ).
Although it was clear that overexpression of these proteins, notably Gsc, could
induce most aspects of organizer function and regulate cell–cell signaling during
axis formation (Niehrs et al. 1993 ), the nature of these signals was not immediately
apparent. In a series of seminal experiments, expression cloning and differential
cDNA screening in Xenopus identified secreted molecules encoded by noggin (nog;
Smith and Harland 1992 ; Lamb et al. 1993 ), chordin (chrd; Sasai et al. 1994 ), and
follistatin (fst; Hemmati-Brivanlou et al. 1994 ) as potent dorsalizing and neuraliz-
ing molecules specifically expressed in the organizer. These genes turned out to
encode extracellular BMP antagonists that bind to secreted BMPs and inhibit the
activation of BMP receptors (Zimmerman et al. 1996 ; Piccolo et al. 1996 ).
Importantly, overexpression of these BMP antagonists mimicked the action of the
organizer in nearly every respect.
Functional interference and genetic manipulations confirmed the requirements for
BMP signaling and BMP antagonism in dorsoventral patterning. Of particular impor-
tance, genetic mutations in zebrafish bmp7 and bmp2b, BMP receptor acvr1 and smad5
(snailhouse, swirl, lost-a-fin, and somitabun mutants, respectively) result in dorsalized
embryos (Kishimoto et al. 1997 ; Nguyen et al. 1998 ; Hild et al. 1999 ; Dick et al. 2000 ;
Schmid et al. 2000 ; Mintzer et al. 2001 ), whereas embryos lacking chrd function
(chordino; Hammerschmidt et al. 1996a, b) are ventralized. Results using antisense
oligo-based loss-of-function in Xenopus similarly showed dorsalization upon reduction
of BMPs and ventralization following inhibition of Nog, Chrd, Fst (Oelgeschläger et al.
2003 ; Khokha et al. 2005 ; Reversade et al. 2005 ). In the mouse and the chick, the roles
of BMPs are more complex owing to their functions in extra embryonic tissues and in
controlling cell proliferation (Hogan 1996 ). Nonetheless, Nog and Chrd are expressed
in the anterior primitive streak and Hensen’s node (Connolly et al. 1997 ; Streit et al.
1998 ; Streit and Stern 1999 ; Bachiller et al. 2000 ), and Nog, Chrd double mouse mutants
exhibit dorsoanterior truncations (Bachiller et al. 2000 ). Additionally, genetic deletion
of Bmp4 in mouse results in ventral mesoderm defects (Winnier et al. 1995 ).
A host of other BMP antagonists as well as antagonists of other ligands were also
discovered and a commonly accepted model emerged; namely that dorsoventral pat-
terning across germ layers is mediated by a gradient of BMP activity originating
6 Vertebrate Axial Patterning: From Egg to Asymmetry