2576.5.1 Cellular and Tissue Basis of Anteroposterior Patterning
6.5.1.1 The Hypoblast/Anterior Visceral Endoderm in Anterior
Patterning
Comparative studies have suggested that the role of the hypoblast/AVE in anterior
patterning is widely conserved, based on analogous gene expression patterns in the
AVE and later anterior definitive endoderm, cell migration and anterior signaling
activities (rabbit hypoblast, Knoetgen et al. 1999 ; chicken hypoblast, Foley et al.
2000 ; teleost YSL, Ho et al. 1999 ; amphibian foregut/anterior endoderm,
Bouwmeester et al. 1996 ; Jones et al. 1999 ). Interestingly, long before it was molec-
ularly characterized, the hypoblast received considerable attention as a potential
regulator of anteroposterior patterning through the control of cell movements.
Classical cell tracking experiments in the chicken showed that the pregastrula epi-
blast undergoes bilaterally symmetrical “polonaise-like” or “double vortex” cell
movements, which head anteriorly along the midline before circling back and con-
verging at the posterior, where the primitive streak will form (Wetzel 1929 ; Gräper
1929 ). This pattern of cell movements was shown to be controlled by the hypoblast
(Waddington 1930 , 1933 ). In these studies, which were repeated and extended by
later authors (e.g., Azar and Eyal-Giladi 1981 ; Foley et al. 2000 ), changing the ori-
entation of the underlying hypoblast changed the orientation and placement of the
forming primitive streak. Although this effect of the hypoblast on cell movements
was clearly recognized by Waddington at the time (Waddington 1933 ), later experi-
ments focused mainly on presumed signaling/inductive properties of the hypoblast
in head and mesoderm induction (Azar and Eyal-Giladi 1981 ).
In mammals, the AVE has been extensively studied at the cellular and genetic
level. The AVE forms prior to gastrulation and is characterized by cells with a dis-
tinct columnar morphology (rabbit, Viebahn et al. 1995 ; mouse, Kimura et al. 2000 ;
Rivera-Pérez et al. 2003 ) as well as specific gene expression patterns (VE-1 antigen,
Rosenquist and Martin 1995 ; Hesx1, Thomas and Beddington 1996 ; Otx2, Acampora
et al. 1995 ; Hhex, Thomas et al. 1998 ; Lhx1, Gsc, Foxa2, Cerl, Belo et al. 1997 ). In
mouse, these genes are expressed in visceral endoderm cells at the distal tip of the
forming egg cylinder (Thomas et al. 1998 ). Cell labeling with DiI (Thomas et al.
1998 ) or with transgenic methods (Kimura et al. 2000 ; Rivera-Pérez et al. 2003 ;
Srinivas et al. 2004 ), showed that these Hhex-expressing AVE cells unilaterally
migrate towards the future anterior region of the embryo. Physical ablation of
patches of AVE compromised development of the underlying forebrain (Thomas
and Beddington 1996 ), demonstrating a requirement for the AVE in anterior pattern-
ing. Also, gene targeting analysis of chimeric embryos showed that a number of
transcription factors are required in the AVE for its proper formation and migration
and for anterior specification (Otx2, Acampora et al. 1995 ; Rhinn et al. 1998 ; Perea-
Gomez et al. 2001 ; Foxa2, Dufort et al. 1998 ; Lhx1, Shawlot et al. 1999 and Hhex,
Martinez Barbera et al. 2000 among others).
6 Vertebrate Axial Patterning: From Egg to Asymmetry