stage,
(13,14)
before stable determination of neural identity has
occurred.
Question 2: What tissues have AP
patterning capacity, and should
the Organizer be redefined?
Dorsal mesoderm (Organizer)
The term ``Spemann's Organizer'' is frequently used in con-
junction with induction of AP pattern and has become one of
the catch-phrases of embryology. The original definitions of
the Organizer, however, may no longer be useful. The
Organizer was initially defined as a piece of early gastrula
dorsal mesoderm which, when transplanted ventrally, could
induce a secondary embryo with a well-patterned AP axis.
(15)
Subsequent experiments, therefore, focused on the dorsal
mesoderm (Fig. 3) and its derivatives. More recently, the
Organizer has been defined as the region of early gastrula
dorsal mesoderm with the highest neural determining capa-
city, or as the 90
wide arc of mesodermal tissue that can
rescue a ventralized embryo to a complete AP axis (Ref. 14
and references therein). Each experiment singles out a
Table 1. Markers of anteroposterior pattern in the gastrula future neurectoderm
Stage of initial
Mid-gastrula dorsal expression
expression class Gene Protein class in gastrula References
Zic1/opl Zinc finger transcription factor Development 1998; 125: 579±587 and 2867±2682.
Throughout Zic3 Zinc finger transcription factor Early PNAS 1997; 94: 11980±11985.
Sox2 HMG box transcription factor Development 1998; 125: 579±587.
SoxD HMG box transcription factor Neuron 1998; 21: 77±85.
Geminin Novel coiled-coil Development 1998; 125: 3247±3258.
Otx2 Paired-class homeobox Early Development 1995; 121: 707±720 and 993±1004.
Anterior dorsal RAHA P450 related Mid
a
Xanf1 Paired-class homeobox Early Dev. Biol. 1992; 152: 373±382.
Xanf2 Paired-class homeobox Early Dev Biol 1995; 171: 641±654.
Posterior dorsal Fkh5 Forkhead-class transcription factor Early
a
Far posterior dorsal and HoxD1 Labial-class homeobox Early Genes Dev 1991; 5: 1321±1332.
posteriolateral Gbx2 Gbx-class homeobox Mid Mech Dev 1996; 54: 149±160.
Xcad2, 3 Caudal-class homeobox Mid Development 1996; 122: 3881±3892; Development
1997; 124: 3805±3814.
Neurogenic Neurogenin bHLH transcription factor Mid Cell 1996; 87: 43±52.
Xash-3 bHLH transcription factor Mid Development 1993; 119: 221±232.
a
J. Gamse and H. Sive, unpublished.
Figure 2. Expression of markers in the early
and mid-gastrula ectoderm reveals the progress
of AP patterning. At early gastrula, a rudimentary
AP ectodermal pattern is present. Pan-neural
markers, such as opl, are expressed throughout
the future neurectoderm. The anterior marker
otx2 is specified anteriorly and expressed
posteriorly. The more posterior marker fkh5 is
also expressed in posterior ectoderm. The most
posterior markers, exemplified by hoxD1, are not
yet expressed, nor are neural identity genes
such as neurogenin. By mid-gastrula, a more
complex AP ectodermal pattern is present. otx2 is expressed anteriorly, while fkh5 is expressed more posteriorly. In the most posterior
ectoderm, hoxD1 and other posteriormost markers are expressed. At this stage, neural identity genes such as neurogenin are
expressed. Dorsal views, with anterior to the top; A, anterior; P, posterior; b, blastopore. Black areas designate gene expression,
crosshatched area designates specification of otx2 expression.
Review articles
978 BioEssays 22.11