CARACTERIZACIÓN DEL SECTOR

Based upon his ectoderm fold experiments, Nieuwkoop proposed a model for AP

neural patterning involving two sets of inducer signals (Nieuwkoop, 1952). An initial

“activator” signal, present in all organiser mesoderm, induces neural tissue with

anterior character. A second “transformer” signal, present in a gradient with a high

point in the posterior mesoderm, then progressively posteriori ses the neural plate to

generate the remaining regions of the CNS. He argued that the transformer is dominant

over the activator, since the proximal part of the fold developed with a more posterior

character, even though it must have experienced both factors. The discovery of the

neural inducer molecules noggin and chordin lend support to this model, since they

both induce neural tissue expressing anterior markers in X enopus animal caps.

Furthermore, three classes of signal- Wnts, FGFs and retinoic acid- have been shown

(reviewed in Doniach, 1995; Sasai and De Robertis, 1997). Other molecules are

probably also involved; tissue grafting experiments have shown that posterior non-

axial mesoderm (that comes to underlie the posterior neural plate) can exert a

posteriorising influence on forebrain regions, but this activity is unlikely to be

mediated by the signals just mentioned (Bang et al., 1997; Woo and Fraser, 1997).

The vitamin A metabolite retinoic acid (RA) is thought to play a role in posteriorising

the CNS by influencing the expression of Hox genes, which are involved in positional

identity along the body axis (reviewed in Maden and Holder, 1992; Conlon, 1995).

Treatment of Xenopus embryos with RA causes a reduction in forebrain volume and

corresponding increase in hindbrain volume (Durston et al., 1989; Sive et a l, 1990).

More subtle effects are also seen in the hindbrain, where anterior rhombomeres are

reduced or compressed (Papalopulu et a l, 1991). Furthermore, constitutively active

RA receptors reduce anterior neural tissue while dominant negative RA receptors

expand anterior neural structures (Blumberg et a l, 1997). Several Hox genes contain

RA response elements, and RA can induce Hox gene expression in mouse ectoderm,

while repressing expression of more anterior genes such as otx2 (Conlon and Rossant,

1992; Ang et a l, 1994). In addition, targeted mutagenesis of RA receptors can

produce homeotic alterations to the axial skeleton similar to certain Hox loss-of-

function phenotypes (Lohnes et a l , 1994), although the majority of these mutant

phenotypes are surprisingly mild. These results have led to the suggestion that there

may be an endogenous gradient of RA within the neural plate, with a high point at the

posterior end, that promotes development of posterior structures while restricting the

tuned analyses of the distribution of endogenous RA during neurulation, and the

localisation of the main enzymes involved in the biosynthesis and degradation of RA,

challenge this concept (Maden et al., 1998; reviewed in Maden, 1999; Gavalas and

Krumlauf, 2000). The highpoint of the RA gradient is in fact at the hindbrain/ spinal

cord boundary, and the RA concentration gradually drops towards the posterior end of

the embryo (Maden et a l, 1998). In addition, treatment of mouse and zebrafish

embryos with RA causes a reduction of anterior rhombomeres but does not affect

forebrain and midbrain development (Holder and Hill, 1991; Morris-Kay, 1991; Woo

and Fraser, 1997), suggesting that global effects of RA on neural structures are unique

to Xenopus, and that a more conserved function of RA is in patterning the hindbrain.

A role for FGF signalling in posteriorising neural tissue is supported by many different

experiments (reviewed in Doniach, 1995; Mason, 1996). bFGF (FGF2) can transform

a frog anterior neural plate explant into posterior CNS in vitro (Cox and Hemmati-

Brivanlou, 1995). When animal caps are co-treated with bFGF and either noggin or

chordin, posterior neural markers are induced in addition to forebrain markers, at

opposite poles of the explant (Lamb and Harland, 1995; Cox and Hemmati-Brivanlou,

1995; Sasai et a l, 1996). Injection of a dominant negative receptor (XFD) at the two

cell stage results in embryos completely lacking posterior trunk and tail structures, but

with relatively normal anterior neural structures (Amaya et a l, 1991; Launay et al.,

1996). Furthermore, over-expression of XFD in Keller explants inhibits posterior

neural markers but not anterior or pan-neural markers (Holowacz and Sokol, 1999).

The authors also found they could achieve similar effects in whole embryos, by

However, in transgenic embryos expressing zygotic XFD in every cell, Kroll and

Amaya (1996) were unable to reveal a role for FGF in either neuralisation or posterior

neural patterning.

Analysis of mouse mutants further supports a role for FGF signalling in AP neural

patterning. F G F8 -/- mice show perturbed patterning of the neural plate; anterior

neuroectoderm markers are widely expressed, while posterior markers are absent (Sun

et a l, 1999). In addition, FGFR-1 deficient mouse embryos frequently exhibit

truncations or disorganisation of posterior embryonic regions (Deng et al., 1994;

Yamaguchi et a l, 1994). Finally, a large number of FGFs have expression patterns

confined to posterior axial and paraxial mesoderm, and not anterior mesoderm,

suggesting that several members of the family could be necessary for posterior

development in vivo. However, it is unlikely that a gradient of FGF signalling is

responsible for producing AP pattern, since no clear correlation between the dose of

FGF and the axial level of the neural marker induced, has been demonstrated. It is

possible instead that the effects of FGF depend upon competence of the responding

tissue, since the age of the responding tissue appears to affect the nature of the

response produced (Lamb and Harland, 1995).

Finally, members of the Wnt family are good candidates for a posterior transformation

signal. In wnt3 -/- mice, the epiblast proliferates in an undifferentiated state that lacks

AP neural patterning (Liu et a l, 1999). In addition, the function of wnt3a has been

wnt3a can synergise with noggin and follistatin to increase the expression of posterior

neural genes in Xenopus animal cap explants (McGrew et a l, 1995).