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Embryonic Induction during Vertebrate Development: Regionalization of
What causes diversity in the middle kingdom?
The mesoderm is a complex array of tissue-types in three primary domains:
dorsal, ventrolateral, and ventral. Let's briefly review the sources of
the signals that generate the regional diversity of the mesoderm.
A general mesoderm-inducing signal from the vegetal hemisphere induces overlying
marginal zone cells to become ventral mesoderm. Within the Nieuwkoop center,
signals are produced that induce the dorsal marginal zone cells to become
axial and head mesoderm. The dorsal marginal zone functions as Spemann's
organizer. One of the properties of Spemann's organizer is to dorsalize
the adjacent lateral mesoderm.
As we have discussed, the Nieuwkoop center is apparently established by
activation of a Wnt signaling pathway, of which ß-catenin is a critical
component. Recently, evidence has been presented that a consequence of activation
of the Wnt pathway is activation of the transcription factor Siamois,
which is located downstream of ß-catenin (Carnac et al., 1996).
One implication of these results may be that the signaling properties of
cells of the Nieuwkoop center depend upon regulation of gene expression
by Siamois. This would imply that Nieuwkoop signaling is dependent
upon zygotic gene transcription, whereas earlier events, including dorsalization
and initiation of mesoderm induction, would be dependent upon maternal messengers.
This is a new way of thinking about mesoderm induction. Time will tell whether
this dichotomy holds.
It is likely given the large number of factors that are capable of inducing
mesoderm that the regional character of the mesoderm is a result of the
combinatorial action of the endogenous correlates of these factors.
We still cannot state unequivocally what the roles of these individual factors
are in generating mesoderm, but we should examine their potential involvement.
The messenger RNAs for Vg-1, BMP-2
FGF-2, FGF-4, Wnt-11, and noggin are present in the egg. Thus, they
are present during the time when mesoderm induction is initiated. As we
shall discuss, noggin is also produced by zygotic gene expression
and may play different roles at different times.
We have discussed extensively the likely involvement of Vg-1
in mesoderm induction. Kessler and Melton (1995) proposed that the rotation
of the cortex at fertilization triggers localized processing of Vg1 precursor,
which imparts on those cells functionality as the Nieuwkoop center. Mature
Vg1 ligand secreted from the Nieuwkoop center would induce the Spemann organizer.
The Spemann organizer, in turn, would induce neural tissue and also dorsalize
The bone morphogenetic proteins (BMPs) are TGF-ß-related proteins.
BMP-4 induces ventral mesoderm, suppresses induction of dorsal mesoderm
by activin and inhibits dorsoanterior development of embryos, suggesting
that it is a ventralizing factor. The messengers encoding the BMPs are not
localized in the vegetal hemisphere, which casts some doubt on their role
in mesoderm induction. However, expression of a dominant-negative BMP-4
receptor blocked mesoderm induction by BMP-4 in isolated animal caps and
prevented ventralization of embryos by BMP-4. Overexpression of the receptor
in embryos caused mesoderm to be dorsalized. These results suggest that
BMP-4 may be involved in ventralization of the mesoderm by counteracting
the dorsalizing signals (for review, see Kessler and Melton, 1994).
The fibroblast growth factors induce ventrolateral mesoderm. Expression
of dominant-negative FGF receptor inhibits mesoderm induction in animal
cap explants and causes defects in trunk and posterior development, while
not affecting anterior development (Fig. 1). This result suggests that FGF
is necessary, but not sufficient, for induction of trunk and posterior mesoderm
(for review, see Kessler and Melton, 1994).
Figure 1 Effect on Xenopus embryo of overexpression
of FGF (top) and of ablation of FGF activity (bottom). Photo courtesy of
Noggin mRNA is oogenic, but - like FGF and BMP mRNA - it is not
localized to the vegetal hemisphere. However, zygotic expression of noggin
is interesting because it begins at the mid-blastula transition (MBT) in
the organizer region and later in the dorsal mid-line of the archenteron
roof. This puts it in a position to play an important role in either dorsalization
and/or neural induction (Slack, 1994). It is noteworthy that when ventral
marginal zone tissue is exposed to noggin protein, it induces muscle (Smith
et al., 1993). Thus, noggin is a possible candidate for the
third signal in the three signal model.
The Wnts are a diverse group of proteins related to the transforming
int proteins of mammals and wingless in Drosophila. They were first
implicated in Xenopus development by the demonstration that mouse
wnt-1 overexpression could cause induction of ectopic dorsal axial
structures (McMahon and Moon, 1989). This stimulated a search for the Xenopus
homologues and discovery of a number of Xwnt genes, including Xwnt-11,
whose transcript is localized to the vegetal hemisphere.
Combinatorial effects of factors involved in mesoderm induction
(Xbra) is the Xenopus homolog of a mouse mutation that causes
absence of notochord and posterior mesoderm. It encodes a nuclear protein
that is a putative transcription factor. Brachyury in Xenopus
is initially transcribed generally in the prospective mesoderm and later
becomes restricted to the notochord and posterior mesoderm. Overexpression
studies have shown that it specifies ventro-posterior mesoderm differentiation.
As we have discussed, noggin
is expressed in the organizer and can dorsalize ventral mesoderm. Xwnt-8
on the other hand, is expressed zygotically in all mesoderm except the organizer
and can ventralize dorsal tissue.
Thus, all three of these factors are expressed concurrently in the mesoderm:
Xbra is expressed throughout and is co-expressed dorsally with noggin
and ventrally with Xwnt-8. What, then might be the consequences of
their co-expression? Remember, the brachyury protein is nuclear, whereas
the other two are ligands. Thus, Xbra may be involved in mediating signaling
events, whereas the others are candidates for signals.
Cunliffe and Smith (1994) demonstrated the effects of injecting the Xbra
messenger and the plasmids encoding the other two factors, either alone
or in combination (Fig. 4). The plasmids had the genes encoding these factors
under the control of a cytoskeletal actin promoter, which becomes activated
at the MBT. This allowed them to investigate how these factors might interact
after the MBT. Xbra by itself specifies ventral mesoderm, and noggin by
itself specifies neural ectoderm. However, in combination, Xbra synergizes
with noggin to specify notochord and muscle. On the other hand, Xbra could
not synergize with Xwnt-8 to specify dorsal mesoderm. These results suggest
that specification of dorsal mesoderm may depend upon synergism between
Xbra and noggin.
The ability of noggin by itself to induce neural tissue makes it a strong
candidate for an endogenous neural inducer.
Cunliffe and Smith have proposed a model to explain how these factors may
be involved in pattern specification during Xenopus development (Fig.
Food for thought
Makoto Asashima and his associates at the University of Tokyo have used
the capacity for activin to induce animal caps to form mesoderm to make
organs and tissues in vitro (Roush, 1997). They have taken advantage
of the differential effects of different concentrations of activin. Low
doses (50 ng/ml) produced notochord, 75 ng/ml produced a beating heart,
whereas 100 ng/ml produced a liver. Combining activin with other factors,
such as retinoic acid and insulin-like growth factors, yielded pronephros,
rudimentary eyes and ears. These intriguing results suggest the possibility
that regeneration of organs (perhaps even human organs) in vitro
may be a realistic possibility.
- What is the proposed role of Siamois in establishment of the
- How is cortical rotation thought to affect Vg-1 function?
- What are the BMPs, and what is their proposed role in regionalization
of the mesoderm? What evidence supports that role?
- What role are the FGFs proposed to play in regionalization of the mesoderm?
What evidence supports that role?
- What is unique about the sources of Noggin mRNA in the embryo?
- What are the effects of exposure of ventral marginal zone tissue to
noggin protein? What is the significance of this result?
- What is X-bra and what is its pattern of expression in the embryo?
- How are X-bra, noggin and Xwnt-8 proposed to interact to specify regional
pattern on mesoderm? Support your statement with experimental evidence.
- How might a gradient of activin be involved in specifying regional
pattern on mesoderm?
Links to Related Material
See also Patterning
of the Mesoderm by Activin in Zygote.
Graff, J.M. 1997. Embryonic patterning: to BMP or not to BMP, that is
the question. Cell 89: 171-174.
Maeda, R., Kobayashi, A., Sekine, R., Lin, J.-J., Kung, H.-f. and Maéno,
M. 1997. Xmsx-1 modifies mesodermal tissue pattern along dorsoventral
axis in Xenopus laevis embryo. Development 124: 2553-2560.
Isaacs, H.V. 1997. New perspectives on the role of the fibroblast growth
factor family in amphibian development. Cell. Mol. Life Sci. 53: 350-361.
LaBonne, C. and Whitman, M. 1997. Localization of map kinase activity in
early Xenopus embryos: implications for endogenous FGF signaling.
Develop. Biol. 183: 9-20.
Joseph, E.M. and Melton, D.A., Xnr4: A Xenopus nodal-related
gene expressed in the Spemann organizer. Develop. Biol. 184: 367-372.
Pownall, M.E., Tucker, A.S., Slack, J.M.W. and Isaacs, H.V. 1996. eFGF,
Xcad3 and Hox genes form a molecular pathway that establishes the
anteroposterior axis in Xenopus. Development 122: 3881-3892.
Tada, M., O'Reilly, M.-A.J. and Smith, J.C. 1997. Analysis of competence
and of Brachyury autoinduction by use of hormone-inducible Xbra.
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McMahon, A.P. and R.T. Moon. 1989. Ectopic expression of the proto-oncogene
int-1 in Xenopus embryos leads to duplication of the embryonic axis.
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Roush, W. 1997. A developmental biology summit in the high country. Science
Slack, J.M.W. 1994. Inducing factors in Xenopus early embryos. Current Biology
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