The cellular mechanisms responsible for the formation of the Xenopus nervous system have been examined in total exogastrula embryos in which the axial mesoderm appears to remain segregated from prospective neural ectoderm and in recombinates of ectoderm and mesoderm. Posterior neural tissue displaying anteroposterior pattern develops in exogastrula ectoderm. This effect may be mediated by planar signals that occur in the absence of underlying mesoderm. The formation of a posterior neural tube may depend on the notoplate, a midline ectodermal cell group which extends along the anteroposterior axis. The induction of neural structures characteristic of the forebrain and of cell types normally found in the ventral region of the posterior neural tube requires additional vertical signals from underlying axial mesoderm. Thus, the formation of the embryonic Xenopus nervous system appears to involve the cooperation of distinct planar and vertical signals derived from midline cell groups.

REFERENCES

Abo
T.
,
Balch
C. M.
(
1981
)
A differentiation antigen of hunam NK and K cells identified by a monoclonal antibody (HNK-1).
J. Immunol
127
,
1024
1029
Clarke
J. D. W.
,
Holder
N.
,
Soffe
S. R.
,
Storm-Mathisen
J.
(
1991
)
Neuroanatomical and functional analysis of neural tube formation innotochordless Xenopus embryos: laterality of the spinal cord is lost.
Development
112
,
499
516
Cooke
J.
,
Smith
E. J.
(
1988
)
The restrictive effect of early exposure to lithium upon body pattern in Xenopus development, studied by quantitative anatomy and immunofluorescence.
Development
102
,
85
99
Davis
C. A.
,
Holmyard
D. P.
,
Millen
K. J.
,
Joyner
A. L.
(
1991
)
Examining pattern formation in mouse, chicken and frog embryos with an En-specific antiserum.
Development
111
,
287
298
Dixon
J.
,
Kintner
C. R.
(
1989
)
Cellular contacts required for neural induction in Xenopus embryos: Evidence for two signals.
Development
106
,
749
757
Dodd
J.
,
Morton
S. B.
,
Karagogeos
D.
,
Yamamoto
M.
,
Jessell
T. M.
(
1988
)
Spatial regulation of axonal glycoprotein expression on subsets of embryonic spinal cord neurons.
Neuron
1
,
105
116
Durston
A. J.
,
Timmermans
J. P. M.
,
Hage
W. J.
,
Hendricks
H. F. J.
,
de Vries
N. J.
,
Heideveld
M.
,
Nieuwkoop
P. D.
(
1989
)
Retinoic acid causes an anteroposterior transformation of the developing central nervous system.
Nature
340
,
140
144
Harland
R. M.
(
1991
)
In situ hybridization: an improved whole mount method for Xenopus embryos.
Methods in Enz
36
,
675
685
Hemmati-Brivanlou
A.
,
de la Torre
J. R.
,
Holt
C.
,
Harland
R. M.
(
1991
)
Cephalic expression and molecular characterization of Xenopus En-2.
Development
111
,
715
724
Hemmati-Brivanlou
A.
,
Frank
D.
,
Bolce
M.E.
,
Brown
B. D.
,
Sive
H. L.
,
Harland
R. M.
(
1990
)
Localization of specific mRNAs in Xenopus embryos by whole-mount in situ hybridization.
Development
110
,
325
330
Hemmati-Brivanlou
A.
,
Harland
R. M.
(
1989
)
Expression of an engrailed-related protein is induced in the anterior ectoderm of early Xenopus embryos.
Development
106
,
611
617
Hemmati-Brivanlou
A.
,
Stewart
R. M.
,
Harland
R. M.
(
1990
)
Region-specific neural induction of an engrailed protein by anterior notochord in Xenopus.
Science
250
,
800
802
Hornbruch
A.
,
Wolpert
L.
(
1986
)
Positional signalling by Hensen's node when grafted to the chick limb bud.
J. Embryol. Exp. Morphol
94
,
257
265
Itoh
K.
,
Kubota
H. Y.
(
1991
)
Homoiogenetic neural induction in Xenopus chimeric explants.
Dev. Growth and Diff
33
,
209
216
Jacobson
A. G.
,
Sater
A. K.
(
1988
)
Features of embryonic induction.
Development
104
,
341
359
Jacobson
M.
,
Hirose
G.
(
1981
)
Clonal organization of the nervous system of the frog. II. Clones stemming from individual blastomeres of the 32-and 64-cell stages.
J. Neurosci
1
,
271
284
Kao
K. R.
,
Elinson
R. P.
(
1988
)
The entire mesodermal mantle behaves as Spemann's organizer in dorso-anterior enhanced Xenopus laevis embryos.
Dev. Biol
127
,
64
77
Kao
K. R.
,
Masui
Y.
,
Elinson
R. P.
(
1986
)
Lithium-induced respecification of pattern in Xenopus laevis embryos.
Nature
322
,
371
373
Keller
R.
,
Danilchik
M.
(
1988
)
Regional expression, pattern and timing of convergence and extension during gastrulation of Xenopus laevis.
Development
103
,
193
209
Kintner
C. R.
,
Brockes
J. P.
(
1984
)
Monoclonal antibodies identify blastema cells derived from differentiating muscle in newt limb regeneration.
Nature
308
,
67
69
Kintner
C. R.
,
Melton
D. A.
(
1987
)
Expression of the Xenopus N-CAM RNA in ectoderm is an early response to neural induction.
Development
99
,
311
325
Lamborghini
J. E.
(
1980
)
Rohon-Beard cells and other large neurons in Xenopus embryos originate during gastrulation.
J. Comp. Neurol
189
,
323
333
Newport
J.
,
Kirschner
M.
(
1982
)
A major developmental transition in early Xenopus embryos: II. Control of the onset of transcription.
Cell
30
,
687
696
Nicolet
G.
(
1971
)
Avian gastrulation.
Adv. in Morph
9
,
231
262
Nieuwkoop
P. D.
(
1989
)
The succesive steps in the pattern formation of the amphibian central nervous system.
Dev. Growth Diff
32
,
149
154
Nieuwkoop
P. D.
,
Albers
B.
(
1990
)
The role of competence in the craniocaudal segregation of the central nervous system.
Dev. Growth Diff
32
,
23
31
Nieuwkoop
P. D.
,
Boterenbrood
E. C.
,
Kremer
A.
,
Bloesma
F. F. S. N.
,
Hoessels
E. L. M. J.
,
Meyer
G.
,
Verheyen
F. J.
(
1952
)
Activation and organization of the central nervous system in amphibians.
J. Exp. Zool
120
,
1
108
Nordlander
R. H.
(
1986
)
Motoneurons of the tail of young Xenopus tadpoles.
J. Comp. Neurol
253
,
403
413
Nordlander
R. H.
(
1989
)
HNK-1 marks earliest axonal outgrowth in Xenopus.
Dev. Brain Res
50
,
147
153
Patel
N. H.
,
Martin-Blanco
E.
,
Coleman
K. G.
,
Poole
S. J.
,
Ellis
M. C.
,
Kornberg
T. B.
,
Goodman
C. S.
(
1989
)
Expression of engrailed proteins in arthropods, annelids and chordates.
Cell
58
,
955
968
Placzek
M.
,
Tessier-Lavigne
M.
,
Yamada
T.
,
Jessell
T.
,
Dodd
J.
(
1990
)
Mesodermal control of neural cell identity: floor plate induction by the notochord.
Science
250
,
985
988
Rathjen
F.
,
Wolff
J. M.
,
Chiquet-Ehrismann
R.
(
1991
)
Restrictin: a chick neural extracellular matrix protein involved in cell attachement co-purifies with cell recognition molecule F11.
Development
113
,
151
164
Roach
F. C.
(
1945
)
Differentiation of the central nervous system after axial reversals of the medullary plate of amblystoma.
J. Exp. Zool
99
,
53
77
Ruiz i Altaba
A.
(
1990
)
Neural expression of the Xenopus homeobox gene Xhox3: Evidence for a patterning neural signal that spreads through the ectoderm.
Development
108
,
595
604
Ruiz i Altaba
A.
,
Jessell
T. M.
(
1991
)
Retinoic acid modifies mesodermal patterning in early Xenopus embryos.
Genes Dev
5
,
175
187
Ruiz i Altaba
A.
,
Jessell
T. M.
(
1991
)
Retinoic acid modifies the pattern of cell differentiation in the central nervous system of neurula stage Xenopus embryos.
Development
112
,
945
958
Ruiz i Altaba
A.
,
Melton
D. A.
(
1989
)
Interaction between peptide growth factors and homeobox genes in the establishment of anterior-posterior polarity in frog embryos.
Nature
341
,
33
38
Schoenwolf
G. C.
,
Smith
J. L.
(
1990
)
Mechanisms of neurulation: traditional viewpoint and recent advances.
Development
109
,
243
270
Servetnick
M.
,
Grainger
R.
(
1991
)
Homeogenetic neural induction in Xenopus.
Dev. Biol
147
,
73
82
Sharpe
C. R.
,
Fritz
A.
,
De Robertis
E. M.
,
Gurdon
J. B.
(
1987
)
A homeobox-containing marker of posterior neural differenttiation shows the importance of predetermination in neural induction.
Cell
50
,
749
758
Smith
J. C.
,
Price
B. M. J.
,
Green
J. B. A.
,
Weigel
D.
,
Herrmann
B. G.
(
1991
)
Expression of a Xenopus homolog of Brachyury (T) is an immediate-early response to mesoderm induction.
Cell
67
,
79
87
Smith
J. C.
,
Malacinski
G. M.
(
1983
)
The origin of the mesoderm in an Anuran, Xenopus laevis, and a Urodele, Ambystoma mexicanum.
Dev. Biol
98
,
250
254
Smith
J. C.
,
Watt
F. M.
(
1985
)
Biochemical specificity of the Xenopus notochord.
Differentiation
29
,
109
115
Sokol
S.
,
Melton
D. A.
(
1991
)
Preexistent pattern in Xenopus animal pole cells revealed by induction with activin.
Nature
361
,
409
411
Spratt
N. T.
,
Haas
H.
(
1965
)
Germ layer formation and the role of the primitive streak in the chick. I. Basic architecture and morphogenetic tissue movements.
J. Exp Zool
158
,
9
38
Suzuki
A. S.
,
Mifune
Y.
,
Kaneda
T.
(
1984
)
Germ layer interactions in pattern formation of amphibian mesoderm during primary embryonic induction.
Dev. Growth Diff
26
,
81
94
Wagner
M.
,
Thaller
C.
,
Jessell
T.
,
Eichele
G.
(
1990
)
Polarizing activity and retinoid synthesis in the floor plate of the neural tube.
Nature
345
,
819
822
Yamada
T.
,
Placzek
M.
,
Tanaka
H.
,
Dodd
J.
,
Jessell
T. M.
(
1991
)
Control of cell pattern in the developing nervous system: polarizing activity of the floor plate and notochord.
Cell
64
,
635
647
This content is only available via PDF.