Several stages in the lens determination process have been defined, though it is not known which gene products control these events. At mid-gastrula stages in Xenopus, ectoderm is transiently competent to respond to lens-inducing signals. Between late gastrula and neural tube stages, the presumptive lens ectoderm acquires a lens-forming bias, becomes specified to form lens and begins differentiation. Several genes have been identified, either by expression pattern, mutant phenotype or involvement in crystallin gene regulation, that may play a role in lens bias and specification, and we focus on these roles here. Fate mapping shows that the transcriptional regulators Otx-2, Pax-6 and Sox-3 are expressed in the presumptive lens ectoderm prior to lens differentiation. Otx-2 appears first, followed by Pax-6, during the stages of lens bias (late neural plate stages); expression of Sox-3 follows neural tube closure and lens specification. We also demonstrate the expression of these genes in competent ectoderm transplanted to the lens-forming region. Expression of these genes is maintained or activated preferentially in ectoderm in response to the anterior head environment. Finally, we examined activation of these genes in response to early and late lens-inducing signals. Activation of Otx-2, Pax-6 and Sox-3 in competent ectoderm occurs in response to the early inducing tissue, the anterior neural plate. Since Sox-3 is activated following neural tube closure, we tested its dependence on the later inducing tissue, the optic vesicle, which contacts lens ectoderm at this stage. Sox-3 is not expressed in lens ectoderm, nor does a lens form, when the optic vesicle anlage is removed at late neural plate stages. Expression of these genes demarcates patterning events preceding differentiation and is tightly coupled to particular phases of lens induction.

REFERENCES

Abdelhak
S.
, et al. 
(
1997
)
A human homologue of the Drosophila eyes absent gene underlies branchio-oto-renal (BOR) syndrome and identifies a novel gene family.
Nature Genet
15
,
157
164
Ang
S. L.
,
Conlon
R. A.
,
Jin
O.
,
Rossant
J.
(
1994
)
Positive and negative signals from mesoderm regulate the expression of mouse Otx2 in ectoderm explants.
Development
120
,
2979
2989
Blitz
I. L.
,
Cho
K. W. Y.
(
1995
)
Anterior neurectoderm is progressively induced during gastrulation: the role of the Xenopus homeobox gene orthodenticle.
Development
121
,
993
1004
Callaerts
P.
,
Halder
G.
,
Gehring
W. J.
(
1997
)
PAX-6 in development and evolution.
Ann. Rev. Neurosci
20
,
483
532
Collignon
J.
,
Sockanathan
S.
,
Hacker
A.
,
Cohen-Tannoudji
M.
,
Norris
D.
,
Rastan
S.
,
Stevanovic
M.
,
Goodfellow
P. N.
,
Lovell-Badge
R.
(
1996
)
A comparison of the properties of Sox-3 with Sry and two related genes, Sox-1 and Sox-2.
Development
122
,
509
520
Cvekl
A.
,
Piatigorsky
J.
(
1996
)
Lens development and crystallin gene expression—many roles for Pax-6.
Bioessays
18
,
621
630
Doniach
T.
,
Musci
T. J.
(
1995
)
Induction of anteroposterior neural pattern in Xenopus: evidence for a quantitative mechanism.
Mech. Dev
53
,
403
413
Eagleson
G. W.
,
Harris
W. A.
(
1990
)
Mapping of the presumptive brain regions in the neural plate of Xenopus laevis.
J. Neurobiol
21
,
427
440
Eagleson
G.
,
Ferreiro
B.
,
Harris
W. A.
(
1995
)
Fate of the anterior neural ridge and the morphogenesis of the Xenopus forebrain.
J. Neurobiol
28
,
146
158
Gans
C.
,
Northcutt
R. G.
(
1983
)
Neural crest and the origin of vertebrates: a new head.
Science
220
,
268
274
Gimlich
R. L.
,
Braun
J.
(
1985
)
Improved fluorescent compounds for tracing cell lineage.
Dev. Biol
109
,
509
514
Gould
S. E.
,
Grainger
R. M.
(
1997
)
Neural induction and antero-posterior patterning in the amphibian embryo: past, present, and future.
Cell. Mol. Life Sci
53
,
319
338
Grainger
R. M.
(
1992
)
Embryonic lens induction: shedding light on vertebrate tissue determination.
Trends Genet
8
,
349
355
Grainger
R. M.
,
Mannion
J. E.
,
Cook
T. L.
Jr.
,
Zygar
C. A.
(
1997
)
Defining intermediate stages in cell determination: acquisition of a lens-forming bias in head ectoderm during lens determination.
Dev. Genetics
20
,
246
257
Halder
G.
,
Callaerts
P.
,
Gehring
W. J.
(
1995
)
Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila.
Science
267
,
1788
92
Harland
R. M.
(
1991
)
In situ hybridization: an improved whole-mount method for Xenopus embryos.
Methods in Cell Biology
36
,
685
695
Henry
J. J.
,
Grainger
R. M.
(
1987
)
Inductive interactions in the spatial and temporal restriction of lens-forming potential in embryonic ectoderm of Xenopus laevis.
Dev. Biol
124
,
200
214
Henry
J. J.
,
Grainger
R. M.
(
1990
)
Early tissue interactions leading to embryonic lens formation in Xenopus laevis.
Dev. Biol
141
,
149
163
Hill
R. E.
, et al. 
(
1991
)
Mouse Small eye results from mutations in a paired-like homeobox-containing gene.
Nature
354
,
522
525
Hirsch
N.
,
Harris
W. A.
(
1997
)
XenopusPax-6 and retinal development.
J. Neurobiol
32
,
45
61
Jacobson
A. G.
(
1963
)
The determination and positioning of the nose, lens, and ear. Interaction within the ectoderm and between the ectoderm and underlying tissues.
J. Exp. Zool
154
,
273
283
Jacobson
A. G.
(
1966
)
Inductive processes in embryonic development.
Science
152
,
25
34
Kamachi
Y.
,
Sockanathan
S.
,
Liu
Q.
,
Breitman
M.
,
Lovell-Badge
R.
,
Kondoh
H.
(
1995
)
Involvement of SOX proteins in lens-specific activation of crystallin genes.
EMBO J
14
,
3510
9
Keller
R. E.
(
1975
)
Vital dye mapping of the gastrula and neurula of Xenopus laevis. I. Prospective areas and morphogenetic movements of the superficial layer.
Dev. Biol
42
,
222
241
Lamb
T. M.
,
Knecht
A. K.
,
Smith
W. C.
,
Stachel
S. E.
,
Economides
A.N.
,
Stahl
N.
,
Yancopolous
G. D.
,
Harland
R. M.
(
1993
)
Neural induction by the secreted polypeptide Noggin.
Science
262
,
713
718
Li
H.-S.
,
Yang
J.-M.
,
Jacobson
R. D.
,
Pasko
D.
,
Sundin
O.
(
1994
)
Pax-6 is first expressed in a region of ectoderm anterior to the early neural plate: implications for stepwise determination of the lens.
Dev. Biol
162
,
181
194
Pannese
M.
,
Polo
C.
,
Andreazzoli
M.
,
Vignali
R.
,
Kablar
B.
,
Barsacchi
G.
,
Boncinelli
E.
(
1995
)
The Xenopus homologue of Otx2 is a maternal homeobox gene that demarcates and specifies anterior body regions.
Development
121
,
707
720
Papalopulu
N.
,
Kintner
C.
(
1993
)
Xenopus Distal-less related homeobox genes are expressed in the developing forebrain and are induced by planar signals.
Development
117
,
961
975
Pevny
L. H.
,
Lovell-Badge
R.
(
1997
)
Sox genes find their feet.
Curr. Opin. Genet. Dev
7
,
338
344
Quiring
R.
,
Walldorf
U.
,
Kloter
U.
,
Gehring
W. J.
(
1994
)
Homology of the eyeless gene of Drosophila to the Small eye gene in mice and Aniridia in humans.
Science
265
,
785
789
Saha
M. S.
,
Spann
C. L.
,
Grainger
R. M.
(
1989
)
Embryonic lens induction: more than meets the optic vesicle.
Cell Diff. Devel
28
,
153
171
Servetnick
M.
,
Grainger
R. M.
(
1991
)
Changes in neural and lenscompetence in Xenopus ectoderm: evidence for an autonomous developmental timer.
Development
112
,
177
188
Servetnick
M.
,
Grainger
R. M.
(
1991
)
Homeogenetic neural induction in Xenopus.
Dev. Biol
147
,
73
82
Simeone
A.
,
Acampora
D.
,
Gulisano
M.
,
Stornaiuolo
A.
,
Boncinelli
E.
(
1992
)
Nested expression domains of four homeobox genes in developing rostral brain.
Nature
358
,
687
690
Slack
J. M.
(
1984
)
Regional biosynthetic markers in the early amphibian embryo.
J. Embryol. Exp. Morph
80
,
289
319
Smolich
B. D.
,
Tarkington
S. K.
,
Saha
M. S.
,
Grainger
R. M.
(
1994
)
Xenopus gamma-crystallin gene expression: evidence that the gamma-crystallin gene family is transcribed in lens and nonlens tissues.
Mol. Cell Biol
14
,
1355
63
Ton
C. C. T.
, et al. 
(
1991
)
Positional cloning and characterization of a paired box-and homeobox-containing gene from the Aniridia region.
Cell
67
,
1059
74
Walther
C.
,
Gruss
P.
(
1991
)
Pax-6, a murine paired box gene, is expressed in the developing CNS.
Development
113
,
1435
1449
Wieschaus
E.
,
Perrimon
N.
,
Finkelstein
R.
(
1992
)
orthodenticle activity is required for the development of medial structures in the larval and adult epidermis of Drosophila.
Development
115
,
801
11
Xu
P.-X.
,
Woo
I.
,
Her
H.
,
Beier
D. R.
,
Maas
R. L.
(
1997
)
Mouse Eya homologues of the Drosophila eyes absent gene require Pax6 for expression in lens and nasal placodes.
Development
124
,
219
231
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