Li H , Tierney C , Wen L , Wu JY , Rao Y .

R01 CA114197-01A2 NCI NIH HHS , R01 EY014576-03 NEI NIH HHS , R01 GM070967-02 NIGMS NIH HHS , R01 CA114197 NCI NIH HHS , R01 EY014576 NEI NIH HHS , R01 GM070967 NIGMS NIH HHS

	Fig. 1. Models for retina primordium formation in vertebrate embryos. The outline illustrates the boundary of the neural plate and the filled circles represent retina primordia. (A) The two separate retina primordia model. (B) A diagram of Stockard’s model of how a single primordium spreads and separates into two retinae. Some of Stockard’s results were directly contradicted by later studies of Adelmann (1929a,b). (C) This is essentially the same as Figs 1, 2 and 4 of LePlat (1919). (D) A schematic representation of Adelmann’s model. It was not determined whether midline cells could migrate laterally. (E) An illustration of the forebrain field model. Eyes are presumably the default fate of this field (Corner, 1966; Boterenbrood, 1970).
	Fig. 2. Alignment of the predicted primary sequence within the T domain of ET with those of other T domain proteins. A dash (-) indicates identity to Xbra. A period (.) indicates a gap. Residues shown in the consensus line are those identical in all known T domains while a star (*) indicates a highly but not absolutely conserved residue. Boxed residues are conserved only between ET and Omb. Omb is the predicted product of the Drosophila gene optomotor blind (Pflugfelderet al., 1992). Trg is that of the Drosophila T related gene involved in hindgut development (Kispert et al., 1994). T, Zf-T (or Ntl) and Xbra are the mouse, zebrafish and Xenopus T proteins respectively (Hermann et al., 1990; Smith et al., 1991; Schulte- Merker et al., 1992). Numbers for the ET sequence indicate positions predicted from the PCR fragment while numbers for known T domain proteins are positions in corresponding full-length proteins.
	Fig. 3. The expression pattern of ET in Xenopus embryos. A to G show anterior views and H is a lateral view. (A) A stage (st.) 12.5 embryo with a band of expression in the anterior neural plate (indicated by the arrow) and one in the primordium of the cement gland (indicated by the arrowhead). Note that the cement gland stays a single structure (A, C-G), whereas the retina band resolves into two primordia in the next few stages. (B) A st. 15 embryo showing retina field expression. The embryo was oriented to maximize visualization of retina expression and the cement gland is thus hardly visible. (C) A st. 16 embryo with ET expression weaker in the median region of the retina field than that in the lateral regions. (D) A st. 18 embryo with two distinct retina primordia. There is no ET expression in the median region. (E) A st. 19 embryo with no midline expression. (F) A st. 20 embryo. Note that weak expression of ET appears in the pineal gland primordium. The intensity of this signal increases in later stages (see G and H). (G) A st. 22 embryo with ET expression in the retina, the cement gland and the pineal gland (the dot between the two eyes). (H) A st. 26 embryo. The staining in the eye is restricted to the dorsal retina, absent from the lens and the ventral retina. Expression in the cephalic ganglia and lateral line organ also appeared. (I) A transverse section of a st. 28 embryo showing ET expression in the dorsal retina. ET is expressed in all layers of the retina, and completely absent from the lens. Dorsal is to the right.
	Fig. 4. Comparison of Pax-6 sequence from Xenopus with those from other species. Alignment of the predicted sequences of full-length Pax-6 proteins from Xenopus, mouse and human. A dash (-) indicates identity to the Xenopus sequence. The paired domain (amino acid residues 3-133) and homeo domain (residues 209-269) are in bold.
	Fig. 5. Distribution of Pax-6 mRNA in Xenopus embryos. A-G show anterior views, H is a dorsal view with the anterior end of the embryo to the left, and I is a lateral view. (A) A st. 12 embryo; note a single band of expression continuous from one side of the embryo to the other in the anterior neural plate (indicated by the red arrow) whereas there are two broad stripes of expression in the primordium of the neural tube (indicated by the red arrowhead). (B) A st. 12.5 embryo. The stripes in the trunk region are thinner and closer to each other than those at st. 12. Note the appearance of lens primordia on each side of the embryo (indicated by the green arrow). (C) A st. 16 embryo. The two red lines demarcate the borders of the retina stripe. The arrow points to a lens primordium. Pax-6 expression has three components: lens primordium, an outer semicircle of the retina field and an inner semicircle of a forebrain structure. The forebrain stripe later resolves into two spots posterior to the eyes shown in D and G, which are the two spots closer to the midline than the eyes in E and F. (D) A st. 18 embryo showing that Pax-6 expression in the median region of the anterior neural plate is turned off. The red line indicates the border between the eye primordium and the forebrain structure. Note also that the lens primordia have moved into the retina primordia to form the eye primordium by this stage (compare D to C). (E) A st. 19 embryo showing distinct Pax 6 expression in the eyes and the forebrain. (F) A stage 26 embryo. (G) Higher magnification of a st. 18 embryo. The line points to the border between the forebrain staining and the eye staining. (H) Dorsal view of a st. 24 embryo showing Pax-6 expression in the neural tube derived from the two broad stripes in the trunk region at st. 12 (the arrowhead in A). (I) A st. 25 embryo. Compare Pax-6 expression in the entire eye to ET expression in only the dorsal retina in Fig. 3H.
	Fig. 6. Fate mapping of midline cells in the retina field of Xenopus embryos. A to D show anterior views. (A) A diagram showing that midline cells in the retina field were labeled by DiI at stages 12 or 12.5 and followed to stage 25 to determine whether the midline cells migrate to the lateral regions. (B) The original location of DiI-labeled cells. This embryo was labeled with DiI at stage 12 and fixed at stage 12.5. DiI was photoconverted to produce the brown spot in the middle of the retina field, which was revealed by in situ hybridization with the Xenopus Pax-6 probe. (C) An image of a stage 25 embryo illuminated by incident light. A yellow arrow points to an eye. (D) A composite of the image shown in C and an image of the same embryo viewed under rhodamine optics to map the location of the fluorescent cells. A yellow arrow points to an eye. This embryo was previously labeled with DiI at stage 12.5. Results were similar with embryos labeled at stage 12. (E) A left side view of another DiI-labeled embryo. Note the absence of DiI labeled cells in the left retina. A white arrow points to DiI-labeled cells. A yellow arrow points to an eye.
	Fig. 7. Effects of removing the prechordal mesoderm on retina formation in Xenopus embryonic explants. (A) A wild-type embryo at st. 35. (B) A st. 35 control explant of the anterior neural plate with the underlying prechordal mesoderm. (C) An anterior neural plate explant without prechordal mesoderm. The explants were isolated at st. 12.5 and cultured until st. 35. (D) A transverse section of a st. 42 control explant after in situ hybridization with a Pax-6 probe. (E) A transverse section of a st. 42 explant without prechordal mesoderm at st. 12.5 and fixed for in situ hybridization with Pax-6 at st. 42. Note the presence of Pax-6 expressing neural retina as well as the retina pigment epithelium.
	Fig. 8. Effects of prechordal mesoderm removal on retina field resolution in Xenopus embryonic explants. (A) A control embryo hybridized with Pax-6 at st. 18. (B) Pax-6 expression in a control anterior neural plate explant with the underlying mesoderm, showing two separate primordia by st. 18. (C) Pax-6 expression in a st. 18 explant of the anterior neural plate without the underlying mesoderm. Note the retina field did not resolve into two separate primordia. (D) A control embryo hybridized with ET at st. 18. (E) ET expression in a st. 18 control explant with the prechordal mesoderm. (F) ET expression in a st. 18 explant without prechordal mesoderm.
	Fig. 11. Retina primordium formation in vertebrate embryos. (A) Our model of retina field development. Dorsal views of the anterior neural plate are illustrated here. There is initially a single retina morphogenetic field in the anterior neural plate of vertebrate embryos. Retina formation is suppressed in its median region, resulting in the resolution of the retina field into two retina primordia. (B) The prechordal plate provides a primary signal for retina field resolution. These are transverse section views of the anterior neural plate and its underlying prechordal mesoderm. Experiments reported here demonstrate a role for the prechordal plate while those performed in zebrafish embryos suggest the midline of the neural plate also plays a role (Hatta et al., 1991, 1994; Hatta, 1992; Macdonald et al., 1995; Ekker et al., 1995b).

Basler, Control of cell pattern in the neural tube: regulation of cell differentiation by dorsalin-1, a novel TGF beta family member. 1993, Pubmed

Basler, Control of cell pattern in the neural tube: regulation of cell differentiation by dorsalin-1, a novel TGF beta family member. 1993, Pubmed
Bollag, An ancient family of embryonically expressed mouse genes sharing a conserved protein motif with the T locus. 1994, Pubmed
Boterenbrood, Differentiation in small grafts of the median region of the presumptive prosencephalon. 1970, Pubmed
Brown, The development of handedness in left/right asymmetry. 1990, Pubmed
Bulfone, T-brain-1: a homolog of Brachyury whose expression defines molecularly distinct domains within the cerebral cortex. 1995, Pubmed
Cohen, Perspectives on holoprosencephaly: Part I. Epidemiology, genetics, and syndromology. 1989, Pubmed
Corner, Morphogenetic field properties of the forebrain area of the neural plate in an anuran. 1966, Pubmed
CORNER, DEVELOPMENT OF THE BRAIN IN XENOPUS LAEVIS AFTER REMOVAL OF PARTS OF THE NEURAL PLATE. 1963, Pubmed , Xenbase
Cunliffe, Ectopic mesoderm formation in Xenopus embryos caused by widespread expression of a Brachyury homologue. 1992, Pubmed , Xenbase
Danos, Linkage of cardiac left-right asymmetry and dorsal-anterior development in Xenopus. 1995, Pubmed , Xenbase
Eagleson, Mapping of the presumptive brain regions in the neural plate of Xenopus laevis. 1990, Pubmed , Xenbase
Eagleson, Fate of the anterior neural ridge and the morphogenesis of the Xenopus forebrain. 1995, Pubmed , Xenbase
Echelard, Sonic hedgehog, a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity. 1993, Pubmed
Ekker, Patterning activities of vertebrate hedgehog proteins in the developing eye and brain. 1995, Pubmed
Ekker, Distinct expression and shared activities of members of the hedgehog gene family of Xenopus laevis. 1995, Pubmed , Xenbase
Ericson, Sonic hedgehog induces the differentiation of ventral forebrain neurons: a common signal for ventral patterning within the neural tube. 1995, Pubmed
Glaser, PAX6 gene dosage effect in a family with congenital cataracts, aniridia, anophthalmia and central nervous system defects. 1994, Pubmed
Goulding, Signals from the notochord and floor plate regulate the region-specific expression of two Pax genes in the developing spinal cord. 1993, Pubmed
GRUNEBERG, Genetical studies on the skeleton of the mouse. XXIII. The development of brachyury and anury. 1958, Pubmed
Halder, Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. 1995, Pubmed
Halpern, Induction of muscle pioneers and floor plate is distinguished by the zebrafish no tail mutation. 1993, Pubmed
HAMBURGER, A series of normal stages in the development of the chick embryo. 1951, Pubmed
Hammerschmidt, Protein kinase A is a common negative regulator of Hedgehog signaling in the vertebrate embryo. 1996, Pubmed
Harland, In situ hybridization: an improved whole-mount method for Xenopus embryos. 1991, Pubmed , Xenbase
Hatta, Role of the floor plate in axonal patterning in the zebrafish CNS. 1992, Pubmed
Hatta, Midline signaling in the primordium of the zebrafish anterior central nervous system. 1994, Pubmed
Hatta, The cyclops mutation blocks specification of the floor plate of the zebrafish central nervous system. 1991, Pubmed
Hemmati-Brivanlou, Follistatin, an antagonist of activin, is expressed in the Spemann organizer and displays direct neuralizing activity. 1994, Pubmed , Xenbase
Herrmann, Cloning of the T gene required in mesoderm formation in the mouse. 1990, Pubmed
Herrmann, The T genes in embryogenesis. 1994, Pubmed , Xenbase
Hill, Mouse small eye results from mutations in a paired-like homeobox-containing gene. , Pubmed
Hogan, Small eye (Sey): a mouse model for the genetic analysis of craniofacial abnormalities. 1988, Pubmed
Izpisúa-Belmonte, The homeobox gene goosecoid and the origin of organizer cells in the early chick blastoderm. 1993, Pubmed , Xenbase
Jacobs, Magnetic resonance microscopy of embryonic cell lineages and movements. 1994, Pubmed , Xenbase
Jacobson, Origin of the retina from both sides of the embryonic brain: a contribution to the problem of crossing at the optic chiasma. 1978, Pubmed , Xenbase
Jessell, Polarity and patterning in the neural tube: the origin and function of the floor plate. 1989, Pubmed
Jordan, The human PAX6 gene is mutated in two patients with aniridia. 1992, Pubmed
Källén, The cyclops and the mermaid: an epidemiological study of two types of rare malformation. 1992, Pubmed
Kispert, The T protein encoded by Brachyury is a tissue-specific transcription factor. 1995, Pubmed
Kispert, The Brachyury gene encodes a novel DNA binding protein. 1993, Pubmed , Xenbase
Kispert, Homologs of the mouse Brachyury gene are involved in the specification of posterior terminal structures in Drosophila, Tribolium, and Locusta. 1994, Pubmed
Klar, A model for specification of the left-right axis in vertebrates. 1994, Pubmed
Krauss, A functionally conserved homolog of the Drosophila segment polarity gene hh is expressed in tissues with polarizing activity in zebrafish embryos. 1993, Pubmed
Krauss, Expression pattern of zebrafish pax genes suggests a role in early brain regionalization. 1991, Pubmed
Lamb, Neural induction by the secreted polypeptide noggin. 1993, Pubmed , Xenbase
Levin, A molecular pathway determining left-right asymmetry in chick embryogenesis. 1995, Pubmed
Li, Pax-6 is first expressed in a region of ectoderm anterior to the early neural plate: implications for stepwise determination of the lens. 1994, Pubmed
Liem, Dorsal differentiation of neural plate cells induced by BMP-mediated signals from epidermal ectoderm. 1995, Pubmed , Xenbase
Macdonald, Midline signalling is required for Pax gene regulation and patterning of the eyes. 1995, Pubmed
Martí, Distribution of Sonic hedgehog peptides in the developing chick and mouse embryo. 1995, Pubmed
Moerman, Holoprosencephaly and postaxial polydactyly: another observation. 1988, Pubmed
Muenke, Linkage of a human brain malformation, familial holoprosencephaly, to chromosome 7 and evidence for genetic heterogeneity. 1994, Pubmed
Pflugfelder, A homology domain shared between Drosophila optomotor-blind and mouse Brachyury is involved in DNA binding. 1992, Pubmed
Pflugfelder, The lethal(1)optomotor-blind gene of Drosophila melanogaster is a major organizer of optic lobe development: isolation and characterization of the gene. 1992, Pubmed
Placzek, Induction of floor plate differentiation by contact-dependent, homeogenetic signals. 1993, Pubmed
Püschel, Sequence and expression pattern of pax-6 are highly conserved between zebrafish and mice. 1992, Pubmed
Quiring, Homology of the eyeless gene of Drosophila to the Small eye gene in mice and Aniridia in humans. 1994, Pubmed
Rao, Conversion of a mesodermalizing molecule, the Xenopus Brachyury gene, into a neuralizing factor. 1994, Pubmed , Xenbase
Riddle, Sonic hedgehog mediates the polarizing activity of the ZPA. 1993, Pubmed
Roelink, Floor plate and motor neuron induction by vhh-1, a vertebrate homolog of hedgehog expressed by the notochord. 1994, Pubmed , Xenbase
Roelink, Floor plate and motor neuron induction by different concentrations of the amino-terminal cleavage product of sonic hedgehog autoproteolysis. 1995, Pubmed
Saldivar, Rhombomere of origin determines autonomous versus environmentally regulated expression of Hoxa-3 in the avian embryo. 1996, Pubmed
Schulte-Merker, The protein product of the zebrafish homologue of the mouse T gene is expressed in nuclei of the germ ring and the notochord of the early embryo. 1992, Pubmed
Schulte-Merker, no tail (ntl) is the zebrafish homologue of the mouse T (Brachyury) gene. 1994, Pubmed
Shih, Distribution of tissue progenitors within the shield region of the zebrafish gastrula. 1995, Pubmed
Shimamura, Longitudinal organization of the anterior neural plate and neural tube. 1995, Pubmed
Shiota, Holoprosencephaly, ventricular septal defect, and postaxial polydactyly in a human embryo. 1988, Pubmed
Smith, Expression of a Xenopus homolog of Brachyury (T) is an immediate-early response to mesoderm induction. 1991, Pubmed , Xenbase
Sperber, Holoprosencephalic synophthalmia (cyclopia) in an 8 week fetus. 1987, Pubmed
Stockard, THE QUESTION OF CYCLOPIA, ONE-EYED MONSTERS. 1908, Pubmed
Sundin, An early marker of axial pattern in the chick embryo and its respecification by retinoic acid. 1992, Pubmed
Ton, Positional cloning and characterization of a paired box- and homeobox-containing gene from the aniridia region. 1991, Pubmed
van Straaten, Effect of the notochord on the differentiation of a floor plate area in the neural tube of the chick embryo. 1988, Pubmed
Walther, Pax-6, a murine paired box gene, is expressed in the developing CNS. 1991, Pubmed
Woo, Order and coherence in the fate map of the zebrafish nervous system. 1995, Pubmed
Yamada, Control of cell pattern in the neural tube: motor neuron induction by diffusible factors from notochord and floor plate. 1993, Pubmed
Yamada, Control of cell pattern in the developing nervous system: polarizing activity of the floor plate and notochord. 1991, Pubmed
Yasuo, An Ascidian Homolog of the Mouse Brachyury (T) Gene is Expressed Exclusively in Notochord Cells at the Fate Restricted Stage: (Ascidians/T (Brachyury) gene/sequence conservation/notochord cells/transient expression). 1994, Pubmed
Yost, Regulation of vertebrate left-right asymmetries by extracellular matrix. 1992, Pubmed , Xenbase
Yost, Vertebrate left-right development. 1995, Pubmed
Yost, Inhibition of proteoglycan synthesis eliminates left-right asymmetry in Xenopus laevis cardiac looping. 1990, Pubmed , Xenbase
Yost, Development of the left-right axis in amphibians. 1991, Pubmed , Xenbase
Young, Unknown syndrome: holoprosencephaly, congenital heart defects, and polydactyly. 1987, Pubmed