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First published online 27 August 2003
doi: 10.1242/dev.00712

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Anterior identity is established in chick epiblast by hypoblast and anterior definitive endoderm

¹ MRC Centre for Developmental Neurobiology, Kings College London, New Hunts House, Guy's Hospital, London SE1 1UL, UK
² University of Utah School of Medicine, Department of Neurobiology and Anatomy, and Children's Health Research Center, Room 401 MREB, 20 North 1900 East, Salt Lake City, UT 84132-3401 USA

^* Author for correspondence (e-mail: susan.chapman{at}kcl.ac.uk)

Accepted 8 July 2003

	SUMMARY

TOP SUMMARY Introduction Materials and methods Results Discussion REFERENCES

 
Previous studies of head induction in the chick have failed to demonstrate a clear role for the hypoblast and anterior definitive endoderm (ADE) in patterning the overlying ectoderm, whereas data from both mouse and rabbit suggest patterning roles for anterior visceral endoderm (AVE) and ADE. Based on similarity of gene expression patterns, fate and a dual role in `protecting' the prospective forebrain from caudalising influences of the organiser, the chick hypoblast has been suggested to be the homologue of the mouse anterior visceral endoderm. In support of this, when transplanted to chick embryos, the rabbit AVE induces anterior markers in the chick epiblast. To reevaluate the role of the hypoblast/ADE (lower layer) in patterning the chick ectoderm, we used rostral blastoderm isolates (RBIs) as an assay, that is, rostral regions of blastoderms transected at levels rostral to the node. RBIs are, therefore, free from the influences of Hensen's node and ingressing axial mesoderm - tissues that are able to induce Ganf, the earliest specific marker of anterior neural plate. We demonstrate, using such RBIs (or RBIs dissected to remove the lower layer with or without tissue replacement), that the hypoblast/ADE (lower layer) is required and sufficient for patterning anterior positional identity in the overlying ectoderm, leading to expression of Ganf in neuroectoderm. Our results suggest that patterning of anterior positional identity and specification of neural identity are separable events operating to pattern the rostral end of the early chick embryo. Based on this new evidence we propose a revised model for establishing anteroposterior polarity, neural specification and head patterning in the early chick that is consonant with that occurring in other vertebrates.

Key words: Blastula, Expression, Forebrain, Gastrula, Head, Induction, Organiser, Markers, Patterning, Specification, Trunk/tail, Visceral endoderm

	Introduction

TOP SUMMARY Introduction Materials and methods Results Discussion REFERENCES

 
Early patterning of the vertebrate central nervous system involves a complex and interwoven set of spatiotemporal inductions, tissue movements and patterning mechanisms. Two major tasks are achieved early on: the definition of anterior positional identity and the segregation of neural tissue identity. In mouse (Beddington and Robertson, 1998; Beddington and Robertson, 1999), zebrafish (Houart et al., 1998; Koshida et al., 1998) and Xenopus (Jones et al., 1999), signalling centres have been identified that are distinct from the classical organiser and capable of establishing anterior positional identity separately from neural specification. In chick, therefore, positional and tissue identity may also be separable. Three mechanisms could exist for establishing initial anterior positional and tissue identity in the epiblast/prospective neural plate: first, neural specification leads to neuralised tissue, which by default is anterior in character (Mangold, 1933; Nieuwkoop et al., 1952; Nieuwkoop and Nigtevecht, 1954; Spemann, 1931; Spemann, 1938); second, positional identity is conferred by a separate mechanism upon neuralised tissue, which is initially positionally neutral (Waddington and Needham, 1936); and third, initial anterior positional identity is established in the epiblast independent of and before neural specification occurs. The latter two possibilities require that anterior positional identity be established separately from neural specification.

In chick, the process of neural induction begins before the onset of gastrulation, with competence being conferred by FGF signals emanating from the posterior of the embryo (Muhr et al., 1999; Streit et al., 2000; Wilson et al., 2001; Wilson and Edlund, 2001; Wilson et al., 2000). The cellular interactions leading to the specification of competent tissue as neural, and the timing over which they occur, remain unclear. Although transplants of posterior epiblast can induce transient expression of pre-neural markers such as Sox3 in epiblast, stable expression of Sox2 in specified neuroectoderm requires both central and posterior epiblast cells to come together at mid-streak stages (3+ or 3c/d), forming a functional organiser (Streit et al., 2000). Further support for the timing of neural specification at mid-streak stages comes from explant studies in which competent tissue at stage 3d, but not 3c, cultured in isolation, was able to self differentiate, developing the columnar neuroepithelial morphology of specified neuroectoderm, as well as having stable expression of Sox2 (Darnell et al., 1999).

The relative spatiotemporal positions of early embryonic tissues (Fig. 1) suggest that several tissues could function potentially in anteroposterior patterning. Candidate tissues able to produce `organising' signals include a population of central epiblast (CE) cells (Darnell et al., 1999; Garcia-Martinez et al., 1993; Hatada and Stern, 1994; Healy et al., 2001; Lawson and Schoenwolf, 2001a; Lawson and Schoenwolf, 2001b; Schoenwolf et al., 1989b; Streit et al., 2000), and the underlying lower layer, the hypoblast and ingressing anterior definitive endoderm (ADE). The CE population is a group of epiblast cells rostral to the tip of the primitive streak between stages 2 and 4. They are in a position equivalent to that of the mouse early gastrula organiser (EGO), which has been shown to have a role in head patterning when combined with epiblast and anterior visceral endoderm (AVE) (Tam and Steiner, 1999). As the primitive streak extends forward, the CE population becomes incorporated into the streak (Garcia-Martinez et al., 1993; Garcia-Martinez and Schoenwolf, 1993; Joubin and Stern, 1999; Lawson and Schoenwolf, 2001a; Lawson and Schoenwolf, 2001b; Schoenwolf and Alvarez, 1989; Schoenwolf et al., 1989a; Schoenwolf et al., 1989b; Schoenwolf et al., 1992; Smith and Schoenwolf, 1991). Early fate-mapping studies used quail/chick chimaeras and fluorescent dye injections to determine the fate of cells in the rostral streak (Garcia-Martinez et al., 1993; Garcia-Martinez and Schoenwolf, 1993; Schoenwolf et al., 1992; Selleck and Stern, 1991). Homotopic and isochronic cell grafts from stage 3a/b rostral streak contributed extensively to head mesenchyme and foregut endoderm, whereas a small proportion was also detected in notochord and the median hinge-point cells (i.e. the future floor plate of the neural tube). Stages 3c-4 rostral streak cells contributed mainly to notochord and median hinge-point cells, although a small number were traced to the head mesenchyme and foregut endoderm.

View larger version (50K): [in this window] [in a new window]   Fig. 1. Early chick embryonic stages and formation of the lower layer. At stage X, detached cells originating from epiblast lie in the subgerminal cavity (green circles). Koller's sickle (black crescent) and the posterior marginal zone produce a sheet of lower layer cells extending rostrally, forming the primary hypoblast (green sheet) by stage XIV. Endoblast (dark green) formation from the caudal part of the embryo then begins. Stage 2, a broad, short triangular streak forms in the caudal part of the embryo. The streak begins to elongate at stage 3a/b, but only reaches the centre of the area pellucida at stage 3c (equivalent to HH stage 3+) with definitive endoderm ingressing through the rostral tip of the streak (blue). Stage 3a and 3b are difficult to separate in practice (stage 3a, a short and broad linear streak with no groove yet visible; stage 3b, a longer narrower linear streak with visible primitive groove). The defining feature of stage 3a/b is that the streak has not yet reached the centre of the area pellucida, the widest zone across the left/right axis of the embryo. Stage 3c, the elongated and grooved streak extends to the centre, with stage 3d characterised by the rostral extension of the streak beyond the centre point. Maximum streak extension is at stage 4, with a noticeable change in the morphological character of the ectoderm, with the onset of neural specification at stage 3d. By stage 4, definitive endoderm has almost completely ingressed and both primary hypoblast and endoblast have been displaced towards the rostral and caudal poles of the embryo, respectively. Definitive endoderm spreads by a polonaise movement; after ingressing through the tip of the streak, the cells spread rostrally and then laterally. A noticeable swelling at the rostralmost part of the streak indicates the formation of Hensen's node. Stage 4+ heralds the beginning of ingression of axial mesoderm, recognised by a triangular-shaped ingression rostral to Hensen's node.

Owing to contradictory results and gaps in our understanding regarding the role of CE, the hypoblast and ADE, we have tested these tissues for a role in establishing anterior positional identity. Previous chick transplant studies have failed overall to show a role for lower layer tissues in determining cell fate (Foley et al., 2000). Removing the lower layer has also met with little previous success, mostly because of the embryos' ability to recover and replace ablated tissues, such as the hypoblast at early stages (Vanroelen et al., 1982), or to tissue being removed later than the time in question (Withington et al., 2001). We used the transection assay (Fig. 2) (Darnell et al., 1999; Healy et al., 2001; Schoenwolf et al., 1989b; Yuan et al., 1995a) to address what the role for these lower layer tissues is, and determined that anterior positional identity seems to be separable from neural specification. Isolating rostral epiblast (prospective anterior neural plate) from the influences of Hensen's node and ingressing axial mesoderm was crucial, because ingressing axial mesoderm from stage 4+ has a role in the induction of Ganf, the earliest specific maker of anterior neural plate (Knoetgen et al., 1999). The expression patterns of Sox2, the most definitive early neural specification marker identified to date (Rex et al., 1997; Streit et al., 2000; Streit et al., 1997; Uchikawa et al., 2003), and Ganf were re-examined and used to define anterior and neural identity. We show that anterior positional identity is established and maintained in the epiblast by the hypoblast at stage 3a/b and ADE at stages 3d and 4, apparently separately from neural specification, and we propose a revised model for establishing anteroposterior polarity, neural specification and head patterning, based on this new evidence.

View larger version (67K): [in this window] [in a new window]   Fig. 2. Transections. At all stages shown, transections were either Type B (broken blue line) or Type C (broken green line). Type B transections were made at the rostral extent of the ingressing primitive streak, excluding the extending streak and later ingressing axial mesoderm from the rostral blastoderm isolates (RBIs). Type C transections were made by measuring 125 µm rostral to the tip of the primitive streak (approximately one node diameter) and then transecting through all layers. The centre of the area pellucida is marked by a red line and assists in staging before transecting the embryo. The yellow circle represents the central epiblast (CE) population of cells. This group of cells is known to have organising properties and is rostral to the extending streak at stage 3a/b. At stages 3c, the CE is becoming incorporated into the extending streak. By stage 3d and 4, CE is incorporated into the rostral streak and forms part of Hensen's node. At each of the stages note the relative positions of the CE, the rostral extent of the primitive streak, the centre of the area pellucida, and the level of type B and type C transections.

	Materials and methods

TOP SUMMARY Introduction Materials and methods Results Discussion REFERENCES

In situ hybridisation
In situ hybridisation was performed as described previously (Chapman et al., 2002). Embryos were then cleared in 80% glycerol/PBS, embedded in 20% gelatin, fixed with 4% PFA and sectioned using a Leica vibratome at 40-50 µm. Embryos were imaged with a SPOT, Coolsnap or Zeiss Axiocam digital camera. The following markers were used: Sox2, specified neuroectoderm (R. Lovell-Badge): Wnt8c, ingressing mesodermal cells (J. Dodd): Ganf, earliest marker of anterior neuroectoderm (A. Zaraisky); Fgf8, primitive streak (G. Martin); Chordin, primitive streak, Hensen's node and ingressing axial mesoderm (A. Graham); Crescent, hypoblast and ADE (P. Pfeffer).

Embryo culture and transection
Transection of embryos was performed as described by Darnell et al. (Darnell et al., 1999). In experiment 1, embryos were transected to determine the effect of separating rostral tissues from the primitive streak, prospective node and ingressing mesoderm. Embryos were transected at the rostralmost level of the streak (Type B), or 125 µm rostral to the streak (Type C), at stages 3a-4+ and cultured on an agar/albumen substrate with no added culture media for 24 hours (Fig. 2). The blastoderm isolates were then processed for Ganf and Sox2 transcripts (Table 1). In experiment 2, the lower layer of rostral isolates was removed to determine whether this layer has a role in patterning the rostral epiblast. Isolates were cultured in collagen: 3.3 mg/ml rat tail collagen (Roche) was prepared in 0.2% acetic acid. 480 µl collagen, 36 µl DEPC-H₂O, 60 µl 10x DMEM and 20 µl 0.75% bicarbonate solution were added together on ice. Rostral and caudal isolates of each transected embryo were embedded, and after 30 minutes at 37°C in a 5% CO₂ incubator, carbonated Neurobasal medium supplemented with Glutamax was added. Embryos were transected (Type B) in saline (123 mM), followed by removal of the lower layer using tungsten needles (0.125 mm tungsten wire, WPI). No enzymatic treatments were used. RBIs with an intact lower layer served as controls (Table 2). To test for mesoderm in the RBIs, in experiment 3, transected embryos were fixed immediately and then processed for Wnt8c expression. Experiments 4 and 5 were designed to test sufficiency of the lower layer to induce Ganf: either rostral (experiment 4) or caudal (experiment 5) lower layer was recombined with rostral epiblast in collagen culture for 24 hours and then processed for Ganf transcripts.

	Results

TOP SUMMARY Introduction Materials and methods Results Discussion REFERENCES

The lower (ventral) layer is formed by polyingression of cells from the overlying epiblast at stage X/XI, forming islands of cells in the subgerminal cavity (Fig. 1) (Harrison et al., 1991; Lawson and Schoenwolf, 2001a). Together with cells moving rostrally from Koller's sickle and the posterior marginal zone (PMZ), a complete lower layer is formed, called the primary hypoblast (Callebaut et al., 1999; Stern and Canning, 1990; Vakaet, 1970). The endoblast (secondary hypoblast) forms at stage XIII/XIV, with cells moving rostrally from Koller's sickle and the PMZ. Primary and secondary hypoblast form a continuous sheet of cells under the epiblast by stage XIV/2 - the primitive endoderm. The primitive streak forms at stage 2, as a triangularly shaped structure that elongates rostrally. Definitive endoderm begins ingression through the rostral end of the primitive streak from stage 3a to stage 4/4+, by which time the lower layer has fully displaced the hypoblast sheet rostral to the embryo, forming the germ cell crescent (Lawson and Schoenwolf, 2003). At stages 3a/b the rostral streak gives rise to the ADE, including the midline prechordal plate endoderm (PCPE) that lies beneath the forebrain. During subsequent development, the prechordal plate endoderm buds off proliferative mesoderm and together with ingressing mesoderm, forms a middle layer, the prechordal plate mesoderm, which contributes to the head mesenchyme (Seifert et al., 1993). Axial mesoderm ingresses through the rostral streak from stage 4+ (the head process), and consists of a mixed cell population of prechordal plate mesoderm and rostral notochord, which intercalates between the neuroectoderm and ADE (Foley et al., 1997; Vesque et al., 2000). The molecular basis for the spatial separation of these two populations is unclear, although SEM studies of the morphological movements have been described (England, 1984; England and Wakely, 1977; England et al., 1978; Wakely and England, 1979). The laying down of more caudal notochord occurs as Hensen's node and the definitive streak regress caudally. Intercalation of the fan-shaped prechordal plate mesoderm results in the prechordal plate mesoderm coming to partially overlie the PCPE.

Sox2 is a pan-neural marker expressed from the onset of neural specification
Sox2 is the earliest pan-neural marker stably expressed in the specified neuroectoderm (Rex et al., 1997; Streit et al., 2000; Streit et al., 1997). Embryos were tested for expression of Sox2 from stage XI/XII (not shown), with expression first detected at stage 3d, as expected (Fig. 3A). Expression began just rostral and lateral to Hensen's node and later expanded rostrally and laterally toward the outer boundary of the neural plate, away from the streak (stages 4/4+) (Fig. 3B). This pattern suggests that neural specification occurs in a spatiotemporal manner across the prospective neuroectoderm. The caudal boundary of expression remained constant, suggesting that the first cells to express Sox2 are not the anteriormost neuroectoderm, but rather are the more caudal neural plate. By stage 5, Sox2 was expressed throughout the neural plate, which is still flat prior to formation of the head fold and neural tube (Fig. 3C). Concomitant with node regression, the caudal boundary of Sox2 expression extended caudally through convergent extension (Fig. 3D). The ventral neural plate was lighter in colour as the neural tube formed, while the neural plate narrowed as the neural folds rose up and moved medially, with varying levels of expression within the rostrocaudal length of the neural plate, with stronger expression of Sox2 in the rostralmost neural plate. In summary, Sox2 is detected from stage 3d onwards and is the earliest available stable marker of specified neuroectoderm.

View larger version (66K): [in this window] [in a new window]   Fig. 3. Sox2 expression enlarges progressively with neural specification. A panel of whole-mount embryos probed for Sox2 transcripts by in situ hybridisation. Anterior is towards the top. (A) At stage 3d, neural specification of the prospective neural ectoderm occurs. The first cells with detectable expression of Sox2 lie in close proximity rostral and lateral to the definitive streak. (B) As neuralisation proceeds and Hensen's node forms, expression of Sox2 expands outward from the streak (stages 4/4+). The caudal boundary of expression remains constant at this stage. (C) By stage 5, the neural plate has pan-neural expression of Sox2 over the whole of the still flat neuroectoderm. (D) At stage 5+, the node regresses, drawing the posterior boundary of expression caudally. The ventral neural plate looks lighter in colour as the neural tube begins to form, becoming narrower as the neural folds move medially. Some variation in transcript levels occurs with stronger expression of Sox2 in the anteriormost neural plate.

View larger version (106K): [in this window] [in a new window]   Fig. 4. Whole-mount in situ hybridisation of Ganf. Embryos stained for Ganf (blue) and Chordin (red). (A,C,E) Whole-mount, dorsal view with anterior towards the top of the page. (B,D,F) Sagittal sections (40-50 µm) with anterior towards the left. (A,B) At stage 4 (18 hours) Ganf is expressed for the first time. Arrows indicate the gap between neuroectodermal Ganf expression and Chordin in Hensen's node and ingressing axial mesoderm. (C,D) At stage 5, axial mesoderm underlies Ganf expressing tissue and there is some suggestion that mesodermal cells may also express Ganf at this point. (E,F) The headfold begins to form at stage 6 (24 hours) and the Ganf-positive zone narrows as convergent extension takes place in advance of neural tube formation.

View larger version (72K): [in this window] [in a new window]   Fig. 5. Ganf is colocalised within the Sox2 expression domain. Anterior is towards the top. Sox2 alone results in a pink colour, and where Ganf and Sox2 overlap a brown colour results. In all the embryos tested, Ganf is always colocalised within the Sox2 expression domain. (A) Stage 4/4+: a broken line marks the rostral extent of Sox2 expression. Ganf is strongest directly rostral to Hensen's node. (B) Sox2 expression extends rostrally as neural specification occurs throughout the neural plate at stages 4+/5. The Ganf expression domain has also enlarged, but remains within the Sox2 expression region. (C) By stage 5+, the streak has begun to regress (note gap between Ganf expression and Hensen's node) and Sox2 expression extends more caudally (pink).

View larger version (70K): [in this window] [in a new window]   Fig. 6. Sox2 expression in transected embryos. (A-C) RBIs to the top and CBI below, processed after 24 hours in agar/albumen culture. (A,C) Stage 3a/b and 3d RBIs have expression of Sox2 in most cases (see Table 1), whereas (B) stage 3c RBIs do not. All caudal isolates have expression of the pan-neural marker.

View larger version (90K): [in this window] [in a new window]   Fig. 7. Ganf expression in intact and lower layer deficient transected embryos. RBIs of type B transections. Isolates are cultured in collagen gel to maintain tissue integrity following lower layer excision. All CBIs were positive for Ganf expression (not shown). (A,B) Stage 3a/b. Intact RBI has Ganf expression (A), whereas RBI with lower layer removed (B) is negative for transcripts. CE is present in RBIs and is required for neural specification to occur, whereas the hypoblast is required for Ganf expression. (C,D) Stage 3c. Both control (C) and experimental (D) RBIs are negative for transcripts. (E,F) Stage 3d. As for stage 3a/b, except that the RBI does not include CE cells. Ganf expression occurs because the rostral tissue is already neuralised. Loss of the lower layer indicates the requirement for hypoblast and ADE.

At stage 3c, the number of RBIs with Ganf transcripts is similar to that of intact embryos, 5/19, compared with 4/15 isolates lacking the lower layer. The lack of neural specification in these RBIs is due to the exclusion of CE cells and results in low numbers of RBIs expressing Ganf. Lower layer removal, now composed of hypoblast and ADE, has no effect on the numbers of RBIs expressing Ganf, indicating that the inducing activity of the lower layer may be no longer required. Interestingly, the ADE expresses only Crescent, Cerberus, Hex and Otx2 at stage 3c, whereas at stage 3d, Lim1 and Hnf3ß are also induced (Chapman et al., 2002). This may indicate that at stage 3c the ADE cannot perform a maintenance role, but by stage 3d has developed sufficiently to do so. At stage 3d, removal of hypoblast and ADE again causes a reduction in the numbers of embryos expressing Ganf, 3/15 (20%) compared with 7/12 intact RBIs. This is indicative of a maintenance function being lost. At stage 4 this effect is even more pronounced with 0/8 isolates (7/12 intact RBIs) positive for Ganf transcripts, suggesting that although the lower layer is responsible for initial induction and maintenance of anterior identity in epiblast, factors from other tissues may be required to maintain and even stabilise the expression. Tissue candidates for this role include Hensen's node and ingressing axial mesoderm, which have been identified previously as important in Ganf expression (Knoetgen et al., 1999). In summary, removal of the lower layer in RBIs demonstrates that vertical signalling by the lower layer is required for the expression of the anterior neuroectoderm marker Ganf, apparently separately from and before neural specification, and that following neural specification the lower layer has a maintenance role, without the involvement of mesoderm.

Mesodermal cells are not detected in RBIs
Patterning of the rostral ectoderm in intact isolates could be due to the presence of mesodermal cells that are inadvertently included in the RBIs when transecting (experiment 3). When the lower layer is removed in these isolates, any mesodermal cells present could conceivably also be stripped away, resulting in loss of Ganf expression. To test this we, transected embryos from stages 3a/b through to stage 4 and tested for Wnt8c expression, which marks ingressing mesoderm (Fig. 8). Isolates were fixed immediately after transection, and in no case were Wnt8c transcripts detected by in situ hybridisation in the RBIs (n=29; stage 3a/b, n=12; stage 3c, n=7; stage 3d, n=7; stage 4, n=3). The caudal blastoderm isolates from these transections acted as controls for the presence of Wnt8c and were positive for mesodermal cells in all cases.

View larger version (141K): [in this window] [in a new window]   Fig. 8. Mesoderm is excluded from RBIs. (A-D) Transected isolates at the level of the node at stage 3a/b-4 tested for the presence of mesoderm in RBIs (top) and CBIs (below) immediately after transecting (asterisks indicate the nodes). In all cases, the CBIs were positive for Wnt8c transcripts, as expected, whereas the RBIs were negative.

View larger version (92K): [in this window] [in a new window]   Fig. 9. The lower layer is sufficient for Ganf induction in rostral epiblast. RBIs (top) and CBIs (below) cultured in collagen gel. (A) After transection, rostral ectoderm recombined with rostral hypoblast is sufficient to induce Ganf expression (top arrow). The CBI is also positive for Ganf expression (bottom arrow). (B) Caudal endoderm taken from a position lateral to the streak is unable to induce Ganf expression (0/6) when recombined with rostral ectoderm. CBI is positive for Ganf expression (arrows).

	Discussion

TOP SUMMARY Introduction Materials and methods Results Discussion REFERENCES

After transection at stage 3a/b, the epiblast still neuralises, as indicated by the expression of the definitive pan-neural marker Sox2 (Rex et al., 1997). Neural specification in RBIs depends on the presence of a population of central epiblast (CE) cells with `organising' ability, acting either as an organiser or inducer of an organiser (Darnell et al., 1999). After removal of Hensen's node in whole embryos and tissue isolates, the organiser reconstitutes (Joubin and Stern, 2001; Psychoyos and Stern, 1996; Yuan et al., 1995a; Yuan et al., 1995b; Yuan and Schoenwolf, 1998; Yuan and Schoenwolf, 1999). We have not determined whether the same mechanism operates in the rostral blastoderm isolates, although markers of notochord (Not1), node (Shh) and primitive streak (Brachyury/T) were detected in RBIs, suggesting that the organiser is reconstituted (Darnell et al., 1999). Therefore, tissue identified as able to specify neural identity in RBIs was present at stage 3a/b and reduced at stage 3c as the primitive streak extended rostrally, incorporating the CE cells (Darnell et al., 1999; Lawson and Schoenwolf, 2001a; Lawson and Schoenwolf, 2001b), although long-range neural specification signalling, prior to transection, cannot be ruled out entirely. When lower layer, composed only of hypoblast, was included in the RBI, Ganf was expressed at the same frequency as Sox2. By contrast, removing lower layer from these RBIs resulted in the loss of Ganf expression, but did not affect neural specification. Ganf was not transiently induced, suggesting that lower layer signals are required to establish positional identity in the overlying epiblast before neural specification at stage 3d.

Hypoblast seems to be required for only a brief period, because in stage 3c transections, removal of the hypoblast does not abolish Ganf expression. Only a small proportion of RBIs undergo neural specification at this stage, as CE cells are excluded from transected RBIs. By contrast, concomitant with neural specification at stage 3d, transection does not affect the neural character of RBIs, whereas removal of lower layer still leads to a reduction in the percentage of RBIs with Ganf expression, similar to that for stage 3a/b. Hypoblast has been displaced rostrally by the ADE (including midline prechordal plate endoderm) now underling the region where Ganf is induced. The ADE may perform a maintenance role from stage 3d when expression of Lim1 and Hnf3ß is induced, in addition to Crescent, Cerberus, Hex and Otx2 (Chapman et al., 2002). Further work will be needed to determine whether the ADE is directly involved in the induction of Ganf, or whether ADE maintains anterior character specified earlier by the inductive interaction with the hypoblast.

Reassessing current models of early chick development
There is an ongoing debate as to which lower layer tissue in the chick is equivalent to the mammalian AVE, and whether the hypoblast and ADE have any patterning role. In mouse, adjoining the rostral boundary of the primitive streak the early gastrula organiser (EGO), together with epiblast and AVE, is required for head formation (Martinez-Barbera and Beddington, 2001). Chick CE cells are in a position equivalent to the EGO and have `head organiser' properties (i.e. the ability to induce neural identity) (Darnell et al., 1999; Garcia-Martinez et al., 1993; Healy et al., 2001; Schoenwolf et al., 1989b). Determining whether CE cells can be considered a true head organiser still requires that roles in neuralising naïve epiblast and re-patterning more caudal areas of the neural plate be demonstrated. At stage 2 and 3a/b, the CE population is rostral to the extending streak, but by stage 3c it becomes incorporated into the rostrally extending streak forming Hensen's node (Schoenwolf et al., 1989b). The node acts like a head organiser, establishing and refining neural identity, and maintaining and embellishing patterning in overlying neuroectoderm. The properties of axial mesoderm as it ingresses through Hensen's node at stage 4+ are reported to be the result of its origin in the node and vertical signals from the definitive endoderm as it intercalates between the upper and lower germ layers (Vesque et al., 2000). An anteriorising signalling centre in the lower layer could act as the source of signals that operate to further pattern the extending axial mesoderm, indicating a relay mechanism operates, where the anterior endoderm patterns, directly or indirectly, the prechordal plate mesoderm, which in turn patterns the overlying neuroectoderm (Dale et al., 1997; Foley et al., 1997; Pera and Kessel, 1997). Our results suggest that hypoblast and ADE also have an earlier role in directly patterning the overlying epiblast.

With ingression of axial mesoderm through Hensen's node, head organiser ability is lost, perhaps allowing remaining cells to perform the role of trunk/tail organiser, refining the patterning of more caudal parts of the neural plate. An important related issue is whether neural identity is a neutral fate, with lower layer providing positional identity. In our experiments, removal of the lower layer does not affect neural specification, because RBIs still express the definitive pan-neural marker Sox2. The significance of the Sox2 expression pattern, which expands progressively from medial tissue adjacent to Hensen's node and then laterally across the neuroectoderm, suggests that neural specification does not occur first in the most anteriorly positioned cells of the prospective neural plate. However, Ganf expression is lost when the lower layer is removed. Therefore, these data together support a model in which loss of anterior identity does not affect the neural character of tissue, suggesting that neural identity itself is neutral with respect to position.

The lower layer signals vertically to the overlying epiblast
Transplanted chick lower layer was unable to induce anterior neuroectodermal (ANE) markers in epiblast, whereas rabbit AVE and chick axial mesoderm induced Ganf (Knoetgen et al., 1999). A heterochronic shift in patterning of the ANE was proposed, with chick prechordal mesoderm taking over the role played by mouse AVE. Why did the transplanted chick tissue not induce expression of Ganf? The signals needed to induce positional identity were either no longer present (transplanted hypoblast was older than stage 3a/b), i.e. necessary signals could have been reduced by enzymatic treatments used to facilitate isolation of the lower layer, or the responding tissue was not neuralised and, therefore, not competent to express Ganf. Our data demonstrate that intact hypoblast at stage 3a/b is required for the neuroectoderm to express Ganf. Extirpation of hypoblast at stage 3 did not lead to loss of Ganf expression at later stages; however, stage 3 is highly dynamic and prospective ANE was not separated from the influence of the node or ingressing axial mesoderm, both of which are sufficient to induce the expression of Ganf (Knoetgen et al., 1999). Transections demonstrate that in RBIs the lack of a node does not affect induction of Ganf, whilst axial mesoderm begins ingressing only after Ganf expression has begun and, therefore, is unlikely to be the initial endogenous inducer. Thus, chick axial mesoderm is probably not the homologue of the mouse AVE and a heterochronic shift is unlikely. The later patterning role of axial mesoderm is important in refining regional neuroectoderm identity (Dale et al., 1997; Foley et al., 1997; Pera and Kessel, 1997), but initial anterior positional identity must be assigned to earlier endodermal tissues.

The modified Nieuwkoop model
An alternative hypothesis, a modified Nieuwkoop model, proposes that AVE and hypoblast at stage XII/XIII are equivalent tissues. However, the lower layer in this model is responsible for cell movements, rather than cell fate, directing cells away from the caudalising influence of Hensen's node (Foley et al., 2000). Transplanted hypoblast induced transient expression of Sox3 and Otx2 in epiblast, indicating signalling capability, but as expression was not maintained the authors suggested that anteriorising the epiblast is not the hypoblast's main role. Our data do not support the interpretation of the proposed early pre-forebrain state, defined by the expression of the pre-neural markers Sox3 and chick ERNI (Streit et al., 2000). ERNI has been shown to be a retrotransposon only present in the Galliform genome, requiring clarification of its biological significance (Acloque et al., 2001), and Sox3 is expressed in a mosaic of epiblast cells from prestreak stages across the entire area pellucida, only becoming restricted to the neuroectoderm after stage 3d (Rex et al., 1997). Rather, in our model successive inductive interactions anteriorises epiblast, with molecular signals establishing and maintaining anterior identity separately from neural specification. An explanation for the failure to maintain the initial Sox3 and Otx2 expression is that hypoblast is unable to either stabilise or maintain this expression, requiring later ADE to provide the necessary signals. More importantly, this early induction suggests that, as in the mouse, the ability to pattern the early embryo is not confined to Hensen's node and its derivatives.

A revised hypothesis for rostral patterning and head induction
We suggest a revised hypothesis, where successive inductive interactions between hypoblast/ADE and epiblast act to promote anterior character (Fig. 10). As hypoblast is displaced rostrally by the ADE, signals from ADE stabilise/maintain this rostral identity in the overlying epiblast/neuroectoderm. Rostrally located hypoblast (stages XII-XIV) is remarkably similar to the mouse AVE, expressing Lim1, Hnf3b, Otx2, Gsc, Cerberus, Hex and Crescent (Chapman et al., 2002). Genes expressed in ADE include Crescent, Cerberus, Hex and Otx2, whereas Lim1 and Hnf3b are detected only after stage 3d (Chapman et al., 2002). Ganf is the earliest marker detected in the rostral epiblast in response to anteriorising signals from the lower layer and neural specification by the head organiser. Head organiser cells leave Hensen's node, as ingressing axial mesoderm, permitting the remaining population to perform the role of trunk/tail organiser. Changing gene expression reflecting this include Otx2, Nodal and Dkk1, which are lost from the streak at stages 5+/6, while Bmp7 is now expressed in rostral streak from which it was previously excluded (Chapman et al., 2002). This novel hypothesis allows for separate signalling pathways to pattern anterior and neural identity, and for the hypoblast to direct cells with a rostral fate away from the caudalising influence of the trunk/tail organiser (Foley et al., 2000). It further takes into account results suggesting that definitive endoderm is required for patterning neural plate, as stage 4+ removal results in loss of the forebrain because of lack of vertical signals to ingressing head process, and also direct maintenance signals to the overlying neuroectoderm (Withington et al., 2001). Loss of these stabilising and maintenance signals results in the loss of forebrain identity. This hypothesis is further supported by the Foxa2 conditional mouse mutant, where loss of Foxa2 results in axial mesoderm losing its identity; anterior neuroectoderm in turn is not stabilised, resulting in forebrain truncation (Hallonet et al., 2002).

View larger version (26K): [in this window] [in a new window]   Fig. 10. Schematic representation of tissue positions in revised early patterning model. Anterior is towards the left, epiblast/neuroectoderm layer in yellow, with the rostralmost level of the primitive streak (hatched yellow) marked by vertical arrows. At stage 3a/b, note that CE is rostral to the streak, but becomes incorporated into the extending streak until it forms part of Hensen's node at stage 4. Definitive endoderm is represented by bar in blue. Only at stage 4+ does axial mesoderm begin ingressing (red). Hypoblast is responsible for establishing anterior identity in overlying epiblast at stage 3a/b (red arrowheads). Only at stage 3d and 4 is the prechordal plate endoderm fully specified and has a maintenance role in the overlying neuroectoderm layer (dark blue arrows). The CE population signals in the plane of the ectoderm and becomes incorporated into the rostral primitive streak, and together with Hensen's node forms the head organiser (horizontal arrows). After ingression of the axial mesoderm at stage 4+, a trunk/tail organiser function for Hensen's node is revealed. am, axial mesoderm; ce, central epiblast; de, definitive endoderm; ds, definitive streak; en, endoblast; ep, epiblast; gc, germ cell crescent; hy, hypoblast; hn, Hensen's node; ne, neuroectoderm; pcpe, prechordal plate endoderm; ps, primitive streak; se, stomodeal ectoderm.

	ACKNOWLEDGMENTS

 
This work was supported by the MRC, a Wellcome Trust Prize Fellowship (S.C.C.), and NIH grants NS18112 and DC04185 (G.C.S.).

	REFERENCES

TOP SUMMARY Introduction Materials and methods Results Discussion REFERENCES

 

Acloque, H., Risson, V., Birot, A., Kunita, R., Pain, B. and Samarut, J. (2001). Identification of a new gene family specifically expressed in chicken embryonic stem cells and early embryo. Mech. Dev. 103,79 -91.[CrossRef][Medline]
Beddington, R. S. P. and Robertson, E. J. (1998). Anterior patterning in mouse. Trends Genet. 14,277 -284.[CrossRef][Medline]
Beddington, R. S. P. and Robertson, E. J. (1999). Axis development and early asymmetry in mammals. Cell 96,195 -209.[CrossRef][Medline]
Callebaut, M., van Neuten, E., Harrisson, F., van Nassauw, L. and Bortier, H. (1999). Endophyll orients and organizes the early head region of the avian embryo. Eur. J. Morphol. 37,37 -52.[CrossRef][Medline]
Chapman, S. C., Schubert, F. R., Schoenwolf, G. C. and Lumsden, A. (2002). Analysis of spatial and temporal gene expression patterns in blastula and gastrula stage chick embryos. Dev. Biol. 245,187 -199.[CrossRef][Medline]
Dale, J. K., Vesque, C., Lints, T. J., Sampath, T. K., Furley, A., Dodd, J. and Placzek, M. (1997). Cooperation of BMP7 and SHH in the induction of forebrain ventral midline cells by prechordal mesoderm. Cell 90,257 -269.[CrossRef][Medline]
Darnell, D. K., Stark, M. R. and Schoenwolf, G. C. (1999). Timing and cell interactions underlying neural induction in the chick embryo. Development 126,2505 -2514.[Abstract]
England, M. A. (1984). Gastrulation in avian embryos. Scanning Electron Microsc. IV,2059 -2065.
England, M. A. and Wakely, J. (1977). Scanning electron microscopy of the development of the mesoderm layer in chick embryos. Anat. Embryol. 150,291 -300.[CrossRef][Medline]
England, M. A., Wakely, J. and Cowper, S. V. (1978). Scanning electron microscopy of the late primitive streak and head process of the chick embryo. Scanning Electron Microsc. II,103 -110.
Eyal-Giladi, H. and Kochav, S. (1976). From cleavage to primitive streak formation: A complementary normal table and a new look at the first stages of the development of the chick. Dev. Biol. 49,321 -337.[CrossRef][Medline]
Foley, A. C., Skromne, I. and Stern, C. D. (2000). Reconciling different models of forebrain induction and patterning: a dual role for the hypoblast. Development 127,3839 -3854.[Abstract]
Foley, A. C., Storey, K. G. and Stern, C. D. (1997). The prechordal region lacks neural inducing ability, but can confer anterior character to more posterior neuroepithelium. Development 124,2983 -2996.[Abstract]
Garcia-Martinez, V. and Schoenwolf, G. C. (1993). Primitive-streak origin of the cardiovascular system in avian embryos. Dev. Biol. 159,706 -719.[CrossRef][Medline]
Garcia-Martinez, V., Alvarez, I. S. and Schoenwolf, G. C. (1993). Locations of the ectodermal and nonectodermal subdivisions of the epiblast at stages 3 and 4 of avian gastrulation and neurulation. J. Exp. Zool. 267,431 -446.[CrossRef][Medline]
Hallonet, M., Kaestner, K. H., Martin-Parras, L., Sasaki, H., Betz, U. A. and Ang, S. L. (2002). Maintenance of the specification of the anterior definitive endoderm and forebrain depends on the axial mesendoderm: a study using HNF3beta/Foxa2 conditional mutants. Dev. Biol. 243,20 -33.[CrossRef][Medline]
Hamburger, V. and Hamilton, H. L. (1951). A series of normal stages in the development of the chick embryo. J. Morphol. 88,49 -92.[CrossRef]
Harrison, F., Callebaut, M. and Vakaet, L. (1991). Features of polyingression and primitive streak ingression through the basal lamina in the chicken blastoderm. Anat. Rec. 229,369 -383.[CrossRef][Medline]
Hatada, Y. and Stern, C. D. (1994). A fate map of the epiblast of the early chick embryo. Development 120,2879 -2889.[Abstract]
Healy, K. H., Schoenwolf, G. C. and Darnell, D. K. (2001). Cell interactions underlying notochord induction and formation in the chick embryo. Dev. Dyn. 222,165 -177.[CrossRef][Medline]
Hermesz, E., Mackem, S. and Mahon, K. A. (1996). Rpx: a novel anterior-restricted homeobox gene progressively activated in the prechordal plate, anterior neural plate and Rathke's pouch of the mouse embryo. Development 122, 41-52.[Abstract]
Houart, C., Westerfield, M. and Wilson, S. W. (1998). A small population of anterior cells patterns the forebrain during zebrafish gastrulation. Nature 391,788 -793.[CrossRef][Medline]
Jones, C. M., Broadbent, J., Thomas, P. Q., Smith, J. C. and Beddington, R. S. P. (1999). An anterior signalling centre in Xenopus revealed by the homeobox gene XHex. Curr. Biol. 9,946 -954.[CrossRef][Medline]
Joubin, K. and Stern, C. D. (1999). Molecular interactions continuously define the organizer during the cell movements of gastrulation. Cell 98,559 -571.[CrossRef][Medline]
Joubin, K. and Stern, C. D. (2001). Formation and maintenance of the organizer among the vertebrates. Int. J. Dev. Biol. 45,165 -175.[Medline]
Kazanskaya, O. V., Severtzova, E. A., Barth, K. A., Ermakova, G. V., Lukyanov, S. A., Benyumov, A. O., Pannese, M., Boncinelli, E., Wilson, S. W. and Zaraisky, A. G. (1997). Anf: a novel class of vertebrate homeobox genes expressed at the anterior end of the main embryonic axis. Gene 200,25 -34.[CrossRef][Medline]
Knoetgen, H., Viebahn, C. and Kessel, M. (1999). Head induction in the chick by primitive endoderm of mammalian, but not avian origin. Development 126,815 -825.[Abstract]
Koshida, S., Shinya, M., Mizuno, T., Kuroiwa, A. and Takeda, H. (1998). Initial anteroposterior pattern of the zebrafish central nervous system is determined by differential competence of the epiblast. Development 125,1957 -1966.[Abstract]
Lawson, A. and Schoenwolf, G. C. (2001a). New insights into critical events of avian gastrulation. Anat. Rec. 262,238 -252.[CrossRef][Medline]
Lawson, A. and Schoenwolf, G. C. (2001b). Cell populations and morphogenetic movements underlying formation of the avian primitive streak and organizer. Genesis 29,188 -195.[CrossRef][Medline]
Lawson, A. and Schoenwolf, G. C. (2003). Epiblast and primitive-streak orgins of the endoderm in the gastrulating chick embryo. Development 130,3491 -3501.[Abstract/Free Full Text]
Mangold, O. (1933). Uber die Induktionsfahighkeit der verschiedenen Bezirke der Neurula von Urodelen. Naturwiss 21,761 -766.[CrossRef]
Martinez-Barbera, J. P. and Beddington, R. S. (2001). Getting your head around Hex and Hesx1: forebrain formation in mouse. Int. J. Dev. Biol. 45,327 -336.[Medline]
Muhr, J., Graziano, E., Wilson, S., Jessell, T. M. and Edlund, T. (1999). Convergent inductive signals specify midbrain, hindbrain, and spinal cord identity in gastrula stage chick embryos. Neuron 23,689 -702.[CrossRef][Medline]
Nieuwkoop, P. D., Botterenbrood, E. C., Kremer, A., Bloesma, F. F. S. N., Hoessels, E. L. M. J., Meyer, G. and Verheyen, F. J. (1952). Activation and organization of the Central Nervous System in Amphibians. J. Exp. Zool. 120, 1-108.[CrossRef]
Nieuwkoop, P. D. and Nigtevecht, G. V. (1954). Neural activation and transformation in explants of competent ectoderm under the influence of fragments of anterior notochord in urodeles. J. Embryol. Exp. Morphol. 2,175 -193.
Pera, E. M. and Kessel, M. (1997). Patterning of the chick forebrain anlage by the prechordal plate. Development 124,4153 -4162.[Abstract]
Psychoyos, D. and Stern, C. D. (1996). Fates and migratory routs of primitive streak cells in the chick embryo. Development 122,1523 -1534.[Abstract]
Rex, M., Orme, A., Uwanogho, D., Tointon, K., Wigmore, P. M., Sharpe, P. T. and Scotting, P. J. (1997). Dynamic expression of chicken Sox2 and Sox3 genes in ectoderm induced to form neural tissue. Dev. Dyn. 209,323 -332.[CrossRef][Medline]
Schoenwolf, G. C. and Alvarez, I. S. (1989). Roles of neuroepithelial cell rearrangement and division in shaping of the avian neural plate. Development 106,427 -439.[Abstract]
Schoenwolf, G. C., Bortier, H. and Vakaet, L. (1989a). Fate mapping the avian neural plate with quail/chick chimeras: origin of prospective median wedge cells. J. Exp. Zool. 249,271 -278.[CrossRef][Medline]
Schoenwolf, G. C., Everaert, S., Bortier, H. and Vakaet, L. (1989b). Neural plate- and neural tube-forming potential of isolated epiblast areas in avian embryos. Anat. Embryol. 179,541 -549.[CrossRef][Medline]
Schoenwolf, G. C., Garcia-Martinez, V. and Dias, M. S. (1992). Mesoderm movement and fate during avian gastrulation and neurulation. Dev. Dyn. 193,235 -248.[Medline]
Seifert, R., Jacob, M. and Jacob, H. J. (1993). The avian prechordal head region: a morphological study. J. Anat. 183,75 -89.
Selleck, M. A. J. and Stern, C. D. (1991). Fate mapping and cell lineage analysis of Hensen's node in the chick embryo. Development 112,615 -626.[Abstract]
Smith, J. L. and Schoenwolf, G. C. (1991). Further evidence of extrinsic forces in bending of the neural plate. J. Comp. Neurol. 307,225 -236.[CrossRef][Medline]
Spemann, H. (1931). Uber den abteil vom implantat und wirtskeime an der orientierung un beschaffenheit der induzierten embryonalanlage. Roux's Arch. Entw.Mech. Org. 123,389 -517.[CrossRef]
Spemann, H. (1938). Embryonic Development and Induction. New Haven: Yale University Press.
Stern, C. D. and Canning, D. R. (1990). Origin of cells giving rise to mesoderm and endoderm in chick embryo. Nature 343,273 -275.[CrossRef][Medline]
Streit, A., Berliner, A. J., Papanayotou, C., Sirulnik, A. and Stern, C. D. (2000). Initiation of neural induction by FGF signalling before gastrulation. Nature 406, 74-78.[CrossRef][Medline]
Streit, A., Sockanathan, S., Perez, L., Rex, M., Scotting, P. J., Sharpe, P. T., Lovell-Badge, R. and Stern, C. D. (1997). Preventing the loss of competence for neural induction: HGF/SF, L5 and Sox-2. Development 124,1191 -1202.[Abstract]
Tam, P. P. and Steiner, K. A. (1999). Anterior patterning by synergistic activity of the early gastrula organizer and the anterior germ layer tissues of the mouse embryo. Development 126,5171 -5179.[Abstract]
Uchikawa, M., Ishida, Y., Takemoto, T., Kamachi, Y. and Kondoh, H. (2003). Functional analysis of chicken Sox2 Enhancers highlights and array of diverse regulatory elements that are conserved in mammals. Dev. Cell 4, 509-519.[CrossRef][Medline]
Vakaet, L. (1970). Cinephotomicrographic investigations of gastrulation in the chick blastoderm. Arch. Biol. 81,387 -426.
Vanroelen, C., Verplanken, P. and Vakaet, L. C. (1982). The effects of partial hypoblast removal on the cell morphology of the epiblast in the chick blastoderm. J. Embryol. Exp. Morphol. 70,189 -196.[Medline]
Vesque, C., Ellis, S., Lee, A., Szabo, M., Thomas, P., Beddington, R. and Placzek, M. (2000). Development of chick axial mesoderm: specification of prechordal mesoderm by anterior endoderm-derived TGFbeta family signalling. Development 127,2795 -2809.[Abstract]
Waddington, C. H. and Needham, J. (1936). Evocation and individuation and competence in amphibian organizer action. Proc Kon Akad Wetensch Amsterdam 39,887 -891.
Wakely, J. and England, M. A. (1979). The chick embryo late primitive streak and head process studied by scanning electron microscopy. J. Anat. 129,615 -622.[Medline]
Wilson, S., Rydstrom, A., Trimborn, T., Willert, K., Nusse, R., Jessell, T. M. and Edlund, T. (2001). The status of Wnt signalling regulates neural and epidermal fates in the chick embryo. Nature 411,325 -330.[CrossRef][Medline]
Wilson, S. I. and Edlund, T. (2001). Neural induction: toward a unifying mechanism. Nat. Neurosci. Suppl 4,1161 -1168.
Wilson, S. I., Graziano, E., Harland, R., Jessell, T. M. and Edlund, T. (2000). An early requirement for FGF signalling in the acquisition of neural cell fate in the chick embryo. Curr. Biol. 10,421 -429.[CrossRef][Medline]
Withington, S., Beddington, R. and Cooke, J. (2001). Foregut endoderm is required at head process stages for anteriormost neural patterning in chick. Development 128,309 -320.[Abstract]
Yuan, S., Darnell, D. K. and Schoenwolf, G. C. (1995a). Identification of inducing, responding, and suppressing regions in an experimental model of notochord formation in avian embryos. Dev. Biol. 172,567 -584.[CrossRef][Medline]
Yuan, S., Darnell, D. K. and Schoenwolf, G. C. (1995b). Mesodermal patterning during avian gastrulation and neurulation: experimental induction of notochord from non-notochordal precursor cells. Dev. Genet. 17, 38-54.[CrossRef][Medline]
Yuan, S. and Schoenwolf, G. C. (1998). De novo induction of the organizer and formation of the primitive streak in an experimental model of notochord reconstitution in avian embryos. Development 125,201 -213.[Abstract]
Yuan, S. and Schoenwolf, G. C. (1999). Reconstitution of the organizer is both sufficient and required to re-establish a fully patterned body plan in avian embryos. Development 126,2461 -2473.[Abstract]

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