Beck CW and Slack JM (1998), Analysis of the developing Xenopus tail bud rev...

XB-ART-15068

Mech Dev 1998 Mar 01;721-2:41-52.

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Analysis of the developing Xenopus tail bud reveals separate phases of gene expression during determination and outgrowth.

Beck CW , Slack JM .

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We have studied Xenopus tail development from the end of gastrulation to the commencement of outgrowth at the tail bud stage. We show that an early group of genes are expressed at the stage of tail bud determination, at the end of gastrulation, and a late group are expressed at around stage 27 just before tail bud outgrowth. Together, these genes define seven distinct regions of the tail bud as outgrowth commences. We have previously shown that formation of a tail bud depends on the interaction of three tissue regions, called N, M and C, at stage 13. Here we show that expression of the late group of genes is dependent on this NMC interaction. We describe molecular correlates of two of these regions, M and C, which were formerly unobservable and whose existence was inferred from embryological experiments.

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Species referenced: Xenopus
Genes referenced: bmp4 cdx4 dll1 dll4 evx1 jag1 lfng lhx1 not notch1 shh tbx2 tbxt wnt3a

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	Fig. 1. Normal development of the Xenopus tail bud. Schematic representation of tail bud development shown in sagittal section. (A) Tail bud initiation at stage 13 (end of gastrulation). Interaction between the caudal notochord and overlying posterior neural plate generates a tail bud. The chordoneural hinge at this stage corresponds to the late dorsal blastopore lip. (B) By stage 22 (early tail bud) the posterior wall has been formed by the closure of the lateral blastopore lips over the blastopore. This produces a lumen, the neuroenteric canal, with openings to the posterior (proctadaeum) and anterior. The anterior opening of the neuroenteric canal is continuous with the neural tube lumen. (C) Enlarged schematic showing the posterior region of (B). The relative positions of the N, M and C regions which together initiate tail bud formation are shown. Note that the relative positioning of the NM boundary in the posterior wall is an approximation. Region C corresponds to the caudal notochord, M represents the most posterior neural plate which is fated to form mesoderm and N is the neural plate anterior to M. Tail extension occurs at the NM junction beginning at around stage 27. All figures are oriented anterior to left and dorsal uppermost.
	Fig. 2. Early gene expression in the Xenopus tail bud. Whole-mount in situ hybridisations to show expression of various early genes in the tail bud just after initiation and during early extension of the tail. Embryos in the left hand column are stage 14. The embryos are viewed from the dorsal side with anterior to the left, tail bud on the right. The centre column shows the expression patterns at stage 30 as the tail begins to extend. Embryos are viewed from the left side with anterior to the left, tail bud to the right and dorsal up. The right hand column shows the tail bud expression at stage 30 in more detail. Embryos are oriented as centre column and are cleared with the exception of (I). (A), X-delta-1; (D), Xlim1; (G), Xnot2; (J), Xbra; (N), Xcad3.
	Fig. 3. Late gene expression in the Xenopus tail bud. Whole-mount in situ hybridisations to show expression of late genes in the tail bud during early extension of the tail. (A), Xwnt3a; (E), lunatic fringe; (I), Xhox3. (A,E,I) show the expression patterns at stage 30 viewed from the left side with anterior to the left, tail bud to the right and dorsal up. (B,F,J) show detail of the tail bud expression at stage 30, in cleared specimens. (C G,K) are sagittal sections through stage 30 embryos. Scale bar, 250 mm. Note the sharp posterior boundaries of lunatic fringe and Xwnt3a expression in the dorsal roof, and restriction of Xhox3 to the distal tip of the posterior wall, (D,H,L) are schematic representations of (C,G,K), respectively. (M) Stage 30 tail bud showing BMP-4 expression in the fin and epidermis. (N) X-serrate-1 expression in the fin. Abbreviations: cnh, chordoneural hinge; end, endoderm; nc, notochord; nt, neural tube; pw, posterior wall.
	Fig. 4. Early genes expressed in the posterior wall of the tail bud. (A,C) Sagittal sections of stage 30 whole mount in situ hybridisations. Scale bar, 250 mm. (A) Expression of Xbra (dark staining) in the posterior wall and chordoneural hinge, with highest levels distally. (C) Expression of X-delta- 1 in the posterior wall, with highest levels distally. X-delta-1 transcripts are absent from the dorsal roof but punctate expression is seen more anteriorly in the neural tube. (B,D) Schematic diagrams of expression based on the sections shown in (A,C), respectively. (E,F) Whole mount in situ hybridisations showing the expression of other genes in the Notch/delta pathway in the tail bud. (E) X-delta-2 expression is primarily in the most posterior somite (*) but is also weakly expressed in the posterior wall. As for Xdelta- 1, expression is lacking from the dorsal roof of the tail bud. (F) XNotch- 1 is expressed widely in the tail bud but is excluded from the same region of the dorsal roof as X-delta-1 and X-delta-2. All figures are oriented with posterior to the right and dorsal upwards. Abbreviations: cnh, chordoneural hinge; end, endoderm; nc, notochord; nt, neural tube; pw, posterior wall.
	Fig. 5. Xcad3 is transiently expressed in the posterior notochord and is a candidate marker for the C region. (A) Whole mount in situ hybridisation of a stage 13 embryo with part of the neural plate removed to show Xcad3 expression (dark staining) in the underlying posterior notochord. View from posterior with dorsal up. (B) Xcad3 in situ hybridisation to a posterior transverse section of an embryo at stage 13 to show presence of transcripts in the neural plate and to a lesser extent in the notochord. Scale bar, 100 mm. (C) Xcad3 expression in the stage 30 tail bud is restricted to the neural tube and posterior wall. Transcripts are absent from the chordoneural hinge and notochord. Scale bar, 250 mm. (D) Schematic diagram to show Xcad3 expression based on (C). Marks E, F and G indicate the relative positions of the transverse sections in (E), respectively. (E) transverse section through tail tip at stage 30 and schematic showing peripheral expression of Xcad3 at a level posterior to the chordoneural hinge. (F) Transverse section and schematic midway through the tail at stage 30 showing restriction of Xcad3 transcripts to the dorsal and lateral periphery. (G) Transverse section through the posterior trunk at the level of the proctodaeum showing Xcad3 in the neural tube and a few cells of the adjacent somite. Expression in the neural tube is excluded from the floor plate and neural crest derived fin mesenchyme and forms a posterior to anterior gradient. Scale bar in (E), 100 mm. Abbreviations: cnh, chordoneural hinge; end, endoderm; me, fin mesenchyme; nc, notochord; nt, neural tube; pw, posterior wall; s, somite.
	Fig. 6. Expression of early genes in the chordoneural hinge. (A) Sagittal section of a stage 32 tail bud whole mount in situ hybridisation to show expression of Xnot2 (dark staining). Transcripts are restricted to the chordoneural hinge and posterior ventral neural tube as previously reported (Gont et al., 1993). (B) schematic diagram of Xnot2 expression based on (A). (C) Sagittal section of stage 30 tail bud to show Xlim1 expression in the chordoneural hinge and posterior notochord. There is also punctate expression of Xlim1 in the neural tube more anteriorly. Xlim1 transcripts are clearly absent from the tip of the chordoneural hinge in the sagittal section, suggesting that expression is excluded from the M region. (D) Schematic diagram of Xlim1 expression in the tail bud based on (C). (E) Sagittal section of stage 30 tail bud to show Xshh expression in the chordoneural hinge, notochord and floorplate of the neural tube and underlying endoderm. (F) Schematic diagram of Xshh expression based on (E). Scale bar in (A,C,E), 250 mm. Abbreviations: cnh, chordoneural hinge; end, endoderm; nc, notochord; nt, neural tube; pw, posterior wall.
	Fig. 7. The expression of late genes depends on correct initiation of the tail bud at stage 13. (A) Schematic diagram of exogastrula to show the position of the tail bud forming region and location of N, M and C on the dorsal side of the waist region. The waist region links the mesoendoderm (ME) and the neuroectoderm (NE) posterior to posterior and the dorsal side corresponds to the chordoneural hinge. (B) whole mount in situ hybridisations to exogastrulae at stage 32. Dorsal views with mesoendoderm uppermost, red boxes mark the hinge region, other blue spotting in the neuroectodermal sac is due to trapping of the probe. (B) Exogastrula showing expression of Xnot2 in the unfolded chordoneural hinge. (C) Expression of Xcad3 in the hinge region. (D) Lack of expression of Xwnt3a in the hinge region. (E) Lack of expression of Xhox3 in the hinge region as previously reported (Ruiz-i-Altaba, 1990).
	Fig. 8. Orientation of late gene expression recapitulates the polarity of the NM junction. (A) Diagram to illustrate the neural plate rotations used to generate an extra tail with reversed orientation of N and M. (A) A region of the neural plate 600 600 mm is peeled from an embryo at stage 14. The region removed is within 5000 mm of the blastopore and therefore contains not only neural plate (N) but a small amount of mesodermal tissue continuous with the neural plate (M). (B) This square of tissue is rotated 180before replacing it onto the same embryo. (C,D) The displaced NM region, which has already interacted with C, is now in reverse orientation with M anterior to N, and a second NM junction has been made over C which generates a new tail at the posterior of the embryo with the correct orientation. Arrows show the direction of extension of the two tail buds. (E) Examples of neural plate rotated embryos stained for lunatic fringe (E), Xwnt3a (F) or Xcad3 (G) transcripts at stage 30. Reverse dorsal ventral polarity staining in the ectopic tail bud is marked with a black arrow.
	Fig. 9. Relative positioning of the 7 tail bud regions at stage 30. Schematic illustration of the stage 30 tail bud in sagittal section with posterior to the right and dorsal uppermost. The expression domains defining the DR and PW regions overlap slightly at the tail tip, shown by cross hatching. Abbreviations: CNH, chordoneural hinge; DR, dorsal roof; MH, tip of chordoneural hinge derived from M region of the former neural plate; PNC, posterior notochord; PW, posterior wall of the neuroenteric canal; T, tip of posterior wall; VNT, ventral neural tube.

	dll1 (delta-like 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 14, dorsal view, anterior left.


	lhx1 (LIM homeobox 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 14, dorsal view, anterior left.

	lim1 gene expression in Xenopus laevis embryo, tail region only, assayed via in situ hybridization, NF stage 29 & 30, lateral view, anterior left, dorsal up.
	not (notochord homeobox) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 14, dorsal view, anterior left.


	t (T, brachyury homolog) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 14, dorsal view, anterior left.


	cdx4 (caudal type homeobox 4 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 14, dorsal view, anterior left.