September 1, 2000;
Xenopus Six1 gene is expressed in neurogenic cranial placodes and maintained in the differentiating lateral lines.
Six genes are homeobox-containing transcription factors, many of which are expressed in head
structures. We isolated a full-length cDNA of a previously unknown Xenopus member of this family. It shares a high sequence homology with mouse and human Six1
, which during development are expressed in mesoderm
. In contrast, XSix1
is prominently expressed in all neurogenic cephalic placodes and lateral
stages. The neurons derived from these placodes do not express XSix1
, but the lateral
line mechanoreceptors maintain expression. XSix1
is weakly expressed in muscle
later in development.
[+] show captions
Fig. 4. Expression of XSix1 in the cranial placodes. (A) Anterior view of stage 17/18 embryo demonstrating XSix1 expression in the primitive placodal region (p) bordering the anterior neural plate (np). Short arrows point to faint staining in the region from which the olfactory placodes originate.(B) Anterior view of stage 21 embryo demonstrating XSix1 expression in olfactory placodes (olf), trigeminal/profundal placode (V) and dorsolateral placodal area (dlp). Note lack of expression in the presumptive lens ectoderm (ple). (C) Horizontal section through stage 21 embryo demonstrating expression in the more anterior trigeminal/profundal placode (V) and the more posterior dorsolateral placodal area (dlp). Arrow points to approximate border between them. nt, neural tube. (D) Horizontal section through stage 21 embryo at level of olfactory placodes (olf). Note lack of staining in ple overlying the optic vesicle (ov). (E) In the stage 32 embryo XSix1 is prominently expressed in the domains corresponding to the olfactory (olf), trigeminal (V), anterodorsal (AD), anteroventral (AV) and facial (VII) placodes. In addition, intense staining is present in the otocyst (oto) and a broad domain associated with both the dorsolateral and epibranchial placodes of the glossopharyngeal and vagus cranial nerves (IX/X), that also includes the posterior lateral line placode. Arrowheads ventral to the somites denote segmented patches of expression that are ectodermal. There also is faint staining in the dorsal and ventral tips of the somites. (F) Coronal section through the olfactory placode (between arrows) of a stage 22 embryo. XSix1 is intensely expressed in both the superficial and deep layers. fb, forebrain. (G) Coronal section through the olfactory placode of a stage 25 embryo. XSix1 is not expressed in the deep germinal layer (arrow). fb, forebrain. (H) Coronal section through the olfactory placode of a stage 39 embryo. XSix1 is confined to the superficial and subjacent cells. Asterisk denotes faintly stained head mesenchymal cells. (I) Transverse section of stage 34 embryo at the most posterior level of the retina (ret and adjacent black pigmented retinal epithelium). The intense XSix1 staining corresponds to the remnant of the trigeminal placode (V), which may be contiguous with the anterodorsal placode (see Fig. 4E). Neither the profundal (Pg) nor trigeminal (Vg) ganglion contains XSix1-expressing cells. (J) Transverse section of stage 34 embryo at level of the otocyst (oto). XSix1 is expressed faintly in the ventromedial wall (arrow) and intensely in the lateral wall of the otocyst. The broad region of intense staining ventral to the otocyst is comprised of several placodes of the IX/X complex. hb, hindbrain. (K) At stage 36/37, faint XSix1 expression is detected in the mesenchyme (*) just deep to the domain of the IX/X placode complex, but differentiating neurons in the vagus ganglion (Xg) do not express XSix1. (L) Stage 28/29 otocyst shows faint staining in the ventromedial wall and intense staining in the lateral wall. Arrow denotes border between otocyst and overlying ectoderm. (M) At stage 38/39, the lateral wall of the otocyst (oto) continues to express high levels of XSix1, as do the epibranchial and lateral line IX/X placodal domains. Arrow denotes border between otocyst and overlying ectoderm. (N) In stage 42 otocyst XSix1 expression is maintained in the lateral wall. The staining in the IX/X placodal domain has resolved into two lateral line primordia (arrows). The IX ganglion (*) does not express the gene.
Fig. 5. Expression of XSix1 in the lateral lines. (A) At stage 37/38 XSix1 expression corresponds to the position of the primordia of the medial trunk lateral line (m) and the ventral trunk lateral line (v). Arrowheads just caudal to the ventral lateral line primordium denote segmental expression in the ectoderm near the ventral border of the somites. (B) Transverse section demonstrating XSix1 expression in an ectodermal thickening corresponding to the medial trunk lateral line primordium. Both superficial and deep layers express the gene. Asterisk denotes faint staining in the dorsal somite. n, nephrotome. (C) At stage 41 the three trunk lateral lines (dorsal, d; middle, m; ventral, v) are distinguished by XSix1 expression. Spots of staining (blue) also are observed near the gills (asterisk). Black spots are pigment cells. (D) A ventral view of embryo in (C) demonstrates XSix1 expression in the anterior branch of the ventral trunk lateral line (v), in the hyomandibular lateral line (hy) and in spots near the gills (asterisk), which may correspond to the hypobranchial placodes. (E) Stage 37 expression of Xkrk1 in the same position as XSix1 (cf. B). Dark splotches in neural tube (nt) are optical artifacts. (F) Stage 39 expression of sox3 in the medial and ventral lateral line primordia posterior to the otocyst, in positions similar to those of XSix1 expressing cells (cf. A). (G) Stage 40 XSix1-expressing cell cluster that histologically appears to be a neuromast. (H) Stage 40 sox3-expressing cell cluster in same position as XSix1-positive cells (cf. G). The superficial cells of the cluster do not express sox3. (I) Stage 40 sox2-expressing cell clusters (arrows) in same position as XSix1-positive cells corresponding to the ventral and medial lateral lines (cf. C). (J) Stage 37/38 XSix1-expressing cell clusters (arrows) in the appropriate positions to represent the supraorbital and infraorbital lateral line primordia. ret, retina; nt, neural tube. (K) Stage 41 XSix1-expressing cell cluster (arrow) in the appropriate position to be a supraorbital neuromast. (L) Stage 41 sox2-expressing cell cluster (arrow) in same position as XSix1-positive cells in the hyomandibular lateral line. (cf. D). (M) Stage 42 XSix1-expressing neuromast of the middle lateral line. (N) Stage 42 sox2-expressing neuromast of the middle lateral line. Arrow points to border between superficial, unstained cells and deep layer of stained cells.