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We have isolated an amphibian homolog of the homeotic gene spalt of Drosophila. Like its Drosophila counterpart the Xenopus Xsal-1 gene encodes a protein that contains three widely separated sets of sequence related double zinc finger motifs of the CC/HH-type as well as a single CC/HH zinc finger. The Xenopus gene encodes a fourth double zinc finger and a single CC/HC zinc finger motif that have no counterpart in the fly protein. Alternative splicing of Xsal-1 transcripts gives rise to RNAs coding for either four, three or two double zinc fingers, respectively. The main expression domains of Xsal-1 in early development are confined to distinct regions along the lateralaxon tracts within the midbrain, hindbrain, and spinal cord. Outside the central nervous system Xsal-1 is expressed in the facio-acoustic ganglion and in the developing limb buds. The pattern of expression suggests that Xsal-1 might be under control of signals emanating from the notochord and/or the floor plate and that it might function in neuronal cell specification.
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8734496 ???displayArticle.link???Mech Dev
Fig. 7. Expression of Xsal-1 in the brain and spinal cord. Sections of stained glycol methacrylate embedded embryos at stage 32 are shown. (A-F) Transversal sections at different levels of the CNS: (A) at the level of the telencephalon (te) (B) level of the anteriormesencephalon (me), (C) level of the posteriormesencephalon, diencephalon (de). The pigment of the eye (eye) is seen in brown. (D) Section through the rhombencephalonanterior of the otic vesicle. Outside the brain staining is detected in the ganglion of the VIIth cranial nerve (Vll.cg). The arrow indicates the lateralaxon tract (at), (nc) notochord. (E) Section through the rhombencephalon at the level of the otic vesicle (ov). (F) Section through the anterior spinal cord. Staining is confined to the region of motor neurons and interneurons. (G-I) Sagittal sections of the head region (G), the trunk region (H) and the tail region (I) of a stained embryo at stage 32. (ba) branchial arches, (hg) lumen of the hindgut, (ne) neuro-enteric canal, (re) rhombencephalon, (sc) spinal cord.
Fig. 5. Expression pattern of Xsal-I during development using whole-mount in situ hybridization and sections of stained embryos. (A) Dorsal/posterior view of an embryo at stage 121/2. Staining is detected in the posterior neural plate and the region of the prospective neural folds. The arrow indicates the closing blastopore. (B) Dorsal view of an embryo at stage 15. Staining is confined to two parallel stripes within the neural groove and two broader stripes that run along the neural folds. At the anterior border of the latter stripes staining extends more laterally. No staining is observed anterior of the prospective midbrain. The short lines numbered 1-3 indicate the plains of vibratome sections shown in B1-B3. (B1) Transversal section of the embryo shown in (B). At the level of the lateral stripes staining is detected in the epithelial as well as in the sensorial layer of the neuroectoderm. (B2) Parasagittal section of an embryo at stage 15. In the anterior part of the Xsal-1 expression domain the epithelial as well as the sensorial layer of the neuroectoderm are stained. Note the sharp anterior border of the expression. (B3). Transverse section of the embryo shown in (B) at the level of the future spinal cord. Staining is confined to the epithelial layer of the neuroectoderm. (C) Dorsal view of an embryo at stage 18. Staining is prominent in two discrete lateral pairs of stripes at the anterior border of expression in the midbrain/hindbrain region. Staining of the neural plate dorsally of the blastopore is more diffuse. (D) Dorso/laterai view of an embryo at stage 27. A cleared embryo is shown. Staining is confined to two parallel stripes in the midbrain (mb), hindbrain (hb), and the spinal cord (sc) Outside the CNS transient staining is observed in the ventral part of the otic vesicle (ov). (E)Lateral view of an embryo at stage 31. A cleared embryo is shown. Prominent staining is seen in the midbrain, hindbraln, spinal cord, the ganglion of the VIIth cranial nerve, and the tail tip. Staining in the tail tip is more diffuse. Weaker staining is detected in the frontal part of the forebraln and in the heart anlage (ha), the pronephros (pn) and the pronephric duct. (F) Dorso/lateral view of an embryo at stage 31. The arrow indicates the position of the VIIth cranial ganglion (VII.cg) that is stained by Xsal-1. Note the transverse stripe within the hindbrain at the level of the otic vesicle and the lack of staining at the midbrain/hindbraln boundary. (G) Horizontal vibratome section of a stained embryo at stage 31. Staining in the midbrain and spinal cord is defined to the lateral parts. The arrow points to the position of the fourth rhombomere (4.rb). The plain of section was chosen to show the extent of the transverse stripe within the hindbrain. The weak expression further rostrai corresponds to the expression within the forebrain as seen in (E,I). (I,H) Isolated brain and spinal cord of an embryo at stage 32. (1) Lateral view, anterior is left, (J) ventral view, the VIIth cranial ganglion of the left side has been removed. (H,K) Isolated hind limb buds of an embryo at stage 52, anterior is left. (H) left and (K) right bud. Xsal-I staining is found
in a belt subterminai of the tip of the bud.
Fig. 6. Spatial relationship of Xsal-1 expression to the expression of En-2 and Krox-20 using whole-mount in situ double staining. (A) Anterior view of an embryo at stage 19 double stained with Xsal-1 (dark blue) and En-2 (red, arrow). En-2 demarcates the future midbrain/hindbrain boundary. (B) Anterior/dorsal view of an embryo at stage 17 double stained with Xsal-1 (darkblue) and Krox-20 (red) Xsal-1 and Krox-20 show an alternating pattern of expression with some overlap of the posterior parts of the Xsal-1 stripes with the anterior parts of the respective Krox-20 stripes. The Krox-20 domains are marked by arrow heads. (C) Lateral view of an embryo at stage 28 double stained with Xsal-1 (darkblue) and En-2 (red, arrow). Note that the Xsal-1 expression is interrupted at the midbrain/hindbrain boundary marked by the En-2 stripe. (D) Lateral view of an embryo at stage 31 double stained with Xsal-1 (darkblue) and Krox-20 (red). The arrowheads point to the Krox-20 expression in rhombomeres three and five.
sall3 ( sal-like 3 ) gene expression in Xenopus laevis embryos, NF stage 27, as assayed by in situ hybridization. Lateral view: anteriorleft, dorsal up.
Image copyrighted by Elsevier
sall3 ( sal-like 3 ) gene expression in Xenopus laevis embryos, NF stage 31, as assayed by in situ hybridization. Dorso-Lateral view: anteriorleft, dorsal up (rotated slightly down along long axis of embryo).
Image copyrighted by Elsevier
