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The mammalian c-kit receptor tyrosine kinase gene is required during embryogenesis for the survival and/or proliferation of three migrating stem cell populations: primordial germ cells, haematopoietic stem cells and neural crest-derived melanoblasts. We have cloned a Xenopus gene, XKrk1, whose closest relative is c-kit. Differences in the expression pattern suggest that XKrk1 is not the Xenopus homologue of c-kit; however, it is expressed in a migrating stem cell population, the precursor cells for the mechanosensory lateral line system. XKrk1 is the first reported marker for lateral line stem cells.
Fig. 6. Whole mount in situ hybridisations showing XKrkI expression in migrating lateral line stem cells. Embryos are albino/wild-type crosses (albino egg, wild-type sperm; these lack maternal pigment but have pigmented melanocytes) except for those in (g), (i) and cj), which are full albinos. Timings given refer to development at 23 deg C. Brown cells are melanocytes; blue indicates positive staining. (a) Stage 31/32 embryo (39 h) hybridised to antisense XKrkl probe, showing staining in a region ventro caudal to the eye. (b) Stage 33/34embryo (44.5 h) hybridised to antisense XKrkl probe, showing staining of lines of cells anterior to the otic vesicle (ov). (c) Stage 33/34embryo hybridised to sense XKrkl probe (control). (d) Stage 35/36 embryo (50 h), showing additional expression in patches adjacent to the cement gland (cg) and posterior to the gill arches (arrowheads). (e) Stage 35/36 sense control. (f) Transverse section through stage 35/36 embryo hybridised to antisense XKrkl probe. nt, neural tube; nc, notochord. Arrowheads indicate blue-staining cell clusters just beneath epidermis. Dark brown cells are melanocytes. (g) cleared stage 37/38 albino embryo (53.5 h) showing expression in bilateral ribbons of cells extending caudally from patch seen in (d). (h) Stage 37/38embryo showing similar expression to that in (g). Diffuse staining around the eye is non-specific. (i) Cleared stage 42 albino embryo (80 h) hybridised to antisense XKrkI probe. Dark- blue cells are positive. (i) Cleared stage 42 albino sense control.
Fig. 7. Whole mount in situ hybridisations showing XKrkI expression in cells in the hindgutendoderm (a-d) and dorsal lateralmesoderm (e-k). Embryos are all albino/wild type crosses except for that in (k) which is a full albino. (a) Tailbud region of a cleared stage 26 embryo (29.5 h) hybridised to an antisense XKrkf probe, showing a cluster of positive cells (blue, arrowed) around the hindgut where it curves ventrally. (b) Tailbud region of a cleared stage 29130embryo (35 h) showing similar staining to (a). (c) Tailbud region of a cleared stage 31 embryo (37.5 h) showing similar staining. (d) Transverse section through posterior region of a stage 29/30 embryo (35 h) showing positive cells in the endodermlateral to the hind gut. g, gut cavity. Ventral cavity (unmarked) is that of the cloaca. (e) Stage 31/32 embryo (39 h) hybridised to antisense XKrkl probe. Arrowheads indicate positive staining in cell clusters ventral to the myotomes. (f) Stage31/32 sense control. (g) Stage 33/34 embryo (44.5h) showing more positive clusters. (h) Transverse section through stage 33/34 embryo hybridised to antisense XKrkl probe. Positive cells (arrowed) are seen in lateralmesoderm, dorsal to melanocytes (brown cells). nt, neural tube; nc, notochord; g, gut cavity. (i) Transverse section through stage 35/36 embryo showing positive staining in cells ventral to the pronephric duct (pd). g, gut cavity. (j) Stage 35/36 embryo (50 h) showing positive staining in U- shaped clusters ventral to the myotomes. Brown cells are melanocytes. (k) Stage 35/36 albino embryo showing positive U-shaped clusters as in (j).
