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We report the expression of a family of Xenopus genes encoding BTB-Kelch repeat proteins related to mouse ENC-1. Xenopus ENC-related-1 (Xencr-1) is expressed throughout the dorsal midline in dorsal endoderm, notochord, and deepneuroectoderm cells during early neurula stages. Later, Xencr-1 expression is downregulated in the notochord in an anterior-to-posterior progression but is maintained in the floorplate and dorsal endoderm. Xencr-3 is expressed strongly in several cranial placodes during the early neurula stages, placing it among the earliest molecular markers of placode precursor populations.
Fig. 2. Early expression of Xencr-1. (A–C) Vegetal view of Xencr-1 during gastrula stages, dorsal side up. (A) st. 11. (B) st. 11.5. (C) st. 12.5. Xencr-1 expression begins at st. 11 in the medial dorsal marginal zone above the blastopore (A) and converges toward the dorsal midline and extends anteroposteriorly during gastrulation (B, C). (D–I) Dorsal view of Xencr-1 during neurulation, anterior side up. (D) st. 13.5. (E) st. 14. (F) st. 15. (G) st. 16. (H) st. 18. (I) st. 20. Xencr-1 expression extends to the prechordal mesoderm by st. 13.5 (E, arrowhead) and by stage 14 begins to be expressed in the anterior and lateral neural folds (F, arrowheads) of the trunk region of the embryo (intermediolateral deep cells of [Schroeder, 1970]. Expression in the pronephric anlage is seen at st. 20 (I).Sagittal section at st. 13.5 (d′) and transverse section at st. 14 (e′) reveal that Xencr-1 is expressed throughout the dorsal midline tissues, the dorsal-most endoderm, notochord, and median deep cells of the neuroectoderm. Red arrows in (e′) indicate the neural ectoderm/notochord boundary. Sagittal section at stage 16 (g′) reveals that expression is being down-regulated in the anterior notochord, while sagittal and transverse sections at st. 18 (h′), (h′) demonstrate that expression is maintained in anteriorhypochord (h′, arrowhead) and floorplate (h′ arrow; median deep cells of [Schroeder, 1970].
Fig. 3. Early expression of Xencr-2 and Xencr-3. (A–C) Dorsal view of Xencr-2 during neurula stages, anterior side up. (A) st. 14. (B) st. 16. (C) st. 20. Xencr-2 is a pan-neural marker with exception to the midline of the neural plate. (D–F) Dorsal view of Xencr-3 during neurulation, anterior side up. (D) st. 14. (E) st. 16. (F) st. 20. Xencr-3 expression is seen in prospective neural crest populations along the neural plate border in intermedio-lateraldeepneuroectoderm cells ([Schroeder, 1970]). (G–L) Anterior view of Xencr-3 during gastrula and neurula stages, dorsal side up. (G) st. 12. (H) st. 14. (I) st. 16. (J) st. 17. (K) st. 18. (L) st. 20. (G) Light expression is detected during mid-gastrulation but (H) is quickly upregulated in the otic and profundal–trigeminal placode primordia (red arrowhead) as well as the epibranchial/lateral line placode primordia (yellow arrowhead) at the beginning of neurulation ([Schlosser and Northcutt, 2000]). (I) Expression in neural crest cells along the anterior neural crest border (orange arrowhead) is seen at st. 16. (J–L) The profundal and trigeminal placodes (bottom red arrowhead) become distinct from the otic placode (top red arrowhead) by the end of neurulation.
Fig. 4. Later expression of Xencr-1, Xencr-2, and Xencr-3. (A–F) Lateral view, anterior to the left. (A) Xencr-1, st. 24 (B) Xencr-1, st. 32. (C) Xencr-2, st. 24. (D) Xencr-2, st. 32. (E) Xencr-3, st. 24. (F) Xencr-3, st. 32. (A–B) Xencr-1 shows expression in the dorsal neural tube, optic vesicle, migrating branchial arch neural crest cells (red arrowhead), pronephric anlage (red arrow), and faint staining in the liver. (C–D) Xencr-2 shows expression in the neural tube, cement gland, optic vesicle, otic vesicle, branchial arches, somites, and faint staining in the lateral line placodes. (E–F) Xencr-3 shows continued expression in the dorsal neural tube, lens placode, otic vesicle, lateral lines, branchial arch and hyoid neural crest streams, somites, and dorsal fin. A transient expression in the lateral plate (black arrow) is seen during early tailbud stages but is downregulated by st. 32.
