Xl-fli (ets-family of transcription factors); endothelial

Xl-fli, nucleic acid

The Xl-fli gene, belonging to the ets-family of transcription factors, is expressed in the zygote from stage 14. The very early expression is observed in the ventro-anterior region, in the putative area for heart and blood islands. Then, the gene is expressed in the neural crest cells and their derivatives, along the pronephric duct migration pathway, in the forming heart and blood vessels. The expression is specific for endothelial cells. Whole-mount in situ hybridization reveals the expression of the Xl-fli gene in several lineages of migrating cells in Xenopus embryos. Images and text supplied by Denise Meyer, Patrick Stiegler, Colette Hindelang, Anne-Marie Mager and Pierre Remy.

A. Stage 22/23 ; dorsal view ; the migrating mandibular, hyoid and branchial (anterior plus posterior) arches (respectively Ma., Hy. and Br. a. + p.) can clearly be seen. PND= pronephric anlage, precursor of the pronephros.
B. Stage 22/23 ; lateral view. E : eye.
C.

C. Stage 32/33 ; lateral view : upper embryo : in addition to the mandibular, hyoid and branchial arches, the heart is now labelled (H). The line along the PND migration pathway now extends to the cloaca (note PND pathway, not the duct itself); tiny spikes regularly spaced, evocative of the developing intersomitic arteries, are observed (small red arrows). This staining probably represents the posterior cardinal vein and its intersomitic branches. Around the cloaca, small islets of positive cells are detected (long red arrows). lower embryo : control embryo hybridized with a sense Xl-fli probe.
D. Horizontal section through a whole-mount hybridized embryo (stage 33/34) : The section shows a strong hybridization in the cells derived from the neural crest at the exclusion of the ectoderm, mesoderm and endoderm.
E.

E. Transversal section through a whole-mount hybridized embryo (stage 29/30), at the level of the cardiac primordia : A to D : The four consecutive 10mm-thick sections, in a caudal to rostral direction, show the fusion of the bilateral cardiac primordia in a single tube. Only the endothelial cells are labelled.

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