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We have isolated a novel gene from Xenopus, called Mespo, which encodes a protein containing a basic helix-loop-helix (bHLH) motif characteristic of a family of transcriptional activators. Mespo expression begins at the gastrula stage and continues throughout tailbud stages; expression occurs in the presomitic mesoderm and the posteriortailbud. Mespo has high similarity to a subfamily of bHLH transcription factors involved in segmentation of the presomitic paraxial mesoderm.
Fig. 3. Whole-mount in situ hybridization analysis of Mespo expression. (1) A stage 10.5, vegetal view, shows Mespo expression in the ventral and lateral
mesoderm. Note that expression is excluded from the region above the dorsal blastopore lip. (2) Stage 11. (3) Stage 11.5. Expression is seen in the lateral
mesoderm extending away from the blastopore lip. (4) Stage 18, a lateral view with anterior oriented towards the left and posterior towards the right, with
dorsal at the top and ventral at the bottom. Strong expression is evident in the tailbud and weak expression in the lateral mesoderm. (5) Stage 20. (6) Stage 25.
Mespo expression in the tailbud is clearly excluded from the dorsal-most region. (7) Stage 26. Expression is seen in the tail blastema. Expression in the head
region is a non-specific artifact in this and the next two panels (7–9). (8) Stage 29. (9) Stage 35/36. Mespo is expressed at the tip of the tail.
Fig. 4. Section in situ hybridization analysis. (1) A stage 10.5 section, oriented with the animal pole at the top and the vegetal pole at the bottom, shows Mespo expression in the lateral mesoderm cells at the endomesodermal border. Note that the blastopore lip is out of the plane of this section. (2) A stage 13 section, oriented with dorsal at the top and ventral at the bottom, with anterior to the left and posterior to the right. Mespo expression is excluded from the involuted dorsal mesoderm but dorsallateral, ventral-lateral, and ventral expression is seen in the mesodermal cells around the yolk plug. The cavity at the top is the archenteron and the cavity at the left is the blastocoel. (3) A stage 17 transverse section, oriented with dorsal at the top and ventral at the bottom, shows Mespo expression in the presomitic mesoderm; expression is not seen in the notochord or archenteron roof cells. (4) A stage 22 transverse section shows expression in the tail region. The cavity shown is the hindgut. (5) A sense control of a stage 10.5 embryo section.