Fig. 4. Abnormal migration of ENS progenitor cells in embryos homozygous for mutant alleles of Ret. (A-D) E9.5 wild-type (A) and Ret.k— (B) embryos and guts dissected from E10.5 wild-type (C) and miRet⁵¹ (D) embryos were hybridised as whole-mount preparations with a Sox10 cRNA probe. In E9.5 wild-type embryos (A), Sox10+ ENS progenitors were detected in the postbranchial region (black arrowhead) and in the foregut (white arrowhead). By contrast, in similar stage Ret.k— embryos (shown in B), no Sox10+ cells were present in the foregut despite the presence of such cells in the immediate postbranchial region (black arrowhead). At this stage, both wild-type and mutant embryos express high levels of Sox10 in cranial ganglia (cg) and in dorsal root ganglia (drg). In the gut of wild-type E10.5 embryos (C), the front of migrating Sox10+ cells (black arrow) has passed the midpoint of the midgut (halfway between the caudal region of the pancreas and the caecum). By contrast, in the gut of miRet⁵¹ homozygous embryos (D), the front of the migrating cells (black arrow) was located more rostrally, in the beginning of the midgut (black arrow). p, pancreas; ce, caecum; co, colon (hindgut).

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