Results

Consequences of Ectoderm Removal on the Epithelialisation of Segmental Plate

In all cases operations were performed at the level of the caudal segmental plate. Ectoderm removal resulted in abolition of somite formation on the operated side. Pax3 is a useful marker since it is expressed at high levels in the epithelialised dorsal compartment of the somite.

Ectoderm removal followed by re-incubation resulted in a dramatic downregulation in the high levels of Pax3 expression associated with epithelialised dermomyotome at the site of operation (Fig. 3 A). Some Pax3 expression was detected directly adjacent to the dorsal neural tube coinciding with a small region that remained in an epithelial state. Examination of a marker of the ventral compartment, Pax1, following ectoderm removal showed robust expression of the gene at the site of operation. Expression levels of Pax1 on the operated side were similar to those on the control side (Fig. 3B). Paraxis, a marker for the epithelialisation of the paraxial mesoderm (Burgess et al. 1995; Barnes et al. 1997; Sosic et al. 1997), was down-regulated like Pax3 at the site of ectoderm removal (Fig. 3C). No evidence of cell death following ectoderm removal could be detected (Schmidt et al. 1998).

Ectoderm removal therefore results in the loss of the epithelial organisation of the somites except in a small region directly adjacent to the dorsal neural tube. At the molecular level, removal of the ectoderm leads to the loss of epithelial somite markers, except in the region adjacent to the neural tube. This, however, is not accompanied by an up-regulation of sclerotome markers.