The unique location, large size, and specific arrangement of the Platynereis mesodermal
midline cells allowed their unambiguous identification after whole - mount in situ hybridization (WMISH) and thus expression
profiling by confocal imaging.
Yet, in the absence of detailed investigations of expression
profile and developmental fate of mesodermal
midline cells in basal ecdysozoans and deuterostome ambulacrarians, the nature of ventral
midline tissue in urbilaterians remains undecided.