Embryonic development of the nervous system of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789)

We have analyzed the embryonic development of the Mesostoma nervous system, using a combination of histology, transmission electron microscopy, and wh olemount immunohistochemistry. Neural progenitors are formed at an early st age when the Mesostoma embryo constitutes a multilayered mesenchymal mass o f cells, A neurectoderm as in vertebrates or arthropods is absent. Only aft er neurons in the deep layers of the embryo have started differentiating do superficial cells reorganize into an epithelium that will give rise to the epidermis. Neurons are clustered in two anterior, bilaterally symmetric br ain hemispheres. An antibody against acetylated beta-tubulin (anti-acTub) t hat labels neurotubules reveals an invariant pattern of pioneer neurons in the brain of midstage embryos. Pioneer neurons are grouped in several small clusters at characteristic positions. They pioneer several commissural tra cts of the brain and two pairs of ventral and dorsal connectives, respectiv ely. (C) 2000 Wiley-Liss, Inc.