"Neuroendocrine cells" in flatworms - Progenitors to metazoan neurons?

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Abstract
The integrative mechanisms in phylum Platyhelminthes are of considerable phylogenetic interest since ancestral flatworms are believed to be progenitors to all "higher" metazoans. The phylum consists of free-living and parasitic worms. In this group of early bilateral animals, the nervous system is condensed and consists of a bilobed brain and longitudinal nerve cords. Endocrine glands and a circulatory system are still lacking. The present review deals mainly with microturbellarians and parasitic worms. Three main types of neurons, all containing vesicles, occur in the worms: neurons containing dense-core vesicles, peptidergic neurons and sensory neurons. All types of neurons react positively to antisera towards invertebrate and vertebrate neuropeptides. Neurons containing dense-core vesicles are the most frequent type. The dense-core vesicles occur both in the cell soma and in their long processes. Neurons of this type form synaptocrine and paracrine release sites. We ask: Is the neuron containing dcv an archaic type of neuron in which the characteristics of the stem cells for neurons have been preserved?

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