Phylogenetic aspects on the neuroendocrine system: A minireview with particular reference to cells storing neurohormonal peptides in some primitive protostomian invertebrates (flatworms, annelids)

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Abstract
The neuroendocrine system, where the messenger substances are peptides of widely varying lengths, comprises three main anatomical components. One is the classical endocrine glands, such as the pituitary, the parathyroids, the thyroid C-cells or the ultimobranchial bodies, the adrenal medulla, and the pancreatic islets of Langerhans. Another is the diffusely scattered epithelial cells in several mucous membranes, like those of the gastrointestinal tract and the bronchial tree. The third component is the peptide-producing neuronal cells of the central and peripheral nervous system. When the phylogeny of the three components of the neuroendocrine system is investigated, it seems as if the origin lies in the neuronal cells, whereas disseminated peptide hormone immunoreactive endocrine cells in the mucous membranes appear only at later evolutionary stages. The classical endocrine glands are restricted to highly developed animals such as the vertebrates. This sequence of events in the phylogeny of the neuroendocrine system is illustrated by a review of some preliminary results of current immunohistochemical investigations in some primitive protostomian invertebrates, namely the flatworms (Platyhelminthes) and the annelids (Annelida). In both these phyla all the neurohormone immunoreactive cells are nerve cells. Annelids, Flatworms, Neuroendcrine system, Neurohormonal peptides, Phylogeny, Protostomian invertebrates.

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