Ontogeny of the ventral nerve cord in malacostracan crustaceans: a common plan for neuronal development in Crustacea, Hexapoda and other Arthropoda?
This review sets out to summarize our current knowledge on the structural layout of the embryonic ventral nerve cord in decapod crustaceans and its development from stem cell to the mature structure. In Decapoda, neuronal stem cells, the neuroblasts, mostly originate from ectodermal stem cells, the ectoteloblast, via a defined lineage. The neuroblasts undergo repeated asymmetric division and generate ganglion mother cells. The ganglion mother cells later divide again to give birth to ganglion cells (neurons) and there is increasing evidence now that ganglion mother cells divide again not only once but repeatedly. Various other aspects of neuroblast proliferation such as their temporal patterns of mitotic activity and spatial arrangement as well as the relation of neurogenesis to the development of the segmental appendages and maturation of motor behaviors are described. The link between cell lineage and cell differentiation in Decapoda so far has only been established for the midline neuroblast. However, there are several other identified early differentiating neurons, the outgrowing neurites of which pioneer the axonal scaffold within the neuromeres of the ventral nerve cord. The maturation of identified neurons as examined by immunohistochemistry against their neurotransmitters or engrailed, is briefly described. These processes are compared to other Arthropoda (including Onychophora, Chelicerata, Diplopoda and Hexapoda) in order to shed light on variations and conserved motifs of the theme 'neurogenesis'. The question of a 'common plan for neuronal development' in the ventral nerve cords of Hexapoda and Crustacea is critically evaluated and the possibility of homologous neurons arising through divergent developmental pathways is discussed.
Harzsch
Steffen
S
Universität Ulm, Fakultät für Naturwissenschaften, Sektion Biosystematische Dokumentation und Abteilung Neurobiologie, Helmholtzstrasse 20, 89081 Ulm, Germany.