title: Evidence for a cordal, not ganglionic, pattern of cephalopod brain neurogenesis.
abstract: BACKGROUND
  From the large-brained cephalopods to the acephalic bivalves, molluscs show a vast range of nervous system centralization patterns. Despite this diversity, molluscan nervous systems, broadly considered, are organized either as medullary cords, as seen in chitons, or as ganglia, which are typical of gastropods and bivalves. The cephalopod brain is exceptional not just in terms of its size; its relationship to a molluscan cordal or ganglionic plan has not been resolved from the study of its compacted adult structure. One approach to clarifying this puzzle is to investigate the patterns of early cephalopod brain neurogenesis, where molecular markers for cephalopod neural development may be informative.
  
  
  RESULTS
  We report here on early brain pattern formation in the California two-spot octopus, Octopus bimaculoides. Employing gene expression analysis with the pan-bilaterian neuronal marker ELAV and the atonal-related neuronal differentiation genes NEUROGENIN and NEUROD, as well as immunostaining using a Distalless-like homeoprotein antibody, we found that the octopus central brain forms from concentric cords rather than bilaterally distributed pairs of ganglia.
  
  
  CONCLUSIONS
  We conclude that the cephalopod brain, despite its great size and elaborate specializations, retains in its development the hypothesized ancestral molluscan nervous system plan of medullary cords, as described for chitons and other aculiferan molluscs.
authors.affiliation: Department of Marine Biodiversity Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, 237-0061 Japan.
authors.forename: Shuichi
authors.initials: S
authors.lastname: Shigeno
authors.affiliation: Department of Neurobiology, The University of Chicago, 947 E 58th Street, Chicago, IL 60637 USA.
authors.forename: Rahul
authors.initials: R
authors.lastname: Parnaik
authors.affiliation: Department of Organismal Biology and Anatomy, The University of Chicago, 1027 E 57th Street, Chicago, IL 60637 USA.
authors.forename: Caroline B
authors.initials: CB
authors.lastname: Albertin
authors.affiliation: Department of Neurobiology, The University of Chicago, 947 E 58th Street, Chicago, IL 60637 USA ; Department of Organismal Biology and Anatomy, The University of Chicago, 1027 E 57th Street, Chicago, IL 60637 USA.
authors.forename: Clifton W
authors.initials: CW
authors.lastname: Ragsdale