P71. Stem cells and gene expression during hemichordate head regeneration

Abstract

Hemichordate acorn worms grow a new head on the anterior wound surface when the body is transected. In Ptychodera flava, a blastema is evident by 3 days and a new full-sized, functional head differentiates on the cut stump by 12 days. The hemichordate nervous system provides the most ancestral extant example of the dorsal hollow brain, formed by neurulation, characteristic of chordates and humans. Only in hemichordates does such a nervous system regenerate significantly. Animals throughout the phylogenetic scale share similar developmental gene regulatory networks, and our work, along with that of others, supports the conclusion that the same gene regulatory networks occur in hemichordate and chordate neural differentiation. Examination of the regeneration process by whole mount in situ hybridization (WISH) of sequences from Yamanaka pluripotency factors or “stem cell” genes, as defined by genes inducing pluripotency in mammalian somatic cells, and developmental organizer genes reveals a cascade of gene expression associated with the steps of regeneration. A subset of the stem cell genes produce the first WISH signals before a blastema is formed. The second set of stem cell genes begins to generate signal as the blastema is established and this signal persists in the blastema as organizer and early development gene signals appear as differentiation proceeds. These results suggest that activation of cells with regenerative capability involves the early expression of stem cell genes in the cells that will form the blastema. Signal from some stem cell genes continues as the expression of typical embryonic gene regulatory networks are activated for the differentiation of the tissues. In planarians, regeneration is based on mobilization of stem cells residing in the tissue of the animal to form a regeneration blastema. During regeneration of an amphibian limb, a blastema is also formed, apparently by dedifferentiation of tissue cells in the cut limb to stem cells which accumulate to establish the blastema. The nature of the cells and the tissue origin of the cells that participate in hemichordate blastema formation and head regeneration are being investigated. Supported by the PBRC Biomedical Fund UHF#12-209-04.