Head regeneration in the simple metazoan Hydra

Ulrich Technau

Molecular Cell Biology; University of Technology Darmstadt; Schnittspahnstr. 10; 64287 Darmstadt, Germany

The cnidarian Hydra belongs to one of the earliest branching metazoan phyla. Unlike the even simpler sponges, Cnidaria possess a defined body axis and a nervous system. Cnidarians are known for their high capacity to regenerate and Hydra has developed into the major model organism to study regeneration. A great amount of classical experiments showed that head regeneration in Hydra is governed by two developmental gradients, i.e. head activation and head inhibition. The molecular nature of these gradients is still unclear, but recently a number of genes have been isolated that appear to play important roles in the process of regeneration. Among the earliest genes expressed during head regeneration are members of the Wnt-signalling cascade as well as the T-box gene Brachyury. The Wnt cascade is known to play important role in axis formation and organizer function in vertebrates and invertebrates. Brachyury is interesting, because this gene is one of the key determinants in early mesoderm differentiation and axis elongation in vertebrates. The evolution of the molecular control of regeneration will be discussed. During the regeneration of the head, the pattern of neuron distribution has to be reestablished. Neurons as well as the other sensory cell type, the nematocytes, are the major derivatives of interstitial stem cells. Stem cells are committed stochastically in the body column, the precursors then migrate towards the ends and differentiate into a specific phenotype according to the epithelial environment. Transcription factors involved in neuronal differentiation in vertebrates also appear to play a role at least in the differentiation of nematocytes in Hydra. These data suggest, that key molecular components of signalling and cell differentiation are conserved from Hydra to humans.