NEURONAL CONTROL OF DEVELOPMENT IN HYDRA

Hydra is an excellent model system for developmental biology, because pattern formation processes can be easily studied in regeneration, tra nsplantation, and reaggregation experiments. At the cellular level hyd ra has the advantage that it contains only a few basic cell types and that differentiation pathways are short, Two types of signals, produce d and released by nerve cells, control the spatial and temporal patter ns of differentiation. Positive signals induce specific local differen tiation events, and negative signals inhibit the spread of such induct ions to larger areas. Head-specific growth and differentiation are con trolled by head activator and head inhibitor, foot-specific processes are regulated by foot activator and foot inhibitor, The activators are peptides, the inhibitors are low-molecular-weight substances. The seq uence of the head activator is known, and it is conserved throughout t he animal kingdom. At the cellular level head activator exerts three t ypes of effects in hydra. It stimulates cells to divide, and it is res ponsible for the determination and the final differentiation of nerve cells and head-specific epithelial cells. For nerve-cell differentiati on the cAMP pathway is used as second messenger system. Components of this pathway were identified in hydra. In mammals head activator is pr oduced by nerve and neuro-endocrine cells, and it acts as mitogen on c ells of neural origin. It is present early in neural development and i n abnormal neural development, such as brain and neuroendocrine tumour s.