PATTERN OF DIFFERENTIATED NERVE-CELLS IN HYDRA IS DETERMINED BY PRECURSOR MIGRATION

The nervous system of the fresh water polyp hydra is built up as a ner ve net spread over the whole body, with higher densities in the head a nd the foot, In adult hydra, as a result of continuous growth, new ner ve cell differentiation takes place continuously, The pattern of nerve cell differentiation and the role of nerve cell precursor migration i n establishing the pattern have been observed in vivo by vitally label ling precursor cells with DiI, The results indicate that nerve cell pr ecursors arise directly from stem cells, complete a final cell cycle a nd divide, giving rise to two daughter cells, which differentiate into nerve cells, A subpopulation of the nerve cell precursors are migrato ry for a brief interval at the onset of the terminal cell cycle, then complete the cell cycle and divide at the site of differentiation. Lab elling small patches of tissue in the head, body column and peduncle/f oot with DiI indicated that formation of nerve cell precursors was nea rly constant at all three positions, However, at least half of the lab elled precursors in the body column migrated to the head or foot befor e differentiating; by contrast, precursors in head and foot differenti ated in situ without significant migration, This redistribution leads to a net increase of nerve cell precursors in head and foot compared t o body column and thus to the higher density of nerve cells in these r egions.