elongation and establishing the pattern of branching may require contact with mesenchyme thus by utilizing the inventors 39 novel model system the inventors have found that in contrast to the widely held view that the complex arborization of the ub during kidney development is dependent upon direct contact between cells of the metanephric mesenchyme and cells of the ub a substantial degree of branching morphogenesis can be mediated by soluble factors lone therefore the branching program exists within the ub itself after it is formed form the wolffian duct and soluble factors can trigger its initiation and continuation no singular soluble factor however appears sufficient a combination of gdnf and an activity or set of activities present in bsn cm is necessary whether this latter activity is the same as that which induces the formation of branching tubules with lumens of ub cells in culture remains to be determined it seems very likely that more direct mesenchymal interactions with the ub are important for establishing the direction of branching events since the cultured ub derived structures lack directionality and only when the culture ub was recombined with metanephric mesenchyme did directionality and elongation occur fig8 h u2013 i epithelial mesenchymal cell cell contact is probably essential for the later steps in the development.