  Many organs of higher organisms are heavily branched structures and arise by
an at first sight similar process of branching morphogenesis. Yet the
regulatory components and local interactions that have been identified differ
greatly in these organs. It is an open question whether the regulatory
processes work according to a common principle and in how far physical and
geometric constraints determine the branching process. Here we review the known
regulatory factors and physical constraints in lung, kidney, pancreas,
prostate, mammary and salivary gland branching morphogenesis, and describe the
models that have been formulated to analyse their impacts.
