Branched structures are common in mammals and exist mainly to solve problems of transport. Branched architectures allow a surface area to be packed into a small volume, minimize the distance of cells from transport systems and from the entrance of a system to its end. For development and evolution, branched structures offer the advantage of being scaleable: tree-shaped systems can grow and add branches without altering their basic nature. Branching morphogenesis takes place by four methods: fusion, clefting, sprouting and intussusception. All are controlled by paracrine factors and take place through changes in the behaviours of cytoskeleton-adhesion systems.

Key concepts:

• The internal anatomy of mammals involves many branched structures.
  
• Branched architectures optimize transport in compact organisms.
  
• Branched architectures can be scaleable, which has evolutionary and developmental implications.
  
• Branching can be by fusion, clefting, sprouting and intussusception.
  
• The largest branches tend to be stereotypical and under precise genetic control, but the finest ones are pseudo-fractal and quite variable.
  
• Branching depends on ramogenic signals from surrounding cells.
  
• Branching structures have endogenous mechanisms to ensure appropriate spacing; at least some rely on repulsive autocrine cues.