Building the house around the plumbing.

Signaling between growing blood vessels and the tissues that they innervate has traditionally been viewed as a one-way conversation, with organs and tissues supplying important cues for the growth and anatomical patterning of the blood vessels supplying them, but not vice-versa. Two recent papers now provide evidence that blood vessels can have an important role in promoting the assembly of organs and tissues. These papers show that proper formation of the pancreas and liver and induction of endocrine and hepatic cell types in these endodermal organs requires inductive signals from blood vessels.