Cornell engineers, taking an engineer's approach to making synthetic blood vessels have designed  tiny, 3-D microchannels in a soft biomaterial and injected human umbilical vein endothelial cells into the channels. They embedded tissue cells from the brain into the surrounding gel and watched the interactions between the "vessels" and cells, which commonly surround microvessels in the body.
Let's remind that human tissue, be it in the heart, brain or bones, can't function without a vascular system — the intricate network of vessels that circulate life-sustaining blood and nutrients.

Here, left picture,  you see a reconstruction of fluorescence confocal micrographs of a microvascular network with endothelial-cell lines channels (red) and perivascular cells (green) in collagen. 

Signals from these tissue cells led to new blood vessels sprouting from the originals — a living network of blood vessels engineered completely in vitro.The results, which could lead to new techniques in regenerative medicine and better drug delivery strategies, are from the lab of Abe Stroock, associate professor of chemical and biomolecular engineering and member of the Kavli Institute at Cornell for Nanoscale Science.

Source: http://www.news.cornell.edu/stories/May12/stroockMicrovessels.html