Researchers from four universities have unveiled a recent accomplishment they have made with 3D bioprinting. One of the main issues with 3D printing actual human organs is the vascular networks needed. Every cell within an organ needs to be within a hair's width of a capillary. What they did was basically print out the fiber like vascualar network, and then build cells around it. They then removed the fiber like structure which they lined with human endothelial cells. After a week, the cells grew around the spaces left by the removed fibers, leaving an entire vascular network. More details about this breakthrough can be found here: http://3dprint.com/7729/3d-print-organs-vascular/

Below is an image of the typical vascular network of a human liver.