One of the major hurdles in tissue engineering is populating tissue with blood vessels sufficient to support it. This is absolutely essential to enable the growth of anything more than a tiny amount of tissue. Decellularization has proven to be a useful way to work around present limits, but that requires donor tissue in order to obtain the guiding extracellular matrix structure. When it comes to building tissue from scratch, researchers are still working on techniques to create the necessary blood vessel networks.
One of the major obstacles to growing new organs - replacement hearts, lungs and kidneys - is the difficulty researchers face in building blood vessels that keep the tissues alive. "It's not just enough to make a piece of tissue that functions like your desired target. If you don't nourish it with blood by vascularizing it, it's only going to be as big as the head of a pen."
Today, biomedical researchers are taking two main approaches to growing new capillaries, the smallest blood vessels and those responsible for exchanging oxygen, carbon dioxide and nutrients between blood and muscles or organs. One group of researchers is developing drug compounds that would signal existing vessels to branch into new tributaries. These compounds - generally protein growth factors - mimic how cancerous tumor cells recruit blood vessels. The other group [is] using a cell-based method. This technique involves injecting cells within a scaffolding carrier near the spot where you want new capillaries to materialize. [Researchers] deliver endothelial cells, which make up the vessel lining and supporting cells. Their scaffolding carrier is fibrin, a protein in the human body that helps blood clot.
"The cells know what to do. You can take these things and mix them and put them in an animal. Literally, it's as easy as a simple injection and over a few days, they spontaneously form new vessels and the animals' own vasculature connects to them. The adult stem cells from fat and bone marrow both work equally well. If we want to use this clinically in five to 10 years, I think it's crucial for the field to focus on a support cell that actually has some stem cell characteristics."