There's much more to tissue than cells: between the cells is an extensive and complex structure known as the extracellular matrix (ECM). Recreating this matrix is one of the challenges facing tissue engineers who aim to produce large segments of replacement tissue, or entire organs grown from a patient's own cells.
Due to its diverse nature and composition, the ECM can serve many functions, such as providing support and anchorage for cells, segregating tissues from one another, and regulating intercellular communication. The ECM regulates a cell's dynamic behavior. In addition, it sequesters a wide range of cellular growth factors, and acts as a local depot for them.
Many research groups are using new capabilities in nanoscale manufacturing to build scaffolds with some of the capabilities of the extracellular matrix: support, ability to release specific biochemicals, and so forth. But it looks very likely that a clever way exists to move straight to the end goal here: a scaffold that works just as well as the real thing because it is the real thing.
One of the more interesting recent developments in tissue engineering is recellularization: removing all the cells from an organ, leaving only the extracellular matrix as a blueprint, and then repopulating that blueprint with cells from the patient who will receive the finished tissue. It's a clever way around our present inability to grow organs from scratch, or generate nanoscale scaffolds as complex as the extracellular matrix. As an added bonus, organs and tissue from animals can be used as the basis for a transplant.
The New Scientist recently published an update on where this research and development initiative stands:
The rat hearts beat just as if they were inside a live animal, but even more remarkable is how each one has been made: by coating the stripped-down "scaffolding" of one rat's heart with tissue grown from another rat's stem cells.
This could lead to a virtually limitless supply of organs for transplantation that are every bit as intricate as those that grow naturally ... "We're already working with heart, kidney, liver, lung, pancreas, gallbladder and muscle," Taylor says. Rival groups are using similar procedures to create new livers and muscle too.
Researchers also note that recellularized organs are a solution to one of the other bugbears of tissue engineering: how to produce the intricate array of blood vessels needed to sustain tissue, and then successfully patch it in to a transplant recipient's existing blood transport system.
Because we've retained the blood vessels, we can take the plumbing and hook it up to the recipient's natural blood supply. That's the beauty of this.
There are a least a few people from early trials and demonstrations presently walking around with not-so-small chunks of recellularized tissue in their bodies, blood vessels and all: heart valves and a trachea for example. It's all very promising - as though we skipped ahead a decade in this one area of medical technology.