Different collagen. In earlier tissue engineering a basic 3D scaffold made using processed animal collagen to which cell cultures were added to build the structure. These scaffolds were, say, in the form of an ear, nose or even a urinary bladder. The cultures grew in this scaffold until they matured and yes, they are implantable - several patients have new bladders that were grown in a lab. This type of scaffold dissolved in time and is replaced by naturally growing collagen. Now they're working on literally 'printing' the structure one 2D layer at a time, including natural collagen tissue, until the full 3D structure is complete. If successful we're talking the ability to manufacture replacement organs or other parts.
Others are working on similar technologies for the production of synthetic meat from cell cultures. Such syn-meat could in principle be tailored to eliminate things like saturated fats etc. in favor of a healthier replacement. It's said not to have the right taste and texture now, but eventually....
Peripheral nerves regrow but slowly. It can take a very long time for them to grow the length of a limb so things are sort-of reconnected, which is why rehab takes so long. Previously it was thought that central nervous system tissue (brain, spinal cord etc.) didn't grow back, but this is no longer the case. Much work is now going into bridging damaged spinal cords and brain tissue, and the results are very promising especially as concerns spinal cord injuries. One development is the use of antibodies to negate the action of NOGO, a family of molecules that prevent nerve fiber regrowth. Also interesting is that olfactory glia cells, normally involved in smell and not present in the cord, can be used at the injury site to promote regeneration of cord nerve fibers.
Things are getting real interesting on all these fronts fronts, but treatments will take a while.