What do you think are the difficulties in tissue engineering of complex organs such as the heart and liver, as compared to skin?
the heart has an extremely complex 3-dimensional structure that consists of muscle fibers that pull in a variety of different directions and which require complex nerve networks to contract hundreds of different fiber bundles in precise sequence in order for it to function as a pump. Those nerves are interconnected in complex ways and triggered by both internally-generated synchronization signals and externally-generated rate signals. Because of the rapid rate at which it performs its work, it also requires a dense network of internal blood vessels to furnish the muscle tissue with a continuous supply of fuel and oxygen. All these things make it extremely difficult if not impossible to "tissue engineer" a synthetic heart based on natural muscle fibers.
In the case of a liver, it also requires a 3-dimensionsal network of blood vessels and liquid ducts that carry away bile acids, and microstructurally it consists of a variety of different cell types that perform different functions essential to its overall "mission". these factors make it difficult to engineer a synthetic liver.