A mold assembly for the in situ formation of a prosthesis in an annulus located in an intervertebral disc space between adjacent vertebrae of a patient. The mold assembly includes at least a first mold having at least one interior cavity adapted to be located in the intervertebral disc space. At least a first lumen has a distal end fluidly coupled to the mold at a first location. One or more discrete reinforcing structures are located in the intervertebral disc space with the mold. One or more biomaterials are provided to be delivered to the interior cavity through the first lumen. The at least partially cured biomaterial, the reinforcing structures and the mold cooperating to form the prosthesis.