( ( Once ( ( sufficient ( cartilage cells ) ) ( have ( been produced ) ) ) ) ( , ( ( the surgeon ) ( ( ( ( makes ( ( a pocket ) ( over ( the ( damaged cartilage ) ) ) ) ) and ) ( ( injects ( the cells ) ) ( into ( the pocket ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN Once) (S (NP (JJ sufficient) (NN cartilage) (NNS cells)) (VP (VBP have) (VP (VBN been) (VP (VBN produced)))))) (, ,) (NP (DT the) (NN surgeon)) (VP (VP (VBZ makes) (NP (NP (DT a) (NN pocket)) (PP (IN over) (NP (DT the) (VBN damaged) (NN cartilage))))) (CC and) (VP (VBZ injects) (NP (DT the) (NNS cells)) (PP (IN into) (NP (DT the) (NN pocket))))) (. .)))
Once sufficient cartilage cells have been produced, the surgeon makes a pocket  over the damaged cartilage and injects the cells into the pocket.
(ROOT (S (NP (NNS Hagfishes)) (VP (VBP have) (NP (NP (DT a) (NN skull)) (VP (VBN made) (PP (IN of) (NP (NN cartilage)))))) (. .)))