Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Utilizing previously established coculture conditions (21), this study shows that 1) rat PAECs but not PASMCs synthesize serotonin, 2) PAEC-derived serotonin passes through myoendothelial gap junctions into PASMCs, and 3) PAEC-derived serotonin activates TGF- signaling and promotes PASMC differentiation.
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