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:To determine the role of endogenous serotonin signaling in the PAEC-induced myoendothelial-dependent differentiation of PASMCs, Sprague-Dawley rat proximal PAECs and PASMCs were cultured alone (monocultured), cocultured on opposite sides of a porous Transwell membrane (touch coculture), or PAECs were cultured on the bottom of a six-well plate and PASMCs on the Transwell membrane (no-touch coculture) as previously described (21).
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