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:It has been described previously that pOPC isolated with this technique differentiate with time (4–6 days in vitro; DIV4-6) into oligodendrocytes and astrocytes [38], however OPC make up the large majority (80–90% of all DAPI+ cells) during the first two days of culture (Fig 1A and 1B).
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