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:Fetal rat lung fibroblasts were isolated from e19 Sprague-Dawley rats and cultured at 37°C in chamber slides, six-well plates, and 100-mm dishes, as described by us previously (21, 36, 38).
Category: