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:…may exist between slow freezing and vitrification protocols, since vitrified whole sheep ovaries in contrast to slow frozen ovaries contained substantially reduced numbers of primordial follicles and blood vessels with some degree of damage (Courbiere et al., 2005, 2006; Baudot et al., 2007).
Category: