I am culturing immortalised mouse embryonic fibroblasts; currently at passage 10. Today I noticed these long thread-like structures floating above the cells (see picture, 40x magnification). The cells themselves are fine, the media isn't cloudy or yellow and these threads are not moving independently. I don't think these structures are coming from the media, as a different cell line growing in the same media isn't affected. We have had sporadic bacterial contaminations in the lab, but these structures don't look anything like the bacterial contamination we've seen previously. Has anyone got any ideas what these are?