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Stanford scientists regrow lost cartilage and reverse arthritis in major breakthrough
(www.sciencedaily.com)
This is a most excellent place for technology news and articles.
They turn into a soup-like homogenate. As is natural in the life cycle of a lab mouse.
And if they weren't euthanized they would almost certainly die of horrible horrible cancerous tumors. Not because of anything the labs do, but because it's a crime against nature for a mouse to live past maybe 2 years.