How can we study a valve ageing or a worn cartilage if we don't even have a valve or a cartilage to watch? Is ageing here defined as: "nutrients in my Petri di

How can we study a valve ageing or a worn cartilage if we don't even have a valve or a cartilage to watch? Is ageing here defined as: "nutrients in my Petri dish are going exhaust so I will go pop"?