Tendons are the strong 'cables' which bind muscle to bone; they develop in carefully-engineered stages. These computer simulations show cross-sections through mouse tendons before and after birth – there has been a remarkable change. In the embryonic tendon (left), individually-coloured cells appear rounded and squashed together; they form a sort of scaffolding for what comes next. By the time the mouse is six weeks old (right), the tendon cells are stretched out into star shapes, providing support for bundles of collagen fibres growing up through the gaps. Collagen fibres run the entire length of human tendons, too, allowing us to cope with the strong forces generated by our muscles. Researchers believe these simulations provide important insights, not just into the development of the skeletal-muscular system, but how the behaviour of cells in living tissues can change dramatically before and after birth.
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