a, Two-dimensional cryoTEM image. b, Slice from a section of the three-dimensional volume along the xy plane (top-most inset), where crystals are visible edge-on (insets 1 and 2, white arrows). Black circle: ACP infiltrating the fibril (see below). c, Computer-generated three-dimensional visualization of mineralized collagen. The fibril is sectioned through the xy plane, revealing plate-shaped apatite crystals (coloured in pink) embedded in the collagen matrix. Scale bars: 100 nm.