(A) E15.5 mouse-tail tendon. A cell membrane is highlighted in blue (top panel, single SBF-SEM transverse image) and reconstructed (middle and bottom panels). A fibril bundle, circled in dark blue (top), is reconstructed, and lies closely associated with the cell along the entire cell length. (B) Newborn mouse-tail tendon. A cell membrane is highlighted in yellow (top panel, single SBF-SEM transverse image). A fibril bundle is highlighted in red and reconstructed in red, together with the corresponding cell (in yellow, middle and bottom panels). The fibril bundle is again closely associated with the cell. Cell protrusions surrounding fibrils are better defined than at E15.5. (C) 6 week old mouse-tail tendon. Two adjacent cell membranes are highlighted blue and pink (top panel, single SBF-SEM transverse section image). The same cells are reconstructed below. A fibril bundle is marked with *. The two cells have elongated processes that reach out and contact each other (highlighted with a rectangle, top reconstruction), enveloping a bundle of collagenous ECM (*). (D) There is a small increase in cell volume between E15.5 and newborn time points, but no further increase in cell volume, which remains relatively constant during postnatal development. (E) Cell surface area increases markedly (1.5 fold and 1.8 fold for E15.5 to newborn and newborn to 6 week old, respectively) as cells develop large cell membrane processes to enlarge the channels containing the fibril bundles. ** Indicates significant difference (p = <0.05), † indicates p = >0.05. * Marks a fibril bundle. Source data in Figure 4—source data 1.