Fig. 3. The fibripositor secretory pathway. (A) Transmission electron microscopy of transverse sections through embryonic mouse tail tendon shows bundles of extracellular collagen fibrils between adjacent cells. One (a), two (b), three (c) or more (not shown) membrane-bounded collagen fibrils are also frequently observed within the cytoplasm. Membrane-bounded collagen fibrils are also observed in plasma membrane (PM) extensions called fibripositors (d). Bar, 500 nm. (B-D) Schematics showing longitudinal representations of collagen fibrils in (B) a Golgi-to-PM transport compartment (GPC^+cf) and within both (C) closed and (D) open fibripositors. The topology of membrane-bounded collagen fibrils observed in cross-section and the PM was determined by serial-section 3D reconstruction. Selected potential planes of section are represented by dotted lines. (a-c) Cross-section through one (a), two (b) and three (c) fibrils located within the cytoplasm; (d) cross-section through a single fibril located within a fibripositor.