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Figure 2: Microstructure of the spike. (A) Representative polished transversal section imaged in BSE-SEM accompanied on the left by a micro-CT reconstruction showing the location of the section. The multilayered architecture of the cuticle, comprising a highly mineralized region (HMR), an outer helicoidal region (OHR), a striated region (STR) and an inner helicoidal region (IHR) is evident. (B) SE-SEM of a longitudinal fracture offering a complementary view of the 4 layers accompanied, on the right by, a micro-CT reconstruction showing the location of the section. Scale bars in (A) and (B) are 200 µm. (C-E) Details of the different layers from the polished surface. In (E), the arrows indicate the transition region (TR) and the dotted lines bound the OHR. In (D), horizontal parallel chitin-protein fibers are cross-sectioned and fiber-rich strands (medium dark) alternate with pore canal lumens (dark) and their densely mineralized sheaths (bright rod-like features). Twisted ribbon-shaped pore canals with unmineralized lumen also appear in (C) the IHR and (E) OHR while they are straight and mineralized in (E) the HMR. Scale bars in (C–E): 10 µm. (F-H) Close-up views of the fractured surface of the different layers highlighting: (F) a rough surface for the IHR in agreement with the twisted plywood organization; (G) sheets of parallel fibers (oriented vertically in the figure) in the STR and (H) a rough stepwise fractured surface in connection with the OHR (bounded by dotted lines), which is in strike contrast with the smoother surface of the HMR. (I) Schematic summarizing the main microstructural features of the spike. Scale bars in (F–H): 8 µm.