Cellular scale: specialized functions acting in a collective At the cellular scale, specialized cell types have been localized and characterized by single-cell sequencing . The core of silk fibers 159 is secreted by three columnar epithelial cell types localized in the tail (zone A). The middle and outer layer coatings are secreted by two cell types in zone B and C of the ampulla (Fig. ). The columnar epithelia are filled with spidroin vesicles, secreted to the lumen from their apical side 187 . Each cell type produces a different combination of spidroins, used to define their identity 159 , and realizes post-translational modifications 197 impacting their self-assembly 198 . Another cell type is scarcely present in both the tail and the ampulla, and does not seem to be directly involved spidroin expression 159 ; we therefore hypothesize its role in supporting its surrounding cells or for tissue homeostasis. The duct consists of a thickening epithelium along its length, as revealed by histology, electron microscopy and single-cell sequencing, which has identified two duct cell types . A cuticular intima separates the cells from the lumen but enables the diffusion of molecules with its pore-like structure 199 . The duct cells regulate the lumen's ChemµEnv, notably the pH with high carbonic anhydrase expression levels 165 ; they are filled with vesicles and have dense apical microvilli . At the posterior side, flask-shaped cells contain secretory granules and may add an additional coating layer 189 . Water is continuously pumped out of the lumen during spinning, notably with terminal tubule cells . The specialized cell functions in spider silk glands exemplify the complexity of material-producing organs, challenging their biomimicry using conventional recombinant approaches.