ECM secretion and bundle formation within the neurospheroid networking in the deep HCWN system. (a,b) Schematic depiction of ECM secretion from the spheroid (a), and ECM secretion from the host and satellite spheroid and bundle formation during NSN formation (b). (c,d) SEM images of the host spheroid and ECM membrane (c) and ECM formation around the spheroids (d). (e,f) Merged image (fluorescence and optical microscope) of laminin (e) expression around a host spheroid. (f) Confocal microscope image of the area around a satellite and host spheroid. (g) Merged image (fluorescence and optical microscope) of laminin expression in a neurite bundle. (h,i) Fluorescence image of GFAP (green) and nestin (red) expression in satellite (h) and host (i) spheroids.