Process extension and BL organization are impaired in ILK mutant SCs. (A and B) EM micrographs of SN ultra-thin sections at P5 (A) and P24 (B). (A) At P5, immature SCs extend processes (white arrowheads) that fully envelop bundles of axons (a and c), which is a typical feature at this stage of maturation. In mutant nerves (b and d), SC processes often fail to completely enwrap axon bundles (black arrowheads). In controls, promyelinating SCs are surrounded by a tightly associated BL (e, white arrows). In contrast, mutant promyelinating SCs display cytoplasmic protrusions at the cell surface (f, gray arrows), frequently associated with a disorganized BL that forms loops detached from the plasma membrane (f, black arrows). Myelinating SCs could be found in controls, and some were also present in mutant nerves (asterisks). Bars: (b) 5 µm; (a and d) 2 µm; (c, e, f) 1 µm. (B) At P24, there are virtually no remaining axon bundles on the control nerves. Large caliber axons have been engaged and are myelinated (a, c, and e), whereas small caliber axons, which will not be myelinated during development, are engaged by nonmyelinating SCs (c, white arrowhead). In mutant nerves, axonal sorting is severely impaired, and many bundles persist. Mutant SCs associated with axon bundles fail to completely surround them (black arrowheads), resembling the phenotype observed at P5 (b and d). Unsorted axons are surrounded by loops of BL (black arrowheads). Control myelinating SCs reveal a tightly associated single BL (g, white arrows) in contrast to ILK-null cells in which multiple cytoplasmic protrusions (gray arrows) are visible (f), which are frequently associated with empty loops of BL (h; black arrows). Boxes indicate regions that are shown at a higher magnification in the panels below. Bars: (b) 5 µm; (a, c, d, and f) 2 µm; (e and h) 1 µm; (g) 0.2 µm.