FIG 4. During the stage of typical fetal lamination, various immunocytochemical markers selectively label discrete contingents of fibers within the stratified periventricular fetal WM (iz): MAP-1b (A, 18-week-old fetus) and SMI-312 (C, 20-week-old fetus) label the cytoskeletal components of many growing axons, whereas the prominent periventricular system of fiber bundles is stained by SNAP-25 antibody (B, 18-week-old fetus, arrows). These markers are also visualizing periventricular fiber crossroad areas (C1, C2, C3, C6). Note that CS–56 antibody (D, 18-week-old fetus) labels the extracellular matrix, but is not expressed within the periventricular fiber system (asterisks). Arrowheads denote the capsula externa system, which marks the border between the fetal WM (iz) and the subplate zone (sp), which is part of the neocortical anlage.