Figure 3
Axonal projection pattern of dI1 neurons. (A) Chick embryos were electroporated at stage 16 (left side) with EdI1 enhancer along with a Cre-dependent GFP plasmid (EdI1::Cre + pCAGG-LoxP-STOP-LoxP-GFP). At E6, spinal cords were removed, fixed and analyzed as an open-book preparation. Whole neural tubes (sacral to cervical) are presented. Confocal images were taken and photomerged utilizing Photoshop software. (B) The schematic illustrates the axonal projection pattern of the dI1 neuronal population. Rostral is up in the image and the schematics. Asterisks represent the levels of the limbs. dI1 neurons have two main rostral axonal projection pathways (A): ipsi-lateral (A4, A5, yellow arrows), and contra-lateral (A, A2, A3, white arrows and arrowheads). After crossing the floor plate (FP), dI1comm neurons elongate along the floor plate at the ventral funiculus (VF; white arrowheads) for several segments and, subsequently, turn toward the lateral funiculus (LF; white arrows). dI1ipsi axons turn longitudinally and rostrally at the LF of the lumbar, thoracic and cervical levels (A, A4, A5). At the caudal sacral level dI1ipsi axons project caudally (A, A1, magenta arrows). (C, D) The relative position of dI1ipsi and dI1comm fascicules at the LF was studied following differential labelling of dI1ipsi and dI1comm axons. At the LF, dI1ipsi and dI1comm form one fascicle (green+red arrow in (D)). (E) Schematic with boxed areas representing the frames of (C, D). Only the rostrally turning axons are illustrated in (E). c, cervical level; b, brachial level; FP, floor plate; l, lumbar level, s, sacral level; t, thoracic level. Scale bars: 150 μm (A, D); 100 μm (A1–A5); 300 μm (C).