Fig. 3. aCC develops dendrites as collateral processes in the embryonic CNS. (A) Schematic of aCC motoneuron in a late-stage embryo. The aCC cell body (aCC) is present near the dorsal midline at each hemisegment in the Drosophila embryonic CNS (CNS). The aCC extends a single axon (axon) laterally that cuts across the longitudinal connective, exits the CNS, grows past proximal muscles including muscle-12, then terminates at its target, muscle-1. As its axon approaches the target, aCC begins to develop ipsilateral dendrites (dendrite) as collateral processes from the CNS portion of the axon. A medially directed dendritic growth cone (gc) extends across the midline, which later develops into contralateral dendrites. A neuropil (neuropil) emerges within the longitudinal connective. (B,C) Tracings of DiI-labeled aCC motoneuron in wild-type embryos at hours 14 (B) and 17 (C). A dotted circle surrounds the main dendrite, and an arrowhead points to the dendritic growth cone that crosses the midline. A star indicates where distal portion of the axon is cropped. Behind the aCC tracing, a shaded region indicates where there is strong anti-HRP staining (see, for example, Fig. 5C left). The width of neuropil is measured as the mean distance between the edges of the HRP-positive longitudinal connective at a point between the centers of anterior and posterior commissures (ac and pc). (D,E) Histogram of the number of aCC dendritic tips (mean±s.e.m. number per 4 µm) at hours 14 (D, n=6 neurons) and 17 (E, n=14) along the medial-lateral axis. Arrow indicates the mean position. Scale bar: 5 µm.