Pioneer growth cone behavior at a differentiating limb segment boundary in the grasshopper embryo.

The first neurons to extend axons through embryonic grasshopper limbs are a pair of sibling pioneer neurons. After migrating proximally along the limb axis, the pioneer growth cones normally make an abrupt ventral turn. In some cases (less than 20%) this turn is directly toward the proximo-ventrally located Cx1 guidepost neurons. However, in the majority of cases (greater than 80%) the pioneer growth cones make a more acute ventral turn along a single circumferential line which lies distal to the Cx1 neurons. Growth cones from other afferent neurons orient along the same line. Growth cones can extend along this line around more than half of the circumference of the limb and can grow in either direction along it. The circumferential line appears to be the prospective trochanter-coxa segment boundary. Afferent axons on the segment boundary leave it and contact the proximo-ventrally located Cx1 neurons. The site at which pioneer growth cones leave the boundary is variable and appears to be the point from which filopodial contact with Cx1 cells is first established. In addition to the trochanter-coxa segment boundary, the pioneer growth cones and axons also respond to the tibia-femur and femur-trochanter segment boundaries. The role of segment boundaries as barriers to growth cone movement and the effect of such barriers on the timing and placement of differentiation of pioneer neurons are discussed.