A detached branch stops being recognized as self by other branches of a neuron

The multiple peripheral projections of a single leech mechanosensory neuron form individual arbors that do not overlap at all with each other, a phenomenon that has been termed "self-avoidance" (Yau, 1976; Kramer and Stent, 1985). This is in marked contrast to the peripheral arbors of adjacent segmental homologues, which partially overlap with each other at their boundaries in target areas of the body wall (Nicholls and Baylor, 1968; Gan and Macagno, 1995). How a neurite differentiates between sibling neurites of the same cell and those of a homologue is not known, but possible mechanisms include the recognition of surface markers of neuronal identity or the detection of cell-specific patterns of activity. In order to test whether this self-recognition requires a neurite to be in direct communication with its soma, we used a laser microbeam to sever a branch of a dye-filled pressure-sensitive (P) neuron in an intact leech embryo. Time-lapse observations of the P cell arbor in the living, unanesthetized, animal for up to 24 h following the surgery showed that the detached branch continued to show dynamic growth behavior throughout the period of observation. However, the detached branch ceased being avoided by the rest of the cell within a few hours, other, attached branches of the neuron overgrowing its territory and directly overlapping with it. Our experiments provide direct evidence for the existence of strong growth-inhibiting interactions between sibling processes, and indicate that self-avoidance by the growing neurites of a cell requires physical continuity between these neurites.