Neuronal and non-neuronal responses to nerve crush in a pulmonate snail, Melampus bidentatus

Immunohistochemical and ultrastructural techniques were used to study sequelae of nerve injury in the pulmonate snail Melampus bidentatus. Either pedal or tentacle nerves were crushed, severing all axons, and recovery was monitored over 15 days. The axons regenerated from the segment attached to the soma, with no evidence of fusion of proximal and distal segments. The medium to large axons of central neurons, including those monitored with serotonin immunohistochemistry, grow distally across the path of smaller axons extending centrally from peripheral somata. The regions into which the growing axons projected were a focus of phagocytic activity. Cells previously labeled by PKH-26PCL, a fluorescent marker for phagocytic activity, were attracted to the crushed nerve within 6 h and were a consistent feature in the vicinity of the injury for at least 9 days, gradually extending their range as repair progressed in both directions from the crush. Repair proceeded within an intact sheath, and many sheath cells survived the crush, although the nuclear dye Hoechst 33258 revealed an initial distortion of their nuclei. The concentration of cells in the sheath in the crushed region increases after the crush, with the packing of nuclei peaking at 3 days and gradually returning to control conditions; this probably reflects migration of resident sheath cells. Cell division is rare in the sheath of intact nerves, but labeling with bromodeoxyuridine increases at the crush site between 4 and 9 days, indicating that cell replacement also occurs at the site.