Donor‐recipient interconnections between giant nerve fibers in transplanted ventral nerve cords of earthworms

Abstract

Twelve segments of ventral nerve cord (VNC) from donor earthworms, Eisenia foetida, were transplanted into recipient worms from which a comparable length of VNC had been removed. Within the first few days after transplantation, bud-like formations, containing outgrowths of the giant nerve fibers, were evident at the ends of transplanted and recipient VNC. Morphological and electrophysiological evidence indicated that by 4-10 days after transplantation, medial (MGF) and lateral (LGF) giant fibers within the transplanted VNC formed cell-specific connections with their counterparts in the recipient VNC. Although the diameters of the giant fiber connections in the transplant-recipient junctions were often larger than normal, spike conduction across the junction was initially slow (approximately 1.0 m/s) but gradually increased over the next 2-3 weeks. Within the transplant, giant fibers were initially normal in appearance, but spike conduction was slow (1-2 m/s). During the next few weeks velocities increased by as much as fourfold and then stabilized for the next several months. However, by 4-5 weeks after transplantation, giant fiber morphology within the transplant was altered significantly, as indicated by the formation of numerous branch-like extensions along the length of each giant fiber. By 9-10 months there were further morphological changes in the transplant, as indicated by decreased branching of the giant fibers and altered neuropile. Despite these morphological changes, through-conduction of giant fiber spikes remained reliable.

Behavioral plasticity and central regeneration of locomotor reflexes in the freshwater oligochaete, Lumbriculus variegatus. II: Ablation studies

“…A body part or tissue, when grafted to an inappropriate site, is assigned a new positional context and could potentially influence neighboring tissue. Most research involving grafted body parts has been done on invertebrates such as hydra, snail, and annelids (Marcum et al 1977;Bode & Bode 1984;Vining & Drewes 1985;Syed et al 1992;Minobe et al 1995). For example, grafted snails neurons survive and integrate into the host system, but proximate effects of these grafts on functions of neighboring host neurons were not determined (Syed et al 1992).…”

“…It is well established that giant nerve fiber pathways (interneuronal in origin) in the ventral nerve cord of oligochaete worms are capable of rapid and reliable reconnection after ventral nerve cord transection (Drewes et al 1988). Previous studies of such reconnection have focused on terrestrial oligochaetes, such as Lumbricus terrestris and Eiseniu foetidu (Balter et al 1980;Vining & Drewes 1985). in these species, the timing of functional recovery (i.e., restoration of giant nerve fiber through-conduction after transection) varies from -1-2 d in Eisenia (small redworm) to 5-18 d in Lurnbricus (large nightcrawler). in the preceding study (Lesiuk & Drewes 2001), functional recovery of giant nerve fiber throughconduction after transection was demonstrated in an aquatic counterpart of earthworms, Lurnbriculus vuriegatus (common name: blackworm).…”

Restoration of sensory and motor function in earthworm escape reflex pathways following ventral nerve cord transplantation

“…Next, a 12 segment long portion of VNC from a donor was transplanted into a recipient animal as described previously (Vining and Drewes, 1985). As shown in Table 1, donor VNC from a posterior origin was placed into a posterior site in Group PP recipients.…”

“…Some of the animals that showed restoration of sensory and motor function were sacrificed for intracellular recordings and histological studies (Vining and Drewes, 1985). For intracellular recordings, animals were anesthetized on ice, dissected to expose the VNC, and covered with a thin layer of saline (Drewes and Pax, 1974).…”

“…Nevertheless the earthworm nervous system, and in particular, escape reflex circuitry, appear well suited for studies involving heterotopic transplantation. First, the giant nerve fibers in transplanted ventral nerve cord survive and form connections with the giant fibers in recipient animals (Vining and Drewes, 1985). Second, although there is little variation in the gross morphology of body segments, there are pronounced intersegmental gradients and discontinuities with respect to the functional properties of the escape reflex, especially those of sensory and motor pathways (Mill, 1975;Drewes, 1984).…”