UC San Diego

During embryonic development, axons that project into the peripheral tissue encounter many different cues that can affect their final phenotype. We have studied the development of individually identified motor neurons in medicinal leeches (Hirudo verbana). We hypothesized that contact with peripheral targets provide signals that direct the formation of central synaptic connections. To test this hypothesis, we transplanted pieces of body wall into ectopic locations to ask if the central connections of motor neurons change when they contact the "wrong" peripheral target. We used embryos at 47-50% of development, a stage at which neurons are just beginning to form their central electrical connections. We transplanted tissue from a donor embryo and implanted it into the opposite region of a host embryo. We then let our embryos develop to a juvenile stage. External pigment patterns indicated that transplanted tissues retained their original fate. Immunostaining for acetylated tubulin revealed that the transplanted tissue became innervated, although less densely than un-manipulated tissue. Injections of AlexaFlour 488 dextran into motor neurons adjacent to an ectopic transplant indicated that the axons of these neurons projected into the periphery similarly to unaltered controls. Injecting Alexa Flour 488 plus Neurobiotin, which crosses gap junctions, revealed no statistically significant difference in the number of cells that were dye-coupled to the filled cell in embryos that received ectopic transplants, although the neurons and connections within the ganglion become markedly un- patterned, suggesting that disrupting the periphery exerts at least some influence in the central nervous system