Date of Award

6-1998

Degree Name

Master of Science

Department

Biological Sciences

First Advisor

Dr. John Jellies

Second Advisor

Dr. Christine Byrd

Third Advisor

Dr. Leonard Beuving

Access Setting

Masters Thesis-Open Access

Abstract

Our lab has been investigating the development of neuronal phenotype and circuitry in Hirudo medicinalis. Early in embryogenesis the heart excitor (HE) motor neuron selectively extends multiple arbors into the body wall prior to heart tube (HT) formation. HE contact with HT primordium at later stages of development results in remodeling such that arbors are elaborated on the HT, while those in the body wall are retracted. HT-derived cues are necessary for HE remodeling (Jellies, 1995. Am. Zool. 35:529; Jellies and Kopp, 1995. Invert. Neurosci. 1:145). HT and body wall explants were co-cultured to investigate the possible influences of target-derived cues in altering axon growth. Axon outgrowth was examined by labeling with a monoclonal antibody (mAb) directed against acetylated tubulin, and nucleated cells were labeled with bisbenzimide. Initially, adult tissues were co-cultured and most showed minimal axonal outgrowth, even in cases where target muscle was in close proximity. These adult axons, however, sprouted and grew upon the perineural sheath. In contrast, coculturing adult ganglia with embryonic or juvenile HT and body wall resulted in profuse axonal outgrowth on the younger target as well as extensive growth on the perineural sheath. The identities of the axons elaborating on the young HT or body wall have not yet been determined, but these results provide strong direct evidence that target-derived factors can effectively stimulate axon outgrowth in vitro. Also, they argue that the adult neurons remain competent to respond to HT-derived factors, but that HT may lose its ability to express or present these factors.

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