Date of Award


Degree Name

Master of Science


Biological Sciences

First Advisor

Dr. John Jellies

Second Advisor

Dr. Cindy Linn

Third Advisor

Dr. Christine Byrd-Jacobs

Fourth Advisor

Dr. John Spitsbergen

Access Setting

Masters Thesis-Campus Only


These studies investigate the developmental strategies used by two segmental homologs in the leech, Hirudo, to partition the heart tube (HT) into non-overlapping territories. The HT is innervated throughout its length by peripheral projections of heart accessory (HA) neurons found only in segments 5 and 6. The general hypothesis is that there is a mutual avoidance between HA5 and HA6. HA5 and HA6 seem to undergo contact-mediated retraction within the connectives during early embryonic development and may avoid fasciculation or overlapping innervation into adulthood. By inhibiting innervation by HA5 neurons, we tested the prediction that segmental homologs in G6 will innervate the unoccupied regions of the heart tube and exclude HA5. Intracellular dye injections of Neurobiotin™ in HA neurons were used to examine morphological differences among surgically manipulated animals and control animals that were raised to adulthood. Results confirm the hypothesis that when HA5 neurons are perturbed during development, HA6 neurons are permitted to reach and elaborate on the heart tube whilst excluding HA5 innervation of the same target. This study supports the proposed theory of homolog avoidance that governs the cell-cell interactions, target selection, pathfinding and generation of innervation territories by HA neurons.

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