Development of Mechanosensory Innervation in the Frog, Xenopus Laevis

Abstract

ABSTRACT DEVELOPMENT OF MECHANOSENSORY INNERVATION IN THE FROG, XENOPUS LAEVIS by Peter Feuk The University of Wisconsin-Milwaukee, 2017 Under the Supervision of Dr. R. David Heathcote This study aims to investigate whether a specific target cell in the epidermis of the African clawed frog, Xenopus laevis, guides the initial outgrowth and pattern of Rohon-Beard (RB) cells and their survival. RB cells are primary mechanosensory neurons present during the early developmental stages of X. laevis. These neurons provide sensory input to the frog throughout embryonic and larval development before initiating apoptosis around the start of metamorphosis. The innervation of embryonic skin cells by RB neurons exhibits a distinct pattern that features encircling of a specific subset of epidermal cells. We hypothesize that encircled cells could be a recently discovered cell type that synthesizes and secretes serotonin. To test whether these small secretory cells (SSC) play a chemoattractive role in the innervation of primary mechanosensory neurons in X. laevis, we used a variety of approaches. By immunolabeling RB processes in the epidermis, we established the wild-type pattern and quantity of innervation. Gain of function and loss of function tests utilizing pharmacological modulators of serotonin, directly examined the role of SSCs. We showed that serotonin levels can be over or under expressed, and that there is a quantifiable response of the resulting mechanosensory innervation. RB neurons innervating larval skin lacking serotonin showed a lower frequency of encircled SSCs as well as a greater number of retraction bulbs. We also showed a correlation between behavior and serotonin levels in the epidermis. Loss of function treatments showed a higher proportion of animals failing to respond to stimuli and an increase in inappropriate escape responses. Finally, we related changes in X. laevis larval skin and mechanosensory neurons during metamorphosis to tissue remodeling and the transition to adult sensory function. This research provides insight into axonal guidance and the patterning of mechanosensory innervation in X. laevis. It stands as an example of how the innervation of the vertebrate skin is established and maintained between developmental stages.