The morphological diversity of trigeminal sensory neurons in the developing zebrafish

The vertebrate trigeminal sensory ganglion (TgSG) is responsible for the detection of chemical, mechanical and thermal stimuli. Individual neurons within this ganglion extend a peripheral axon into the skin to detect cues from the environment. Meanwhile, a central axon projects to the hindbrain to relay this information. These neurons can be divided into large diameter neurons capable of mechanical detection and small diameter neurons capable of noxious stimuli and innocuous temperature detection. Considerable research has been focused on discerning patterns in morphology and axon organization amongst these different functional subsets. However, little is known about how these patterns are established during early development. My thesis work examined the development of peripheral axon arbors expressing the noxious chemical receptor, Trpa1b. Generation of a trpa1b reporter revealed that these arbors are large and more branched when compared to previously described isl1SS sensory arbors. Furthermore, unlike isl1SS, peripheral axons commonly invade into the other side of the head. Despite this contralateral coverage, transplantation experiments indicate that trpa1b arbor territories are still limited by mutual repulsion from neighboring arbors. Interestingly, time-lapse measurements of developing arbors indicate that trpa1b axons may arborize a greater territory by traveling through the skin faster. This remains true even when peripheral axons grow in the absence of neighboring neurons, suggesting that these speed differences are independent of mutual repulsion. Collectively, these studies show that while mutual repulsion serves as a general inhibitory cue, differential outgrowth speeds contribute to the larger territories attributed to trpa1b arbors. Furthermore, my studies also demonstrate the usefulness of functional receptor markers in revealing early and unique patterns in peripheral morphology and receptive field coverage.