SPLTRAK Abstract Submission
Morphological Classification of Taste Fibers in the Mouse Tongue
Tao Huang & Robin Krimm
University of Louisville, Louisville, KY, United States

The morphological characteristics of peripheral taste neurons is unknown. So we traced 96 taste fibers from the mouse tongue using genetic sparse labeling. We observed tremendous variation in taste neuron complexity, some neurons branch little (a fiber entering a single taste bud with a total of one branch end), while others branch heavily (innervating seven taste buds with 40 total branch ends).  A total of 16 morphological characteristics of the traced fibers were analyzed using K-means clustering analyses, which revealed four categories of the traced fibers: category 1 (44%), category 2 (37%), category 3 (14), and category 4 (5%). Total branch length within taste buds was the single morphological feature that provided the best separation between the categories (p<0.01).  Other measures of branching complexity like the total number of branch ends and the largest branch order were also different across the four categories (p<0.05). The simplest branches (Category 1) were shorter (147±9.8 vs 475±28.9 μm) and innervated fewer taste buds (1.88±0.14 μm), compared to the other three categories (3.81±0.26, p<0.001). The number of separate fibers entering the taste bud predicts differences in total branch length (= 0.75), while the mean branch length within the taste bud does not (= -0.29). If each separate fiber contacts an average of 1.6 taste receptor cells (as EM data suggests), category 1 branches should connect with approximately 4 receptor cells while category 4 fibers would connect with 16 receptor cells. Taken together, the categorization of taste fibers in the tongue suggests increasing convergence across categories, which likely explains differences in degree of tuning.