Figure 6

Abnormal calyx formation and hair cell innervations in absence of a functional TrkB/PLCγ site. (A-F) P8 utricular afferent projections labelled with lipophilic dye injected either into the cerebellum (red) or the brainstem (green) were compared. Note that the overall sorting of afferent fibers within the utricle is comparable between the Trkb^PLC/PLC mutant mice and controls. However, many more fibers overlap in the Trkb^PLC/PLC mutants compared to control (yellow in A, B). (C) Closer examination shows numerous hair cells surrounded by partial or complete calyces (indicated by dashed white circles in the striola region) in Trkb^WT/WT controls, white circles indicate those near the edge of the striola region that are found only in Trkb^WT/WT control mice. White arrow points to a peripheral calyx with nerve fiber coming off (so called mixed calyx/bouton fiber in the non-striola region). (D) Note absence of calyces outside the striola region in Trkb^PLC/PLC mutants, and only occasional calyces inside the striola region of the Trkb^PLC/PLC mutant mice (indicated by dashed white circles). Moreover, the long distances fibers are running inside the sensory epithelium as well as along the perimeter in the Trkb^PLC/PLC mutant mice (arrowhead) (B, D), whereas much shorter and branched trajectories are typical in control mice (C). (E-F) Schematic drawings showing hair cell innervation within the striola region of the utricular sensory epithelium (E-F); the striola region is characterized by polarity reversal of hair cells and the presence of calyces around type 1 hair cells. Note that calyces in Trkb^PLC/PLC mutants tend to be smaller, less frequent in the striola region and incomplete or absent near the perimeter (F); whereas Trkb^WT/WT controls form calyces both inside and outside of the striola region (E). Moreover, unique to Trkb^PLC/PLC mutants are afferent fibers that run along the perimeter of the sensory epithelia for long distance. Scale bars, 100 μm.