Absence of Meissner corpuscles in the digital pads of mice lacking functional TrkB.

The TrkB-expressing sensory neurons seem to be involved in touch and other discriminative sensibilities. Thus, several slowly and rapidly adapting cutaneous mechanoreceptors, as well as muscle spindles, are reduced or absent in the territory of the trigeminal nerve in functionally TrkB-deficient mice. Whether this also occurs in the cutaneous or muscular territories of dorsal root ganglia has not been analyzed. Here we used immunohistochemistry and transmission-electron microscopy to analyze the impact of a mutation in the gene coding for TrkB on Meissner and Pacinian corpuscles, and muscle spindles. The animals were studied at the post-natal days 15 and 25, because at this time all the mechanoreceptors examined are fully developed. Typical Meissner's corpuscles, displaying S-100 protein immunoreactivity, were found in the digital pads of wild-type and TrkB+/- mice whereas they were absent in the TrkB-/- animals. Regarding Pacinian corpuscles, the mutation in the trkB gene does not alter either the immunohistochemical or the ultrastructural characteristics. Finally, in muscle spindles the arrangement of the intrafusal muscle fibers and nerve fibers was unchanged in the mutated animals. Nevertheless, about 10% of muscle spindles showed increased number of the intrafusal cells (between 6 and 12) and were supplied by more than one large myelinic nerve fiber. The present results strongly suggest that TrkB-expressing sensory neurons in dorsal root ganglia, like those of the trigeminal ganglion, are responsible for the development and maintenance of several rapidly adapting cutaneous mechanoreceptors, i.e. Meissner's corpuscles.