The effect of destroying the whisker follicles in mice on the sensory nerve, the thalamocortical radiation and cortical barrel development

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Abstract
Electrolytic destruction of whisker follicles in mice on the day of birth caused degeneration in the sensory nerve fibers supplying the follicles. The severity of the degeneration was assessed in animals between 2 and 20 days old by counting the total number of myelinated fibers in the maxillary nerves on both normal and lesioned sides. The degeneration was apparent after 2 days and by 20 days the nerve on the lesioned side contained only 38% of the normal fiber content. This degeneration also involved the trigeminal root, central to the ganglion. The lesioning procedure modifies the terminations of thalamocortical fibers in the barrel region of the sensory cortex. These terminations are normally in clusters, each corresponding to a barrel, but, after lesioning the follicles, the terminals appear to be evenly distributed in layer IV and cortical barrel structures no longer develop. In postnatal mice, electrolytic destruction of whisker follicles had less effect on maxillary nerve fibers and cortical barrels. The number of myelinated axons surviving until day 20 increased progressively with later lesioning to reach nearly 80% of the control level when lesions were made on day 10. Cortical barrels became secure earlier than the maxillary nerve, for a normal number of cortical barrles was present at day 12 when follicles were destroyed on day 4. The implications of these results for the formation of cortical barrels is discussed.