Innervation of the tylotrich-touch dome complexes in rat skin: changing patterns during postnatal development.

The tylotrich-touch dome complexes of the rat were studied in detail at thoracic level, with two objectives: to follow the pattern of innervation of the individual complexes from birth to maturity and to determine the extent of overlap of the segmental nerves supplying them. Techniques included light and electron microscopy and histological observations following section of intercostal nerves. The touch domes were nearly always supplied from a single stem axon; as expected, their terminals increased in number in association with the differentiation of target Merkel cells from the epidermis. In general, they were supplied from the nearest segmental nerves. The tylotrich follicles were each supplied by several stem fibres. The number of palisade terminals applied to the epithelial root sheaths reached a maximum during the 2nd and 3rd postnatal weeks and declined during the following 2 wk. This overshoot can be regarded as another example of hyperinnervation found in the juvenile peripheral nervous system. During the period of decline, the stem fibres extended their territory, resulting in considerable overlap of the territories of the segmental nerves. By the beginning of the 8th week, overlap was relatively scanty, with an irregular distribution.