Successive waves of differentiation of cutaneous afferents in rat mystacial skin.

The present study has traced the sequence of maturation of sensory receptors in the mystacial pad of postnatal rats. At birth the follicle-sinus complexes (F-SC) are well innervated by deep vibrissal nerves although the number of axons entering the sinus is less than that in the adult. The innervation of the F-SC by the conus or superficial vibrissal nerves derived from skin nerves that form the superficial dermal nerve plexus is limited to the Merkel rete ridge collar at birth, and the innervation to the inner conical body is conspicuously absent. The inner conical body innervation begins to appear 3-4 days after birth and rapidly matures over the week. By 3 weeks of age the F-SCs have a mature sensory innervation. At birth small guard hairs are present in the intervibrissal pelage and are associated with scant axons of the superficial dermal nerve plexus, but no mature sensory terminals are present. The sensory innervation of the intervening pelage begins to differentiate during the second week and mature piloneural complexes can be recognized by 3 weeks of age. Innervation to vellus hairs is still developing at 3-4 weeks of age. These maturational changes in peripheral sensory innervation correlate with gradual changes in the structure of barrels in the first somatosensory cortex (SI). Sequential waves of differentiation of sensory receptors appear to be a general feature of neural development.