Embryonic maturation of sensory terminals of primate facial hairs

The present study examines the sequence of maturation of sensory nerve terminals that can be identified on primate facial guard hairs. At birth, the sensory innervation of both guard and vellus hairs is mature in that lanceolate, Ruffini, and free nerve ending (FNE) terminals can be identified and resemble those of the adult. Presumptive lanceolate terminals can be identified at the beginning of the third trimester of gestation, and other axons present are either Ruffini or FNEs, but definite identification is not possible. In the latter half of the second trimester only axons arranged circumferentially are present, resembling FNE or Ruffini terminals. Some of these axons directly abut the hair follicle and might eventually become lanceolate endings. The earliest nerve terminals associated with hairs cytologically resemble FNE or Ruffini terminals. At the onset of hair differentiation in the early part of the second trimester, nerves were always associated with developing epithelial hair placodes. Branching Schwann cells in the axons radiating toward the epidermis in these youngest embryos studied were best identified by electron microscopy. The early development of afferent nerve fibers in hairy skin provides an anatomical substrate for the known reflexogenic activity of primate embryos. Adequate cytologic criteria are thus available for the identification of sensory terminals in growing, differentiating, and presumably regenerating axons in primate hairy skin.