Immunohistochemical study of cutaneous nerves in the emu.

The distribution and chemical content of cutaneous nerves in 3- to 13-day-old emu chicks (Dromaius novaehollandiae) were examined by using double-labelling immunohistochemistry. Seven different subpopulations of cutaneous nerves were identified based on their neurochemistry. No intraepidermal nerve fibres were found. However, axons were located within the dermis and were often associated with blood vessels, pennamotor muscles and feather follicles or innervated Herbst corpuscles. Both similarities and differences exist between subpopulations of cutaneous nerves in the emu and volant birds. As in volant birds, a subpopulation of cutaneous axons innervates the superficial skin layers and contains immunoreactivity to both substance P and calcitonin gene-related peptide (CGRP). This suggests that the neuropeptide content of these presumptive free nerve endings is conserved throughout the evolution of birds. In contrast, Herbst corpuscles in the emu are innervated by axons that contain immunoreactivity for CGRP or neuropeptide Y (NPY) but that lack the calbindin D-28k immunoreactivity found in fibres innervating Herbst corpuscles of volant birds. Herbst corpuscles therefore may have a different chemical content in a flightless species from that in volant birds.