Development of vertebrate cutaneous end-organ complexes.

The glabrous skin of vertebrates is populated by cutaneous end-organ complexes, sensory corpuscles that are the sites at which the qualities of contacting objects (form, sharpness, pressure, hardness or vibration) are transduced into electrical signals. Structurally, these mechanotransducers are comprised of an axon, glial cells, and connective tissue sheaths. The axon is the peripheral prolongation of an Aβ low-threshold mechanoreceptor neuron; the glial cells are represented by non-myelinating terminal glial cells; and the connective sheaths are specializations of the endoneurium and/or perineurium. The variable arrangement of these three elements gives rise to the morphotypes of the cutaneous end-organ complexes typical of mammals: Meissner, Pacini and Ruffini corpuscles, and those of birds: the Grandry and Herbst corpuscles. In this review, an update is made on the development of the individual cellular components of cutaneous sensory corpuscles, and of the cutaneous endo-organ complexes as a whole. In general, cutaneous endo-organ complexes develop through complex multidirectional interactions between the Aβ-axons of mechanoreceptors and the terminal glial cells (both of which are neural crest derivatives) and the surrounding mesenchyme. The development of cutaneous endo-organ complexes in birds, rodents and humans, and the molecular mechanisms that regulate them are detailed.