Three-dimensional analyses of touch domes in the hairy skin of the cat paw reveal morphological substrates for complex sensory processing

The three-dimensional morphology of the innervation of touch domes in the hairy skin folds of cat forepaws was investigated by the confocal laser scanning microscopic analyses of sections stained immunocytochemically with primary antibodies for protein gene product 9.5, neurofilament 200 and cytokeratin 20 in combination with transmission electron microscopic observations. One square centimeter of interdigital skin can contain as many as 68 touch domes. Each touch dome can have up to 150 Merkel cells and all are innervated by a single large-caliber afferent myelinated nerve fiber at the level of the palisade endings around the guard hair. It gives rise to multiple long, myelinated branches. Each final myelinated branch gives rise to several short and fine unmyelinated branches, supplying approximately 15 Merkel cell–axon complexes. Each Merkel cell is typically contacted by multiple small discoid endings instead of by a large single one. Discoid endings on separate Merkel cells were usually the distal ends of the unmyelinated branches, although, some were en-passant swellings of the branches. Only a few Merkel cell–axon complexes at the marginal zone of each territory could also be supplied by adjacent final myelinated branches. Each Merkel cell is surrounded by protrusions of keratinocytes that are penetrated by several collagen bundles of the dermis. This intricate pattern of innervation may explain the unique irregular discharges of action potentials typical for this type of mechanoreceptor.