Postnatal loss of Merkel cells, but not of slowly adapting mechanoreceptors in mice lacking the neurotrophin receptor p75.

Merkel cells are specialized epidermal cells which are abundantly found in touch-sensitive areas and which are innervated by slowly adapting mechanosensitive afferent fibres with large myelinated (Abeta) axons. The role of Merkel cells in mechanosensation, their developmental regulation and their influence on sensory neuron function are, however, incompletely understood. Here, we used mice lacking the neurotrophin receptor p75 which is expressed on Merkel cells to investigate their postnatal development and that of their innervating sensory neurons. Using morphological studies we now show that Merkel cells develop normally in both hairy and glabrous skin in these animals until 2 weeks old, but are progressively lost thereafter and have almost completely disappeared 2 months after birth. Using standard extracellular electrophysiological recording techniques we find that despite the profound loss of Merkel cells there is no corresponding reduction in the number of myelinated slowly adapting afferent fibres. Moreover, the mean mechanical threshold of these neurons and their average stimulus response function to suprathreshold mechanical stimuli does not change during the time period when more than 99% of Merkel cells are lost. We conclude that Merkel cells require p75 during the late postnatal development. However, neither the survival nor the mechanical sensitivity of slowly adapting mechanoreceptive Abeta-fibres depends on the presence of Merkel cells.