The role of merkel cells in mechano-transduction

Abstract

In whatever receptor system Merkel cells are found tney are always associated with a characteristic, slowly adapting response. The role of Merkel cells in the transduction process of slowly adapting Type I cutaneous mechanoreceptors (SAI receptors or touch domes) of rats and cats was investigated by mecnanical and electrical stimulation of SAI receptors and their afferent fibres in various experimental conditions. In an hypoxic environment, touch domes eventually failed to respond to mechanical stimulation. For receptor failure to occur it was necessary to have an 0^ depleted external environment around the limb as well as an 02 deficient blood supply. Exposure of the limb surface to 02 was sufficient to bring about receptor recovery. Correlated with receptor failure was a significant reduction in the number of densecored vesicles normally found in the Merkel cell cytoplasm adjacent to the nerve ending innervating the cell. Receptor recovery was associated with a return in the numbers of dense-cored vesicles back to that found in control cells.In an hypoxic environment, touch domes eventually failed to respond to mechanical stimulation. For receptor failure to occur it was necessary to have an 0₂ depleted external environment around the limb as well as an 0₂ deficient blood supply. Exposure of the limb surface to 0₂ was sufficient to bring about receptor recovery. Correlated with receptor failure was a significant reduction in the number of densecored vesicles normally found in the Merkel cell cytoplasm adjacent to the nerve ending innervating the cell. Receptor recovery was associated with a return in the numbers of dense-cored vesicles back to that found in control cells.When stimulated in the presence of the known calcium channel blockers cobalt and verapamil hydrochloride, SAI receptors again eventually failed to respond to mechanical stimulation. Correlated with receptor failure was a reduction in the number of Merkel cell vesicles as well as an increase in the number of 'synaptic-like' junctions oDserved between Merkel cells and their associated nerve endings.In hypoxic conditions and in the presence of the calcium channel blockers a response could still be produced in the afferent fibre by electrical stimulation of the fibre close to the touch dome.The results indicate that Merkel cell dense-cored vesicles are necessary for the characteristic slowly adapting response of SAI mechanoreceptors possibly by the secretion of a transmitter substance stored within the vesicles.