The specialized junctions between Merkel cell and neurite: an electron microscopic study.

Longitudinal serial sections of one half of the entire sinus hair of a mouse were examined by the electron microscope. Three neurites entering the outer root sheath from the perifollicular blood sinus were encountered. These were separate nerve trunks from those connected with perifollicular tactile nerve endings and exclusively innervated intrafollicular Merkel cells. Two types of specialized junctions were observed at the contact regions between Merkel cell plasma membrane and neurite plasma membrane: (i) desmosome-like structures in which small clear vesicles and/or the large cored vesicles of neurite and thicker membrane (post-synaptic?) of apposed Merkel cell were found ant (ii) synapse-like structures in which Merkel cell granules were concentrated near the plasma membrane and the membrane of the apposed neurite was usually thicker (post-synaptic?). In some of the synapse-like junctions the limiting membrane of Merkel cell granules fused with the Merkel cell plasma membrane and its content seemed to be discharged into the intercellular space. This suggested actual exocytotic secretion of Merkel cell granules. Juxtaposition of 2 types of junctions, i.e. (i) and (ii) above, was also found. This suggested the possibility that the reciprocal synapse would be present between Merkel cells and afferent neurites.