Localization of Merkel cells in the monkey skin: an anatomical model

The Merkel cell-neurite complex is considered to be one class of mechanoreceptors in the skin. Merkel cells are innervated by slowly adapting type I (SAI) tactile nerve fibers. In this paper, the detailed distribution of Merkel cells is studied by immunohistochemical labeling of the monkey (Macaca fascicularis) digital glabrous skin. Specific morphometric variables (density of intermediate epidermal ridges and Merkel cells, distance between skin surface and ridge tips and bases, maximum and average cell counts per ridge, distance between cells and ridges) were measured by a combination of light/fluorescence microscopy and computer-image analysis. The morphometric results were similar for each digit of the monkey's hand. Next, the anatomical data were used to form a three-dimensional reconstruction of the Merkel-cell distribution in the fingertip skin. A patch of the distal-pad surface was then computationally flattened to obtain the two-dimensional distribution of Merkel cells. Based on previous anatomical and physiological data, SAI fibers were simulated to innervate clusters of Merkel cells in the distal-pad surface. On average, 28 cells were innervated by a single fiber. The resulting anatomical model may be used to estimate the population response of SAI fibers by incorporating spike generation.