Detection of calcium binding sites in Merkel cell granules of fetal rat skin

We utilized a cytochemical calcium-staining technique to detect a potential calcium binding site within Merkel cell granules. Calcium has been shown to be essential for exocytotic release in various neuroendocrine cells and recent investigations strongly support the hypothesis that Merkel cells should be considered to be neuroendocrine cells which release their specific granules as a part of their physiological functions. After the addition of 50 mM CaCl2 to glutaraldehyde as the primary fixative, buffer, and OsO4 as a postfixative, electron-dense particles with a characteristic staining pattern were found within many MCGs. Successful removal of these particles after incubation of sections in a solution containing EDTA indicated that these particles were formed by calcium deposits. The occurrence of calcium binding structures within MCGs may imply that they play a role in the process of granule release and/or granule maturation.