Pathway of granule formation in Merkel cells: an ultrastructural study.

Merkel cells (MC) are characterized by dense core granules morphologically similar to secretory granules of endocrine cells, as well as the presence in their cytoplasm of organelles involved in polypeptide synthesis. The aim of this study was to follow the pathway of granule formation in MC. Four compartments, the cis and trans Golgi cisternae, the condensing granules in the Golgi stacks, the bristle-coated granules, and the noncoated granules, are observed by transmission electron microscopy in MC. This suggests that: (1) the organelles usually involved in the formation of secretory polypeptides are present in the cytoplasm of MC; (2) the pathway of Merkel cell granule (MCG) formation appears to be similar to that of secretory endocrine or exocrine cells; and (3) MC might be involved in a secretory process.