The origin and fate of Merkel cell granules--an ultrastructural study.

In order to elucidate the origin and fate of Merkel cell granules (MCGs), electron microscopic studies were carried out in fetal rat skin at day 20 of gestation. In addition to the ordinary processing of the tissue, we incubated a part of the tissue with a solution containing ionophore A23187 in order to capture the rarely observable exocytotic event. Based on our findings, a hypothetical model for the life cycle of Merkel cell granules is proposed as follows: I) Granule morphogenesis takes place in the rough endoplasmic reticulum and the GERL, from where immature granules are budding off, thereby exhibiting a bristle-like coat. II) Maturation and storage of MCGs takes place in the cytoplasm. III) Following stimulation, MCGs are released. IV) After the exocytotic granule release, MCG membranes are retrieved in the form of coated pits.