Ultrastructural study of the Merkel cell and its expression of met-enkephalin immunoreactivity during fetal and postnatal development in mice.

The morphological changes and the expression of met-enkephalin immunoreactivity of Merkel cells during fetal and postnatal development were investigated in touch domes and sinus hair follicles of mice by transmission electron microscopy. In prenatal fetal mice, the Merkel cells were mainly oval in shape and had slightly lobulated nuclei. These fetal Merkel cells (14, 16, 18 d gestation) which were not innervated showed a large number of accumulated dense-core granules in their cytoplasm as compared with the innervated Merkel cells which appeared in adult mice. No Merkel cells could be found in d 10 and d 12 fetuses. Innervation of Merkel cells was found to increase with age. The location of Merkel cells in juvenile, adult and even old mice was very similar, cells being found mainly in the basal layer of the epithelium. Using the electron-microscopic immunogold method, met-enkephalin-like substance was consistently located in the dense-core granule region of both innervated and noninnervated Merkel cells throughout the whole developmental stage. Interestingly, it was also found that the labelling intensity of met-enkephalin immunoreactivity was significantly higher in Merkel cells of younger age groups than in adult and old age groups. None of the nerve terminals associated with Merkel cells were labelled. The present study supports the theory of an epidermal origin of Merkel cells followed by the trophic growth of nerve fibers induced by the peptides.