Merkel cells in human fetal eccrine glands.

The presence of human Merkel cells in the eccrine ridges and eccrine germs was studied, using antibodies to simple epithelial keratins, in separated epidermal sheets with attached eccrine ducts. The localization of Merkel cells could be analysed three-dimensionally in the wet, whole-mount of the stained sheets. In the plantar skin of a 12-week-old human fetus, immunoreactive (ir-) Merkel cells were randomly located in the flattened epidermis. In the plantar skin of a 15-week-old human fetus, there was early development of eccrine germs, and Merkel cells were concentrated in eccrine gland ridges. In the plantar skin of a 20-week-old human fetus, eccrine germs were well formed and ir-Merkel cells were located within the developing eccrine ridges and ducts. In the plantar skin of adults, the eccrine concentration of Merkel cells was markedly reduced. Concentration of Merkel cells on the eccrine structures was also observed in the scalp skin of human fetuses. This tendency continued into adult life, although there was a marked reduction in the total number of Merkel cells. These findings suggest that epidermal Merkel cells move down into the eccrine ducts as eccrine germs extend into the mesenchyme. Alternatively, they may develop de novo from the keratinocytes of the eccrine duct. In view of the expression of nerve growth factor receptor in fetal Merkel cells, it is postulated that these eccrine gland Merkel cells play a role in the formation of the periglandular nerve plexus.