The appearance, density, and distribution of Merkel cells in human embryonic and fetal skin: their relation to sweat gland and hair follicle development.

The density and distribution of Merkel cells in human embryonic and fetal skin were studied using an immunolabeling technique on epidermal and dermal sheets obtained by ethylenediamine tetraacetic acid separation. Merkel cells were identified by the known cytokeratin markers CK20 and CK18. Merkel cells showed CK20 immunoreactivity as early as 56 d estimated gestational age (EGA) in the palmar epidermis (133.11 +/- 44.27 cells/mm2). The density increased rapidly, reaching a maximum of more than 1400 cells/mm2 at 80-90 d EGA. At this stage, the cells became distributed along the primary epidermal ridges. In the palmar epidermis of fetuses older than 100 d EGA, the distribution of Merkel cells showed the same pattern, but the density then decreased gradually. Merkel cells were not observed in ductal and glandular portions of eccrine sweat glands. In the epidermal sheets of hairy skin, a few cells were first seen in the fetus at 75 d EGA. At 100 d EGA, only a few Merkel cells were observed, mostly in the hair pegs and bulbous hair pegs. In the older fetus, ring-like arrangements and aggregates of Merkel cells were prominent in the infundibulum and bulge of hair follicles, respectively. Merkel cells were both globular and dendritic in shape. The ratio of dendritic to globular cells increased gradually until the period of highest Merkel cell density in both the glabrous and hairy skin. All Merkel cells located in the dermis were globular in shape. In accord with the results obtained, we postulate that Merkel cells may have some functional role in the formation and proliferation of eccrine sweat glands and hair follicle anlagen in developing skin.