Relationship between Merkel cells and nerve endings during embryogenesis in the mouse epidermis

Close relationships between Merkel cells (MC) and nerve endings (NE) exist in the adult mouse. Because MC may serve as targets for the ingrowth of NE during embryogenesis, the purpose of the present study was to analyze the relationship between MC and NE during embryogenesis. Frozen tissue from whisker pads and backs of NMRI mouse embryos (12-17 d gestational age) were studied by double-labeling indirect immunofluorescence (IIF) with a cytokeratin monoclonal antibody that recognizes MC and with a neurofilament anti-serum. Such an approach allowed the analysis of a large number of MC (up to 5000), thus yielding quantitative data. At day 12 of gestational age, no MC were observed by IIF. From day 13 to 17, the number of MC, as well as their association with NE, progressively increased. On day 13, only 57% of whisker pad MC were NE associated, whereas by day 17, 95% were NE associated. These results were confirmed by electron microscopic (EM) observations. On the back, the same chronologic relationship between MC and NE was observed, but was later in the course of embryogenesis. There was also a time- and zone-dependent increase in MC association with NE in the epidermal zones studied (isthmic, parafollicular, interfollicular). These observations 1) establish the time course of MC and NE contacts during embryogenesis in the mouse epidermis, 2) show that MC are present in the epidermis and appendages before NE reach the epithelium, and 3) support the hypothesis that MC could act as targets for the growing NE.