Immunoreactivity to serotonin was observed in Merkel cells as well as the afferent type I nerves terminating upon them in touch domes excised from the belly skin of rats. Type I nerves were strongly immunoreactive and could be traced through the dermis of the domal papilla. Merkel cell immunoreactivity was sometimes seen in the entire cell, but was often localized in the Merkel cell cytoplasm adjacent to nerve terminals and may have been in the terminals themselves. Domes were fixed by immersion in 4% paraformaldehyde-lysine-sodium-m-periodate (PLP) fixative at 4 degrees C for 2.5-3 hours and cryoprotected in 30% sucrose overnight. Sections were processed with the avidin-biotin complex peroxidase (ABC), peroxidase-antiperoxidase (PAP), and indirect immunofluorescence techniques with rabbit antiserum generated against serotonin.

No abstract

The ultrastructure of Merkel cells and cutaneous nerves was examined in fetal, newborn, and 7-day-old rats. The earliest observation of cells having some, but not all, of the features of mature Merkel cells was at 16 days gestation in snout skin. These early presumptive Merkel cells resembled the neighboring keratocytes, except that they contained dense-cored vesicles scattered in the cytoplasm. Presumptive Merkel cells were seen only in the epidermis, although a careful search was made of the dermis. Developing neurons were not observed to penetrate the epidermal basal lamina when presumptive Merkel cells were first seen. The earliest observation of identifiable nerve axons in the snout epidermis was at 17-171/2 days gestation. Study of the presumptive Merkel cells through successively older gestational stages showed that the cells became innervated and progressively developed the characteristics of adult Merkel cells. We suggest that Merkel cells arise from keratocyte-like precursors in rat epidermis, at a time when skin nerves may still be several micrometers away.

Touch domes are clearly delineated mechanoreceptors that are visible on the depilated skin of mammals. These structures consist of a sharply circumscribed disk of thickened epithelium surmounting a group of Merkel cells that are innervated by type I sensory neurons. These characteristic cutaneous structures provide an ideal opportunity for investigating whether the localization of nerve growth factor (NGF) in the skin is related to sites of sensory axon termination. For these reasons, we have used immunocytochemistry to study the distribution of NGF and the low-affinity NGF receptor (p75NGFR) in the touch domes of adult rat skin. Intense NGF-like immunoreactivity was sharply restricted to keratinocytes (excluding the stratum corneum) of the thickened epidermis of touch domes. The epidermis immediately surrounding touch domes and the epidermis of the tylotrich hair follicle associated with touch domes were not stained by anti-NGF antiserum. Merkel cells of the basal epidermis of touch domes were immunonegative for NGF but were immunopositive for p75NGFR as were the type I nerve endings innervating these cells. Quantitative Northern blotting revealed that the level of NGF mRNA was substantially higher in keratinocytes isolated from the stratum granulosum and stratum spinosum than in keratinocytes isolated from the stratum germinativum. These findings indicate that NGF synthesis in mature skin has a highly restricted regional distribution that is primarily associated with the innervation of a specialized touch receptor.

Denervation of specialized cutaneous mechanoreceptors (Haarscheiben or domes) in cats was followed after 20 and 25 days by the following alterations in receptor structure: (1) reduced numbers of Merkel cells, (2) Merkel cells degenerating in situ, (3) fewer dense-core granules in the cytoplasm of Merkel cells, (4) an increased number of agranular dendritic cells and Langerhans cells in the dome, (5) the apparent phagocytosis of Merkel cells by Schwann and Langerhans cells, (6) fewer epithelial cell layers over the dome, and (7) a decrease in the number of transitional cells. Skin excised between the domes in the denervated nerve field appeared normal when compared to innervated skin, and it was considered unlikely that the alterations in dome structure were due to generalized nutritional changes in the skin caused by transection of sympathetic axons or to some other side effect of denervation. Since domes are formed in new locations on the skin after nerves have regenerated (Burgess et al., '74), changes in dome structure following nerve transection are probably due to loss of the "trophic" influence of the nerves supplying the dome.