Touch dome properties as a function of age

Merkel cells

Merkel cells are post-mitotic cells scattered throughout the epidermis of vertebrates. They are particularly interesting because of the close connections that they develop with sensory nerve endings and the number of peptides they can secrete. These features suggest that they may make an important contribution to skin homeostasis and cutaneous nerve development. However, these cells remain mysterious because they are difficult to study. They have not been successfully cultured and cannot be isolated, severely hampering molecular biology and functional analysis. Merkel cells probably originate in the neural crest of avians and mammalians, and their "spontaneous" appearance in the epidermis may be caused by a neuron-independent epidermal differentiation process. Their functions are still unclear: they take part in mechanoreception or at least interact with neurons, but little is known about their interactions with other epidermal cells. This review provides a new look at these least-known cells of the skin. The numerous peptides they synthesize and release may allow them to communicate with many cells other than neurons, and it is plausible that Merkel cells play a key role in skin physiology and physiopathology.

Encoding of compressive stress during indentation by slowly adapting type I mechanoreceptors in rat hairy skin

The mechanical state encoded by slowly adapting type 1 mechanoreceptors (SAI) during indentation was examined using an isolated preparation in a rat model. Skin and its intact innervation were harvested from the medial thigh of the rat hindlimb and placed in a dish, with the corium side down, containing synthetic interstitial fluid. The margins of the skin were coupled to an apparatus that could stretch and apply compression to the skin. Using a standard teased nerve preparation, the neural responses of single SAIs were identified. SAIs were stimulated, using controlled compressive stress while simultaneously measuring displacement, by compressing the skin between indenters (flat cylinders) of different diameters and a hard platform. SAIs were subcategorized according to whether their neural response saturated above or below 10 kPa compressive stress (SAI-H or SAI-L, respectively). Linear regression was used to evaluate the relationships between neuron response and stress and force and displacement. For all SAIs, the mean neural response was significantly and substantially more highly correlated with compressive stress than force or displacement. For the SAI-L subcategory, the mean correlation coefficient was significantly and substantially greater for stress than for force but not significantly different for displacement. The data from this study support the hypothesis that SAI mechanoreceptors stimulated by indentation encode compressive stress rather than force, displacement, or strain.

Serotonin-like immunoreactivity in Merkel cells and their afferent neurons in touch domes from the hairy skin of rats

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.

Effects of vincristine sulfate on touch dome function in the rat

The responses of touch domes in hairy skin of the rat to mechanical stimulation were examined after single doses of vincristine sulfate. Within 24 h of drug administration, the mean thresholds of domes to brief mechanical pulses had increased threefold, from 5.2 +/- 2.0 to 14.2 +/- 8.8 microns. This elevated threshold was maintained for 2 weeks but by the 3rd week the domes had recovered normal excitability. Measurements of response latency suggested that the increase in receptor thresholds occurred without impulse propagation being impaired in the axons.