Modulation of keratinocyte proliferation by skin innervation

Degeneration of nociceptive nerve terminals in human peripheral neuropathy

Patients with peripheral neuropathy have symptoms involving small-diameter nociceptive nerves and elevated thermal thresholds. Nociceptive nerves terminate in the epidermis of the skin and are readily demonstrated with the neuronal marker, protein gene product 9.5 (PGP 9.5). To investigate the pathological characteristics of elevated thermal thresholds, we performed PGP 9.5 immunocytochemistry on 3 mm punch skin biopsies (the forearm and the leg) from 55 normal subjects and 35 neuropathic patients. Skin innervation was evaluated by quantifying epidermal nerve densities. Epidermal nerve densities were reduced in neuropathic patients compared to normal subjects. Epidermal nerve densities were variably correlated with thermal thresholds. The proportion of neuropathic patients with reduced epidermal nerve densities was larger than the proportion of neuropathic patients with elevated thermal thresholds. These results indicated that degeneration of epidermal nerve terminals preceded the elevation of thermal thresholds. Skin biopsy together with immunocytochemical demonstration of epidermal innervation offers a new approach to evaluate small-fiber sensory neuropathy.

Neuroendocrinology of the skin

The classical observations of the skin as a target for melanotropins have been complemented by the discovery of their actual production at the local level. In fact, all of the elements controlling the activity of the hypothalamus-pituitary-adrenal axis are expressed in the skin including CRH, urocortin, and POMC, with its products ACTH, alpha-MSH, and beta-endorphin. Demonstration of the corresponding receptors in the same cells suggests para- or autocrine mechanisms of action. These findings, together with the demonstration of cutaneous production of numerous other hormones including vitamin D3, PTH-related protein (PTHrP), catecholamines, and acetylcholine that share regulation by environmental stressors such as UV light, underlie a role for these agents in the skin response to stress. The endocrine mediators with their receptors are organized into dermal and epidermal units that allow precise control of their activity in a field-restricted manner. The skin neuroendocrine system communicates with itself and with the systemic level through humoral and neural pathways to induce vascular, immune, or pigmentary changes, to directly buffer noxious agents or neutralize the elicited local reactions. Therefore, we suggest that the skin neuroendocrine system acts by preserving and maintaining the skin structural and functional integrity and, by inference, systemic homeostasis.

Pathology of nerve terminal degeneration in the skin

To characterize the pathology of epidermal nerve degeneration and regeneration, we investigated temporal and spatial changes in skin innervation of the mouse footpad. Within 24 hours after sciatic nerve axotomy, terminals of epidermal nerves appeared swollen and there was a mild reduction in epidermal nerve density (5.7 +/- 2.8 vs 12.7 +/- 2.2 fibers/mm, p < 0.04). Epidermal nerves completely disappeared by 48 hours (0.2 +/- 0.2 vs 14.2 +/- 0.9 fibers/mm, p < 0.001). Concomitant with the disappearance of epidermal nerves, the immunocytochemical pattern of the subepidermal nerve plexus became fragmented. At the electron microscopic level, the axoplasm of degenerating dermal nerves was distended with organelles and later became amorphous. Beginning from day 28 after axotomy, collateral sprouts from the adjacent saphenous nerve territory extended into the denervated area with a beaded appearance. They never penetrated the epidermal-dermal junction to innervate the epidermis. In contrast, 3 months after nerve crushing, the epidermis on the surgery side resumed a normal innervation pattern as the epidermis on the control side (10.3 +/- 3.9 vs 10.6 +/- 1.5 fibers/mm, p = 0.1). This study demonstrates the characteristics of degenerating and regenerating nerves, and suggests that successful reinnervation mainly originates from regenerating nerves of the original nerve trunks. All these findings provide qualitative and quantitative information for interpreting the pathology of cutaneous nerves.