Epidermal Merkel cells in psoriatic lesions: immunohistochemical investigations on neuroendocrine antigen expression

Biopsy specimens from lesional psoriatic skin and from normal controls were investigated by immunohistochemistry for the presence of epidermal Merkel cells (MC). MC were defined as epidermal cells expressing simple-type keratins, i.e. nos. 8, 18, and 19. A significant number of MC could be found at the bottom of the rete ridges of psoriatic lesions (about 19.6 MC per square mm skin surface area) and of normal skin (about 14.0 MC per square mm surface area). In contrast to normal skin, MC of psoriatic lesions were positive for synaptophysin (21.7% of simple-type keratin positive epidermal cells, i.e. MC), pancreatic polypeptide (14.8%), somatostatin (7.0%), and chromogranin A (less than 3%). The immunostaining was rather faint though significantly different from normal skin. The findings suggest that in psoriasis, epidermal MC show variations of the expression of neuropeptides compared to normal skin. Since some of the neuropeptides are thought to be involved in hyperproliferation and/or skin immunology, our findings might suggest a functional activity of epidermal MC in psoriatic lesions different from normal controls.