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A frequent cause of contact urticaria is skin exposure to the common stinging nettle (Urtica dioica). The urticaria is accompanied by a stinging sensation lasting longer than 12 h. Little is known of the cellular and molecular mechanism of stinging-nettle urticaria. After preliminary pharmacological analysis of pro-inflammatory activity in nettle stings, the cellular response of mononuclear cells, polymorphonuclear cells and mast cells was examined in six people 5 min and 12 h after nettle contact. Only mast cell numbers were significantly increased at 12 h. Ultrastructurally, some mast cells showed evidence of degranulation at 5 min and 12 h. At 12 h mast cells were closely associated with dermal dendritic cells and lymphocytes suggesting a functional unit. The mean histamine and serotonin contents of a nettle hair were found to be 6.1 ng and 33.25 pg, respectively. Nettle-sting extracts did not demonstrate histamine release from dispersed rat mast cells in vitro. These results suggest that part of the immediate reaction to nettle stings is due to histamine introduced by the nettle. However, the persistence of the stinging sensation might suggest the presence of substances in nettle fluid directly toxic to nerves or capable of secondary release of other mediators.

Development and differentiation of the single free cells of mesenchyme and dermis of human embryos and fetuses from week 6 to term is described. From week 6 to week 14, three cell types are present: stellate general mesenchymal cells with long processes, phagocytic macrophages of probable yolk-sac origin, and a granule-secretory type of cell, which could be either a melanoblast or a mast stem cell. From week 14 to week 21, fibroblasts are numerous and active, and perineurial cells, pericytes, melanoblasts, mast cells, and Merkel cells can be individually identified. There is also present another cell type, possible of bone marrow origin, that may be ancestral to the Langerhans cell and that may be carried over into postanal dermis as the "histiocyte" or fixed dermal macrophage. From week 24 to term there is little change apart from the development of fat cells in the deeper dermis. Neither lymphocyte nor plasma cell was observed at any stage of development. These observations were used in the specification and identification of cells of fully developed postnatal dermis.