Ultrastructure of halo nevi.

An ultrastructural study was conducted on eight halo nevi. Some of them were of recent onset. Others had an evolution of several months. The distribution of the population of the dendritic cells in the depigmented epidermis of the halo was more polymorphous at the early stage when few melanocytes with abundant cytofilaments and some Langerhans cells were seen. At an advanced clinical stage, only Langerhans cells were present in the basal layer. In the dermis abnormal nerves as in vitiligo were observed. The inflammatory infiltrate does not seem to appear at the early stage of involution of the nevus. At the advanced clinical stage, small mononuclear cells were also noticed among the inflammatory cells (lymphocyte-like cells, macrophages, mast cells). It is suggested that an initial stage of inhibition of both the melanocytes and the nevus cells may be responsible for the development of the depigmented halo. This stage precedes the appearance of a dermal lymphomacrophagic infiltrate that leads to the destruction of the nevus.