Novel Histiocytoses Considered in the Context of Histiocyte Subset Differentiation

The article on a solitary congenital indeterminate cell histiocytoma by Levisohn et al1in the January issue of theArchivesrepresents the first recognized congenital variant of a cutaneous histiocytic disorder exhibiting the differentiation of so-called "indeterminate cells." These cells are one member of a family of bone marrow-derived histiocytic dendritic cells that are distinguished from monocytes or macrophages based, in part, on certain key features summarized in Table 1. As discussed below, additional ultrastructural and immunophenotypic characteristics can also be helpful for differentiating these cell types. The purpose of this article is to describe histiocyte subsets, to classify histiocytoses, and to briefly review some of the recently recognized forms of these diseases. At the outset, it must be acknowledged that use of the termhistiocytecan be problematic because it has not been defined uniformly in the past.2Nevertheless, at present, it appears to be a term