Xanthogranuloma is the archetype of non-Langerhans cell histiocytoses.

Abstract

Sir, We read with great interest the recent editorial about ‘The confusing state of the histiocytoses’ by Dr Chu.1 We feel that several points arising from this editorial require a comment and further elucidation. For some years now it has been recognized that the application and use of the term ‘histiocytoses’ is indeed confusing. Historically, the term was first introduced by Kiyono and then made popular in the early 1930s by Aschoff who regarded histiocytes as part of the reticuloendothelial system.2 These cells, capable of phagocytosis and ameboid movement, were wrongly thought to derive from locoregionary connective tissue, hence the term ‘histiocyte’ (from the Greek ‘ιστοoν, meaning tissue). However, there are many more ‘histiocytes’, such as mast cells, fibrocytes, muscle cells, nerve cells and endothelial cells, and one could almost equally speak of ‘somatocytes’. Later, van Furth et al.3 recognized that this is a much more dynamic system. Macrophages derive from stem cells in the bone marrow, circulate through the organism as monocytes via blood, leave the blood system into the various tissues for phagocytosis, or alternatively stay there for some time, and then finally die; other variants such as Langerhans cells4 home into the epidermis or other epithelia, digest and process antigens, leave the skin, and migrate via lymphatics to the T‐cell dependent zones of lymph nodes. While there is continuous recruitment of all types of macrophages from the bone marrow, those cells having performed their function die: this can be seen in tuberculoid or palisading granulomas of conditions such as tuberculosis, granuloma annulare, necrobiosis lipoidica or necrobiotic xanthogranuloma. Some of these cells may derive from locally resting tissue macrophages in the sense of ‘histiocytes’ as originally defined;2 most macrophages are derived from the circulation. The term ‘histiocyte’ does not clarify if macrophages were already in the tissue before tissue damage or were recruited from the blood afterwards. In our view ‘dermal dendrocytes’5 are just another evasion of this question, which cannot be answered by pure morphological criteria. A wide variety of cells, and not only ‘precursors of macrophages’,5 may have dendritic morphology, including Langerhans cells, melanocytes, fibrocytes and endothelial cells. Their heterogeneity has been outlined by immunohistochemistry which shows a factor XIIIa‐positive variant in the papillary dermis, but a CD34‐positive variant in the reticular dermis. Moreover, both of these cell types are by no means restricted to the dermis, but may be seen as factor XIIIa‐positive macrophages in the lung for the former, or CD34‐positive dendrocytes in perivascular subcutaneous tissue for the latter, respectively.