Introduction

Histiocytes, represent the cells of the mononuclear phagocyte system (MPS) that can give rise to histiocytoses when they accumulate pathogenically in tissues due to dysregulation of proliferation or survival. This nomenclature may be somewhat misleading as histiocytes, in the strict sense, are macrophages located in connective tissue. However, macrophages and dendritic cells (DC) of different origins, which now are joined in the MPS, can give rise to different forms of histiocytosis, and this deviation is not restricted to connective tissue macrophages. As no alternative term exists that better describes the aberrant accumulation of mononuclear phagocytes, the term ‘histiocytes’ in the current context thus indicates mononuclear phagocytes in general.

Heterogeneity of the MPS

The MPS consists of hematopoietic cells with very diverse characteristics, originally recognized as related cells on the basis of a common lineage derivation and primary function of endocytosis (van Furth et al., 1972; van Furth, 1980). In general, cells of the MPS originate in bone marrow and migrate as monocytes through the blood to peripheral tissues, where they develop into mature cells with different features depending on tissue-specific environmental conditions. Macrophages occur virtually in all organs. Moreover, each organ contains multiple, phenotypically different macrophage subpopulations. For example, lung hosts at least two different macrophage types: alveolar macrophages in alveolar spaces and interstitial macrophages integrated in the tissue. Microglia are phagocytes of the nervous tissue, osteoclasts of bone and Kupffer cells of liver.