R

van Furth. 1969

Bone marrow

Peripheral blood

Tissues

CorreclivC tissue ¬°histiocytes) Skin (tusllocytesj Liver (Kupfler cells) Spleen (macrophages) Lymph nodes (free and ll*ed macrophages) Thymus

Bono marrow (rssiderit macrophages)

Bone (osteoclasts)

Synovia (type A cell)

Lung (alveolar and tissue macrophages)

Mueose-associaled lymphoid tissues

GesHointestinel trscl

Genito-unnary Irracl

Endocrine organs

Central nervous system (macrophages, (reactnro) microglia, cerebrospinal llutd macrophages)

Body cavIIIc*

Pleural macrophages Peritoneal macrophages

Inllsmmalfon

Exudate macrophages Epithelioid ceils Multinucleated giant cells

Figure 1 Mononuclear phagocyte system.

marrow origin, and their morphological and/or functional characteristics Dendritic cells occur in lymphoid organs, e.g., lymph nodes, thymus, and spleen, but also in organs such as lungs, skin, liver, small intestine, etc., originate from precursor cells in the bone marrow. These precursor cells are not yet characterized nor are the mononuclear cells in the circulation that differentiate in the tissues into dendritic cells. In cultures of bone marrow and blood mononuclear cells, in the presence of GM-CSF and IL-4, dendritic cells develop, which supports the bone-marrow origin of these cells. Stem cell factor (e-kit ligand) increases the yield of dendritic cells, and maturation of dendritic cell is promoted by TNF, LPS and other agents. Furthermore, glucocortico-

steroid, which induce a rapid decline of circulating mononuclear cells, including monocytes, cause also a rapid disappearance of dendritic cells from the tissues. Together, these findings support the view that dendritic cells derive from precursors in the bone marrow, but whether monocytes can differentiate in dendritic cells is still uncertain. Although dendritic cells have also many features in common with macrophages, such as cell-surface markers, functions like endocytosis, antigen processing and presentation, these cells can still not yet be considered as bona fide mononuclear phagocytes, because their derivation from monocytes has not yet been firmly established.

Langerhans cells have the same life history and functions as dendritic cells. They derive also from precursors in the bone marrow cells, and circulating mononuclear cells that migrate to the skin differentiate into Langerhans cells. The sojourn time of Langerhans cells in the skin is short and next the cells are transported, via the lymph draining the skin, as veiled cells, which are cells with long cytoplamic viels, to the lymph nodes were they reside as inrer-digitating cells.

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