[Origin of epithelioid cells in sarcoid granuloma]

Sarcoid epithelioid cells are believed to be a variant of tissue macrophages, which are derived from circulating monocytes in blood. Formation of monocytes and macrophages are controlled by hematopoietic growth factors, i.e.; colony-stimulating factors in the bone marrow. In the presence of colony-stimulating factors and/or vitamin D3, blood monocytes can proliferate and differentiate into epithelioid cells and multinucleated giant cells. Recent observations that sarcoid granulomas themselves produce colony-stimulating factors and vitamin D3 suggest vitamin D3 and colony-stimulating factors produced by sarcoid granulomas stimulate the proliferation and differentiation of circulating monocytes into macrophage-epithelioid cells, which form new sarcoid granulomas.