Sources of tissue factor.

Tissue factor (TF) exhibits a distinct nonuniform tissue distribution. Thus, high levels are found in highly vascularized organs such as the lung, brain, and placenta; intermediate levels in the heart, kidney, intestine, testes, and uterus; and low levels in the spleen, thymus, and liver. Several cell types are known to express TF constitutively, such as astrocytes in the brain, epithelial cells enveloping organs and body surfaces, adventitial fibroblasts and pericytes, and cardial myocytes in the heart. Smooth muscle cells in the media of the vessel wall and monocytes/macrophages contain small amounts of TF, which is enhanced substantially upon activation of the cells. Endothelial cells probably do not express TF. The popular concept of TF serving predominantly as a hemostatic envelope encapsulating the vascular bed has been challenged recently by the observation that blood of healthy individuals may form TF-induced thrombi under conditions entailing shear stress and activated platelets, corroborating the notion of blood-borne TF. Accordingly, small amounts of decrypted TF activity is detected in calcium ionophore-stimulated monocytes, and microparticles from plasma of healthy subjects possess TF-like activity subject to partial inactivation by anti-TF antibody. In addition to microparticles, plasma TF also comprises the soluble alternatively spliced human TF and truncated TF, both of which probably require factor VIIa to be physiologically active. Although it has been suggested that activated platelets possess active TF, the notion of TF as an integral platelet component is contested by more recent data. Rather, platelets may be very important in decrypting monocyte TF activity in a process entailing transfer of TF to activated platelets.