Tissue Factor and Thromboplastin

Tissue Factor (TF) (aka Factor 3 or CD142) is a protein with a key role in coagulation. TF is present in large concentrations in subendothelial tissue of vessels (so by smooth muscle cells and fibroblasts). The endothelium itself can also express TF but only in imflammatory states, same for circulating monocytes.
TF is responsible for triggering the so called extrinsic pathway of coagulation (or TF pathway as its also known). This path is triggered when the vessel is damaged and there is exposure of the underlying TF. It acts as a receptor for Factor 7 which leads to the cleavage of Factor 10 and the activation of the common pathway of coagulation and thrombin activation.

Thromboplastin is the name of a lab reagent and is actually the combination of TF and phospholipids. TF and phospholipids together can trigger coagulation. Partial thromboplastin is just the phospholipids by themselves and can trigger the intrinsic pathway of coagulation (TF not needed to activate intrinsic pathway). So when an 'activated partial thromboplastin time' APTT test is order it is a measure of the intrinsic path of coagulation.

Picture highlighting TF (yellow box), the extrinsic pathway (red box) and common pathway of coagulation (green box). #medicine #coagulation #labs #tissuefactor #bleeding #medicalschool #medicalblog #medED #FOAMed #study