Every prothrombin-time result in a coagulation lab traces back to a tissue extract. Thromboplastin, the reagent that starts the extrinsic clotting pathway in the test tube, has been prepared from animal brain for a century, and rabbit brain is still the reference source. A look at what thromboplastin and its phospholipid cousin cephalin actually are, and at the other tissue extracts that turn up in functional assays.
Thromboplastin
Thromboplastin is a complex of tissue factor, the membrane protein that triggers coagulation when blood meets damaged tissue, together with the phospholipid it needs to work. Added to citrated plasma with calcium, it drives the extrinsic pathway to a fibrin clot. The time that takes is the prothrombin time (PT), the test used to monitor warfarin therapy and to screen for factor deficiencies.
The reagent is prepared by extracting brain tissue, classically rabbit brain acetone powder, although bovine brain and lung are also used. How sensitive a given thromboplastin is to each clotting factor depends on the tissue source and on the preparation, and is expressed as its International Sensitivity Index (ISI). Two lots with different ISI values give different clotting times on the same plasma, which is why reagent manufacturers care so much about the consistency of the starting tissue (see why quality and traceability matter in coagulation reagents).
Cephalin
The activated partial thromboplastin time (aPTT) tests the intrinsic pathway and needs phospholipid without tissue factor. That phospholipid fraction, rabbit brain cephalin, stands in for the platelet surface on which the intrinsic factors assemble. Paired with a contact activator such as kaolin or silica, it's the working reagent in aPTT kits and in the mixing studies used to tell factor deficiencies from inhibitors.
Other extracts
Not every tissue extract is about clotting. Bovine pituitary extract is the standard supplement for keratinocyte and epithelial cell culture, supplying the hormones and growth factors those cells need to proliferate in serum-free media. Thymus and other organ extracts serve as antigen sources, as controls in immunoassays, and as starting material for enzyme work.
Getting consistent results
For coagulation reagents, buy the extract, not just the tissue, from a source that can tell you how it was prepared, and calibrate each lot against a reference plasma before you rely on it. For culture supplements, qualify a new lot on your own cells before you switch. We harvest rabbit brain in house and prepare both the acetone powder and the cephalin ourselves, so the chain from animal to vial is documented. If you're developing a coagulation reagent and need a reserved lot or a bulk quantity, contact us to discuss it.