3 DIFFERENT CELL-TYPES CAN SYNTHESIZE FACTOR-XIII SUBUNIT-A IN THE HUMAN LIVER

The origin of human Factor XIII subunit A (FXIII A) has been a subject of intense speculation and investigation during the last decade. The major question under dispute is whether hepatocytes can produce this c lotting factor. Experimental evidence obtained by FXIII A phenotype an alysis in bone marrow transplant patients clearly identified hemopoiet ic cells (monocytes/macrophages and/or megakaryocytes/platelets) as a source of FXIII A, and also showed that additional extra-hemopoietic s ite(s) of synthesis also exist, The liver has been suggested as a poss ible extrahemopoietic source of plasma FXIII A, but the cells responsi ble for synthesizing FXIII A were not identified yet. Our present stud y was designed to determine the cellular distribution of both FXIII A and its encoding mRNA in human liver samples by using light- and elect ron-microscopic immuno-morphological and in situ hybridization techniq ues, In paraformaldehyde/glutaraldehyde (PA/GA) fixed, araldite-embedd ed semithin sections that were immunostained by an ABC/DAB based poste mbedding immunocytochemical method, FXIII A could be detected in Kupff er cells, connective tissue histiocytes and hepatocytes. Immunoreactiv e hepatocytes were observed almost exclusively around the venae centra les, By using postembedding immunogold labeling, FXIII A could be elec tron-microscopically visualized in these hepatocytes in the immediate vicinity of the lamellae of the endoplasmic reticulum. By in situ hybr idization using a mixture of five 32-40mer biotinylated oligonucleotid es (ONs) to mRNA regions encoded by exons VI, XI and XV and a labeling system containing streptavidin conjugated with alb:aline phosphatase/ BCIP/NBT, the message for FXIII A could he detected in the same cell t ypes. These results show that in human liver three different types of cells can synthesize FXIII A, but the extent of their contribution to the plasma FXIII A level will require further studies.