Structural features of isolated, fractionated bone marrow endothelium compared to sinus endothelium in situ.

Structural features of isolated, fractionated rat bone marrow endothelium were compared to those of marrow sinus endothelium in situ. Marrow endothelium was purified, first by density gradient sedimentation on Percoll and then subjected to centrifugal elutriation. Using antifactor VIII antibody staining (indirect immunofluorescent method), preparations of greater than 50% purified endothelium were obtained. By SEM, these cells were about 10 microns in size and showed smooth surface and numerous invaginations. These features were also observed in the in situ endothelium obtained by perfusion-fixation and freeze-cracking. In addition, in situ endothelium displayed numerous hemopoietic cells in migration through the endothelium. By TEM, isolated endothelium showed numerous vesiculations, giving the cell sponge-like appearance. This corresponded to numerous intracellular vesicles in sinus endothelium in situ, reflecting high magnitude of fluid and molecular transport across the endothelium. Weibel-Palade bodies were not seen in either form of the endothelium, despite the positive reaction for factor VIII-related antigen. This finding suggested that the cell, while possessing factor VIII-related antigen, does not store this protein.