Endothelial cells (EC) possess receptors for and metabolize low-density lipoprotein (LDL); acetylation of LDL promotes EC metabolism and the intracellular deposition of its cholesteryl ester component. In turn, acetylated LDL linked to the fluorescent dye, Dil (Dil-Ac-LDL), has been used as a marker for endothelial cells, including cerebral microvascular endothelial cells (CEC). Due to the low electron density of fluorescent dyes, however, detection of this marker is not possible ultrastructurally. We demonstrate here the photo-conversion of the chromogen, 3,3'-diaminobenzidine tetrahydrochloride (DAB), into a stable, electron-dense reaction product in subcultured human fetal CEC containing the fluorescent marker, Dil-Ac-LDL, thus promoting identification of these cells via light and electron microscopy. This method, in turn, may be useful in identifying and tracking human fetal CEC grafted into SCID mice as a model for blood-brain barrier angiogenesis.

Isolated human fetal CEC were subcultured and labeled with Dil-Ac-LDL (10 ug/ml).