Effects of psoralen on the structural integrity of cultured osteoblasts. Phase contrast, immunofluorescent, and electronmicroscopic evaluation.

The 72-h dark interaction of cultured osteoblasts with 0.5-1.0 microgram/ml 8-methoxypsoralen (8-MOP) resulted in the accumulation of cytoplasmic lipid droplets (steatosis) in target cells. These methanol-extractable lipid droplets and 8-MOP, however, had no microscopically detectable effect on the organization of alpha-actin and beta-tubulin-containing filaments. On the ultrastructural level, psoralen effects ranged from negligible to unambiguous structural alterations of target cells. The latter consisted of blebbing, segmental deletions of the nuclear envelope, and structural unraveling of nucleoli. Moreover, the dilatation of the rough endoplasmic reticulum, the decrease in the number of ribosomes, and the extensive vacuolation of cytoplasm constituted yet another hallmark of the 8-MOP effect in the absence of light. Whereas psoralen has induced a number of structural alterations in some, but not in all osteoblasts, more than 95% of the target cells have, nonetheless, remained viable. Taken together, our results suggest that the dark reaction of psoralen with osteoblasts results mainly in transient structural alterations of affected cells.