Introduction Cell death is an important part of the cell cycle. The cell lives, consumes energy, and divides itself repeatedly until a moment when these activities are no longer sustained and the cell dies. Upon this death, it is possible to detect diffe- rent morphological, biochemical, and molecular features that allow us to distinguish the two basic types of cell death – apoptosis and necrosis. While necrosis and various me- chanisms inducing it have long been known and under- stood, it is the intricate mechanism of apoptosis that remains unresolved and has led scientists to focus on it du- ring the last decades (10). Apoptosis is defined as programmed cell death, thus, so- mething quite natural and physiological. According to this definition, every eukaryotic cell seems to have an internal ge- netic program which serves to destroy its bearer when ne- cessary. Its importance is best illustrated by the fact that different pathways and mechanisms of this program are still being identified and every year new facts appear (15). Apoptosis or apoptosis-like processes are also induced by many external factors, a fact adding to the overall com- plexity of this phenomena. Although in this case it is pro- blematic to use the definition “programmed cell death,” the behavior and appearance of externally treated cells resem- ble very closely those of naturally dying cells. Since the int- roduction of apoptosis as one of the fundamental types of cell death, tens of external factors have been reported to be apoptotic inducers. These so called xenobiotics, include many chemicals, drugs, and physical factors, some known to be beneficial, while others known to be harmful to living organisms (2). This is also the case of hexavalent chromium. While the cytotoxic, genotoxic, and carcinogenic potential of hexava- lent chromium (8,12,14) has been known for almost eighty years, its ability to induce apoptosis was not recognized un- til recently. Several studies published over the last five years (3,5,13) showed that hexavalent chromium compounds cau- sed cells to express the typical apoptotic features: i.e. cell shrinkage, formation of membrane blebs, nucleus and DNA fragmentation and marginalization. Still, the expres- sion of these features in the studied cells is generalized and considered by many universal, despite the growing eviden- ce of possibly varying cell behavior in response to apopto- tic stimuli. How the specific chromium compound interacts with the particular cells is described rather well (initial as well as terminal stages) (13), but the course of its cytotoxic activi- ty leading to apoptosis, especially its dynamic aspects re- main relatively unexplored. To find out, we lined up several experiments that would lead us to some of the answers con- cerning the dynamics of cell death after exposure to potas- sium chromate, an important environmental xenobiotic not often included in studies. Materials and Methods Materials Potassium chromate - K 2 CrO 4 was purchased from Sigma (Cat. No. P 0454), dissolved in DMEM without se- rum and kept at low temperature as 1.0 mol/l stock solu- tion. 83 ORIGINAL ARTICLE THE DYNAMICS OF THE HEXAVALENT CHROMIUM INDUCED APOPTOTIC PATTERNS IN VITRO Emil Rudolf, Jan Peychl, Miroslav Červinka Charles University in Prague, Faculty of Medicine in Hradec Králové: Department of Medical Biology and Genetics Summary: Although hexavalent chromium has been shown to induce apoptosis in cells cultivated in vitro, there appear to be no studies focusing on the dynamics of this process. To find out about dynamic patterns of hexavalent chromium-in- duced apoptosis, we treated Hep2 cells with 150 µg/ml potassium chromate and recorded their behavior as well as appe- arance of some crucial organelles using different morphological and biochemical methods. We found that Hep2 cells showed the earliest observable changes at 6 hours after the treatment (blebbing, chromatin shrinkage), with the entire apo- ptotic process lasting up to 24 hours. While all the observed cell features clearly prove apoptosis induced by hexavalent chromium, a typical apoptotic hallmark, DNA ladder, seems not to occur in this type of cells. On the other hand, in HL- 60 cells, used as a control, this ladder was observable. Key words: Potassium chromate; Apoptosis; Morphological methods; DNA ladder; Nuclear changes ACTA MEDICA (Hradec Králové) 2000;43(3):83-89