Ultrastructural and X-ray microanalysis of U-937 cells in hypertonia-induced apoptosis

Abstract

The results of this study of the ultrastructural changes in U-937 cells in the apoptotic state are largely consistent with analogous data available in the literature. However, we also obtained original data on the ultrastructural changes in cellular organelles and immunocytochemical localization and distribution of proteasomes. It was shown that, when apoptosis was induced by incubation of cells in a hypertonic sucrose solution (200–400 mM), the Golgi apparatus (GA) was localized close to the plasma membrane. This fact is indirect evidence that cytoskeletal elements (particularly microtubules, which hold GA in the center of the cell) are depolymerized already at the early stages of apoptosis. At the late stages of apoptosis, the distance between individual GA cisterns significantly increases, which is apparently due to the cleavage of their binding proteins, golgins. Mitochondria in apoptotic cells do not change significantly. They have regularly spaced cristae and a fairly dense matrix containing single vacuoles. At all stages of apoptosis, the nucleus and cytoplasm of U-937 cells contained proteasomes, which were represented by small rod-shaped osmiophilic particles approximately 12 × 30 nm in size. These particles formed aggregates of varying density and size, not covered with a membrane. We assumed that they are similar to the “processing bodies,” or aggresomes, which were described in the literature. The latter are detected in cells under conditions of suppressed transcription in the nucleus and, according to some researchers, are involved in the storage and degradation of various mRNAs, RNPs, and proteins. X-ray spectral analysis revealed changes in the intracellular content of Na+ and K+ at the level of single cells during apoptosis induced by osmotic shock. An increased ratio of intracellular Na+/K+ compared to the control for the majority of cells in apoptosis was shown.