SUMMARY Short-term hypertonic (HT) stress induces apop- totic cell death in human EUE cells in culture, as observed by electron microscopy, agarose-gel electrophoresis of low-molecular-weight DNA, DNA flow cytometry and annexin-V-propidium iodide double-staining. During HT-induced apop- tosis, nuclear ribonucleoprotein (RNP)-containing structures undergo rearrangement, with the forma- tion of Heterogeneous Ectopic RNP-Derived Structures (HERDS) which pass into the cyto- plasm, as already reported for other examples of spontaneous and drug-induced apoptosis. Of spe- cial interest was the observation that nucleolus- like bodies (NLBs) which resemble morphologi- cally nuclear functional nucleoli may be extruded into the cytoplasm of apoptotic cells and are observed inside the cytoplasmic fragments bleb- bing-out at the cell surface; these NLBs still con- tain immunodetectable nucleolar proteins (such as fibrillarin). This is an additional example of RNP- containing structures of nuclear origin which are extruded from the nucleus, in an almost “native” form, during apoptosis. 247 INTRODUCTION Eukaryotic cells are able to react to a wide spec- trum of environmental stresses through changes in gene expression and the synthesis of the so-called stress proteins (Morimoto et al., 1994). These stress-induced functional alterations may be paral- leled by changes in the structural and kinetic fea- tures of cells, like those observed in human embry- onic epithelial (EUE) cells as a consequence of the hypertonic (HT) stress (Pellicciari et al., 1989, 1991, 1995). In fact, upon long-lasting (at least a few days) exposure to a HT culture medium, EUE cells increase their size and protein content, modi- fy their gene expression with an increased synthe- sis of a 33 kDa stress protein (which proved to be aldose reductase: Giuliani et al., 1991; Ferraretto et al., 1993; Pellicciari et al., 1993), and decrease their proliferation potential with the exit of most of the cells from the cycle into a G0 quiescent state (Pellicciari et al., 1989, 1991). At least some of these features relate to the resistance against the HT stress: namely, the increased expression of aldose reductase may directly influence the syn- thesis of organic osmolytes necessary for resisting Rearrangement of nuclear ribonucleoproteins and extrusion of nucleolus-like bodies during apoptosis induced by hypertonic stress C. Pellicciari 1 , M.G. Bottone 1 , A.I. Scovassi 2 , T.E. Martin 3 , and M. Biggiogera 1 1 Dipartimento di Biologia Animale and Centro di Studio per l’Istochimica del CNR, University of Pavia, Piazza Bot- ta 10; 2 Istituto di Genetica Biochimica ed Evoluzionistica del CNR, Via Abbiategrasso 207, I-27100 Pavia (Italy) and 3 Department of Molecular Genetics and Cell Biology, University of Chicago, 1103 East 57th Street, Chicago, IL 60637 (USA) Accepted: 16/6/00 Key words: apoptosis, nucleolus, ribonucleoproteins, hypertonicity, immunocytochemistry, electron microscopy. ORIGINAL PAPER Eur. J. Histochem. 44: 247-254, 2000 © Luigi Ponzio e figlio - Editori in Pavia Correspondence to: C. Pellicciari E-mail: pelli@unipv.it 04. Pellicciari 18-02-2002 13:47 Pagina 247