The role of lung epithelial stem cells in maintenance and repair of the adult lung is ill-defined, and their identity remains contentious because of the lack of definitive markers for their prospective isolation and the absence of clonogenic assays able to measure their stem/progenitor cell potential. In this study, we show that replication of epithelial-mesenchymal interactions in a previously undescribed matrigel-based clonogenic assay enables the identification of lung epithelial stem/progenitor cells by their colonyforming potential in vitro. We describe a population of EpCAM hi CD49f pos CD104 pos CD24 low epithelial cfus that generate colonies comprising airway, alveolar, or mixed lung epithelial cell lineages when cocultured with EpCAM neg Sca-1 pos lung mesenchymal cells. We show that soluble fibroblast growth factor-10 and hepatocyte growth factor partially replace the requirement for mesenchymal support of epithelial colony formation, allowing clonal passaging and demonstration of their capacity for self-renewal. These data support a model in which the adult mouse lung contains a minor population of multipotent epithelial stem/progenitor cells with the capacity for self-renewal and whose descendants give rise to airway and alveolar epithelial cell lineages in vitro.colony-forming assay | lung epithelium | lineage specificity | differentiation | EpCAM

Originally identified as a marker specifying murine hematopoietic stem cells, the Sca-1 antigen has since been shown to be differentially expressed by candidate stem cells in tissues including vascular endothelium, skeletal muscle, mammary gland, and prostate of adult mice. In the adult murine lung, Sca-1 has previously been identified as a selectable marker for the isolation of candidate nonhematopoietic (CD45 Sca-1 1 CD34 1 cell phenotype does define endogenous clonogenic progenitor cells in the adult murine lung, our data indicate that these progenitors are predominantly representative of mesenchymal cell lineages, and highlights the pressing need for the identification of alternative markers and robust functional assays for the identification and characterization of epithelial and fibroblastic stem and progenitor cell populations in the adult lung. STEM CELLS 2009;27:623-633 Disclosure of potential conflicts of interest is found at the end of this article.

Apoptosis is a cell suicide programme characterised by unique cellular events such as mitochondrial fragmentation and dysfunction, nuclear condensation, cytoplasmic shrinkage and activation of apoptotic protease caspases, and these serve as the noticeable apoptotic markers for the commitment of cell demise. Here, we show that, however, the characterised apoptotic dying cancer cells can regain their normal morphology and proliferate after removal of apoptotic inducers. In addition, we demonstrate that reversibility of apoptosis occurs in various cancer cell lines, and in different apoptotic stimuli. Our findings show that cancer cells can survive after initiation of apoptosis, thereby revealing an unexpected potential escape mechanism of cancer cells from chemotherapy.

Radiation-induced loss of mouse brain endothelial cells has been examined in mice given an intravenous injection of the DNA-binding radioprotector Hoechst 33342 (80 mg kg-1). At the time of irradiation, 10 min after injection, Hoechst fluorescence in the brain was confined to the endothelial cells. Endothelial cell density was measured using a histochemical fluorescence technique that had been used previously to monitor post-irradiation changes in endothelial cell density in rat brain, in which it was shown that a sensitive subpopulation comprising about 15% of the endothelial cells was lost within 24 h of radiation exposure. The present study shows a similar dose-response for the control mice, with depletion of the sensitive subpopulation to 85% being almost complete after a dose of 2.5 Gy gamma-rays. However, in mice irradiated 10 min after Hoechst 33342 administration, doses between 12 Gy and 20 Gy were required to ablate these cells. The kinetics of cell loss and the rather large dose modification factor suggests that Hoechst 33342 may be suppressing an apoptotic response in this subpopulation. Whatever the mechanism involved, Hoechst 33342 clearly provides substantial protection against early radiation-induced endothelial cell loss. Further studies are necessary to determine the extent to which this initial protection translates into an improved long-term survival of the "protected" cells and, especially, to see whether this endothelial cell protection can ameliorate the later consequences of central nervous system irradiation, namely necrosis and paralysis.

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