Lungs are composed of a system of highly branched tubes that bring air into the alveoli, where gas exchange takes place. The proximal and distal regions of the lung contain epithelial cells specialized for different functions: basal, secretory and ciliated cells in the conducting airways and type II and type I cells lining the alveoli. Basal, secretory and type II cells can be grown in three-dimensional culture, with or without supporting stromal cells, and under these conditions they give rise to self-organizing structures known as organoids. This Review summarizes the different methods for generating organoids from cells isolated from human and mouse lungs, and compares their final structure and cellular composition with that of the airways or alveoli of the adult lung. We also discuss the potential and limitations of organoids for addressing outstanding questions in lung biology and for developing new drugs for disorders such as cystic fibrosis and asthma.

SUMMARY Cardiac tissue undergoes renewal with low rates. Although resident stem cell populations have been identified to support cardiomyocyte turnover, the source of the cardiac stem cells and their niche remain elusive. Using Cre/Lox-based cell lineage tracing strategies, we discovered that labeling of endothelial cells in the adult heart yields progeny with cardiac stem cell characteristics that express Gata4 and Sca1. Endothelial-derived cardiac progenitor cells were localized in the arterial coronary walls with quiescent and proliferative cells in the media and adventitia layers, respectively. Within myocardium, we identified labeled cardiomyocytes organized in clusters of single-cell origin. Pulse-chase experiments showed that generation of individual clusters was rapid, but confined to specific regions of the heart, primarily in the right anterior and left posterior ventricular walls and the junctions between the two ventricles. Our data demonstrate that endothelial cells are an intrinsic component of the cardiac renewal process.

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Vitamin A (retinol) is a necessary nutrient for all higher animals, including many invertebrates which use it as their visual chromophore. All of these animals must get vitamin A in their diet and must be able not only to transport this hydrophobic vitamin to target cells but must also convert it into active forms of the vitamin, such as retinal and retinoic acid. In mammals, retinol is converted into retinyl esters for export from the small intestine, for storage in the liver, and as part of the visual cycle. Little is known about these processes in invertebrates. Mammals have two enzymes that can catalyze the conversion of retinol to retinyl‐esters: lecithin:retinol acyltransferase (LRAT) and acyl‐coA:retinol acyltransferase (ARAT). LRAT is the physiologically important one in mammals. We have identified an ARAT activity in microsomes from the hepatopancrease of the freshwater crayfish, Procambarus clarkii. We were unable to detect any LRAT activity, suggesting crayfish only have one enzyme that can catalyze this reaction. This activity shows a linear dependence on time and microsome concentration. It shows temperature dependence and an optimal pH of 6.5. We have characterized its acyl‐CoA preference and determined Km and Vmax's for both retinol and the acyl‐CoA.