This general distribution of epithelial cell types is conserved between humans and model organisms such as rodents. However, there are notable differences. For example, the transition from a pseudostratified to columnar epithelium occurs more proximally in rodents, so only the trachea and mainstem bronchi are lined with a pseudostratified epithelium. Nearly all intralobar airways in mice are lined with a simple columnar or cuboidal epithelium with few basal cells. In mice, the abrupt transition from a conducting airway to the alveoli is known as a broncho-alveolar duct junction. In humans, terminal bronchioles give rise to respiratory bronchioles from which many alveolar ducts terminate ultimately in alveoli. Considering essential role of epithelial compartment in the lung much effort has been done to identify epithelial stem and progenitor cells responsible for regenerative and reparative functions.