Airway epithelial cells: current concepts and challenges

Abstract

The adult human bronchial tree is covered with a continuous layer of epithelial cells that play a critical role in maintaining the conduit for air, and which are central to the defenses of the lung against inhaled environmental concomitants. The epithelial sheet functions as an interdependent unit with the other lung components. Importantly, the structure and/or function of airway epithelium is deranged in major lung disorders, including chronic obstructive pulmonary disease, asthma, and bronchogenic carcinoma. Investigations regarding the airway epithelium have led to many advances over the past few decades, but new developments in genetics and stem cell/progenitor cell biology have opened the door to understanding how the airway epithelium is developed and maintained, and how it responds to environmental stress. This article provides an overview of the current state of knowledge regarding airway epithelial stem/progenitor cells, gene expression, cell-cell interactions, and less frequent cell types, and discusses the challenges for future areas of investigation regarding the airway epithelium in health and disease.

Figure 1.

Major cell types of the lung epithelium. In the large airways (2⁰ to 2⁵ branches), the major cells types are ciliated, undifferentiated columnar, secretory, and basal cells. In the small airways (2⁶ to 2²³ branches), the cell types are similar, with relatively more ciliated cells, and the secretory cells shift to the Clara cell type. After 2²³ branches, the airway epithelium merges with the alveolar epithelium, with type I and type II cells. Not shown are a variety of less common cell types, such as cartilage cells and mucus glands in the large airways, and neuroendocrine cells.