However, very complex cellular composition of the lung, comprising more than 40 different cell types, makes this objective challenging. The epithelial lining of the respiratory tract, composed of conducting and respiratory parts, varies along its proximo-distal axis. The conducting airways from the trachea to bronchioles of human lungs consist of pseudostratified epithelium, comprising equal proportions of basal cells, secretory cells, and ciliated cells, as well as some neuroendocrine cells. The smallest bronchioles, known as terminal and respiratory bronchioles, are lined with a simple columnar or cuboidal epithelium containing secretory and ciliated cells with fewer basal cells. The epithelia of these conducting airways form a tight barrier against the outside world and are specialized for the process of mucociliary clearance. The alveoli are lined by type 1 and 2 alveolar epithelial cells, called AT1 and AT2, respectively, hereafter. These cells are also specialized for barrier function and the extremely thin AEC1s share a basement membrane with the surrounding network of pulmonary capillaries to facilitate the diffusion of gases between the atmosphere and the circulation.