Normal Anatomy, Tissue Artifacts, and Incidental Structures

Normal Anatomy

The lungs occupy most of the volume of the thoracic cavity. The average weights of male and female lungs are approximately 850 grams and 750 grams, respectively. The right lung is composed of ten distinct segments comprising three lobes (upper, middle, and lower), and the left lung has ten segments organized into two lobes (upper and lower). Each lobe is covered with pleura (visceral pleura) and separated from the other lobes by fissures. At the microscopic level, the lungs feature distinct yet integrated components, including conducting airways, distal airspaces, blood vessels and lymphatics, and other cellular constituents ( Table 1.1 ).

Airways

Not only do conducting airways form the passageways through which air enters and exits the lungs, but they also warm, humidify, and aid in sterilizing incoming air. The trachea bifurcates into the left and right mainstem bronchi, which bifurcate into additional bronchi that undergo further bifurcations into smaller bronchi and then bronchioles. Airways in adult lungs usually undergo 23 divisions to finally merge with the gas exchange units, the alveoli.

Airways are classified as either bronchi or bronchioles. Bronchi have cartilaginous walls and measure more than 0.1 cm in diameter, whereas bronchioles measure less than 0.1 cm in diameter and lack cartilage. In the mainstem bronchi, hyaline cartilage is C -shaped, but as the airways enter the lung tissue, the cartilage becomes discontinuous. As the bronchial diameter decreases, the cartilage plates become smaller. Unlike bronchioles, bronchi also have submucosal salivary-type glands with both serous and mucous cells ( Fig. 1.1 ).

Terminal bronchioles are the smallest pure conducting airways; about 30,000 terminal bronchioles are found within the lungs. The terminal bronchioles bifurcate into respiratory bronchioles, whose walls consist partially of alveoli ( Fig. 1.2 ). Bronchioles also give rise to alveolar ducts, which terminate in alveolar sacs.

Airways are composed of mucosa, submucosa, muscularis propria, and adventitia. Bronchial epithelium lines the airway lumen and includes pseudostratified ciliated columnar epithelial cells, interspersed goblet cells and neuroendocrine cells, and underlying basal cells. The ciliated respiratory epithelial cells and goblet cells are specialized cells that function in mucociliary clearance mechanisms. Goblet cells secrete mucus, which is important for trapping inhaled particles, and the cilia propel the mucus and entrapped particles toward the pharynx, where they can be eliminated. Bronchi also feature basal cells, pluripotential reserve cells that can regenerate a damaged bronchial mucosa. Scattered neuroendocrine cells are also interspersed in the respiratory epithelium. Clusters of neuroendocrine cells can occasionally be found at airway bifurcations and are termed neuroepithelial bodies. Neuroendocrine cells may not be recognizable in routine hematoxylin and eosin-stained tissue sections but can be highlighted by immunohistochemical staining using antibodies directed against chromogranin or synaptophysin antigens. Neuroendocrine cells may play a role in lung development and/or ventilation/perfusion regulation.

In bronchioles, goblet cells are replaced by nonciliated columnar cells with prominent apical cytoplasm (Clara cells). Clara cells produce surfactant-like material, accumulate and detoxify inhaled toxins, and serve as progenitor cells for regeneration of damaged bronchiolar epithelium.

All airways feature a basement membrane composed of type III collagen and underlying elastic fibers and smooth muscle bundles. Airways are richly innervated by parasympathetic and sympathetic nerves. Blood vessels and lymphatics also course through the airway submucosa.