The lungs consist of principally four compartments: the large airways (bronchi), small airways and airspaces (bronchioles and alveoli), interstitium, and vessels. As in most organs, inflammatory processes tend to preferentially involve one or two compartments, so identifying the most affected area is key to the differential diagnosis. Normal histologic features include the following:
Bronchi: The bronchi are lined with ciliated or columnar epithelium with scattered goblet cells. Goblet cell metaplasia is an indication of irritation, such as in bronchitis or asthma. Squamous cell metaplasia is common in smokers. Under the epithelium you should find seromucinous (salivary-type) glands, cartilage, smooth muscle, and branches of the bronchial arteries (Figure 22.1).
Bronchioles: Bronchioles should have a cuboidal epithelium without goblet cells (Figure 22.2). The Clara cells are probably secretory and reserve cells, but they are difficult to see. There is no cartilage.
Alveoli: The alveoli are the terminal air sacs and therefore have extremely thin walls (see Figure 22.2); in atelectasis, a common biopsy artifact, it is difficult to pick out the collapsed airspaces. Normally they are lined by nearly invisible flat type I epithelium. The presence of a cuboidal epithelium indicates type II hyperplasia (surfactant and reserve cells, which are normally sparse), seen in chronic inflammation or repair. Alveolar macrophages are often scattered throughout but macrophages packing the alveoli is pathologic (see later discussion of desquamative interstitial pneumonia).
Vessels: Pulmonary arterioles run with bronchioles and have two elastic layers on Movats stain (train track appearance). Veins run in interlobular septa and have one irregular elastic lamina. Lymphatics run with arteries, veins, and in pleura.
KeywordsIdiopathic Pulmonary Fibrosis Usual Interstitial Pneumonia Diffuse Alveolar Damage Fibroblast Focus Pulmonary Blastoma
Unable to display preview. Download preview PDF.