The respiratory bronchiole has a wall composed of two components: One consists of recesses that have a wall similar to that of the alveoli and are thus capable of gas exchange; the other has a wall formed by small cuboidal cells that appear to rest on a small bundle of eosinophilic material. This is smooth muscle surrounded by a thin investment of connective tissue. Both of these components are shown at higher magnification in Plate 73.
Alveoli, lungs, human, H&E ×75. The outer surface of lung tissue is the serosa (S); it consists of a lining of mesothelial cells resting on a small amount of connective tissue. This is the
The most distal component of the respiratory passage is the layer that gross anatomists refer to as the visceral pleura.
alveolus. Groups of alveoli clustered together and sharing a common opening are referred to as an alveolar sac (AS). Alveoli that form a tube are referred to as alveolar ducts (AD).
KEY AD, alveolar ducts AS, alveolar sacs B, bronchiole BV, blood vessels C, cartilage L, nodule of lymphocytes RB, respiratory bronchiole S, serosa SM, smooth muscle arrows, end of terminal bronchiole
PLATE 73 Terminal Bronchiole, Respiratory Bronchiole, and Alveolus
Respiratory bronchioles continue to divide to form alveolar ducts, passages lined solely with rows of alveoli that have rings of smooth muscle in knob-like interalveolar septa. The alveolar ducts terminate in alveolar sacs, enlarged spaces surrounded by clusters of alveoli that open into the spaces. The alveoli are lined with type I alveolar cells, extremely thin squamous cells that cover about 95% of the alveolar surface, and with type II alveolar cells, cuboidal cells that secrete surfactant, a surface-active agent that reduces surface tension at the air-epithelium surface. The tissue between adjacent alveoli is called the alveolar septum. This consists of the alveolar epithelial cells and their basal lamina, the basal lamina of the underlying capillary endothelium and the endothelial cells, themselves, and any other connective tissue elements that may lie between the two basal laminae. The alveolar septum is the site of the air-blood barrier.
Terminal bronchiole, lungs, human, H&E ×550.
The histologic features of the terminal bronchiolar wall are shown here. Ciliated epithelium extends from the top of the figure to the diamond. This is ciliated pseudostratified columnar epithelium (PsEp). Some basal cells are still present and, therefore, the designation pseudostratified columnar. Elsewhere, the epithelium might be ciliated simple columnar, and just be
Respiratory bronchiole, lungs, human, H&E ×550.
The wall of a respiratory bronchiole is shown here and in lower left figure. The alveoli (A) are terminal air spaces on the left in each of the two figures. The lumen of the respiratory bronchiole is on the right. Characteristically, the wall of the respiratory bronchiole consists of alternating thick and fore it becomes a respiratory bronchiole, the epithelium may include cuboidal or low columnar nonciliated cells. These nonciliated cells are Clara cells (CC, beyond the diamond). Clara cells produce a surface-active agent that is instrumental in expansion of the lungs. The smooth muscle (SM) in the bronchiolar wall is organized in bundles; other cells under the epithelium and around the smooth muscle belong to the connective tissue.