General Plan of Wall Structure
The wall of the proximal conducting airways (trachea and large bronchi) classically consists of four layers. The main distinction from the overlying larynx is the absence of skeletal striated musculature in the wall.
- Mucous membrane (tunica mucosa). Includes the epithelium, lamina propria (rich in elastic fibers), and muscular layer (lamina muscularis). Smooth myocytes in the trachea lie in scattered circular bundles, whereas in the bronchi they form a continuous layer. This musculature regulates the lumen caliber by narrowing the entrance at sites of bronchial division.
- Submucosa (tela submucosa). Present up to the middle bronchi. It contains seromucous glands, the secretion of which contains the antibacterial enzyme lysozyme.
- Fibrocartilaginous layer (tunica fibromusculocartilaginea). Formed by cartilage tissue and perichondrium. The perichondrium has two layers: an outer fibrous layer and an inner cellular layer (containing cambial chondroblasts). In the hyaline cartilage itself, a superficial zone with solitary young cells and a deep zone where mature chondrocytes lie in isogenous groups of 2–6 cells are distinguished.
- Adventitia (tunica adventitia). The outer layer made of loose fibrous connective tissue. In intrapulmonary bronchi, it merges directly into the interstitium of the pulmonary parenchyma.
Cellular Composition of the Epithelium
In the large bronchi and trachea, the epithelium is classified as pseudostratified ciliated columnar. All of its cells contact the basement membrane, but their nuclei lie at different levels. Major (principal) cells include ciliated, goblet, and basal (stem) cells.
Throughout the airways, specific minor cells are encountered:
- Endocrinocytes (K cells): Elements of the diffuse neuroendocrine system (APUD). They secrete biogenic amines (e.g., serotonin, norepinephrine) and hormone-like peptides basally.
- Langerhans cells: Intraepithelial macrophages containing specific Birbeck granules (tennis-racket shaped). They capture antigens and present information to immune cells.
- Non-ciliated cells: Analogs of intestinal M cells involved in transepithelial antigen transport.
In the distal regions (small and terminal bronchioles), the cellular profile changes. Goblet cells disappear, replaced by:
- Brush cells: Sensory elements with microvilli acting as chemoreceptors.
- Club cells (bronchiolar exocrinocytes): Large cells with dome-shaped apical regions. They metabolize xenobiotics, secrete surfactant-like components to prevent luminal adhesion, modulate local immune responses, and act as stem cells for the regeneration of the distal epithelium.
Morphological Gradient: From Main Bronchi to Terminals
As airways dive deeper into the lung tissue, bronchial caliber decreases. This is accompanied by a strict histological gradient: epithelial height decreases, the cartilaginous framework is reduced, glands disappear, and the relative proportion of smooth muscle tissue increases markedly.
| Structure | Large Bronchi | Medium Bronchi | Small Bronchi |
|---|---|---|---|
| Epithelium | Pseudostratified ciliated | Pseudostratified (reduced height) | Simple columnar/cuboidal ciliated |
| Cartilaginous framework | Large hyaline cartilage plates | Small islands (elastic or hyaline cartilage) | Completely absent |
| Glands | Present in submucosa | Often displaced between cartilage islands | Absent |
| Muscular layer | Thin continuous layer | Proportion increases | Maximally developed |
| Wall layers | 4 layers | 4 layers | 2 layers (mucosa, adventitia) |