General Plan of Wall Structure
The wall of the proximal airways (trachea and large bronchi) traditionally consists of four layers. The main difference from the overlying larynx is the absence of striated skeletal muscle in the wall.
- Mucous Membrane (tunica mucosa). Includes epithelium, lamina propria (rich in elastic fibers), and muscularis mucosae. Smooth myocytes in the trachea lie in scattered circular bundles, whereas in the bronchi they form a continuous layer. This musculature regulates the airway lumen by narrowing the entrance at the sites of bronchial division.
- Submucosa (tela submucosa). Present down to the medium bronchi. It contains mucous and serous 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). The hyaline cartilage itself exhibits a superficial zone with isolated young cells and a deep zone where mature chondrocytes lie in isogenous groups of 2–6 cells.
- Adventitia (tunica adventitia). Outer layer of loose fibrous connective tissue. In intrapulmonary bronchi, it directly transitions into the interstitium of the pulmonary parenchyma.
Cellular Composition of the Epithelium
In large bronchi and the trachea, the epithelium is classified as pseudostratified ciliated. All of its cells contact the basement membrane, but their nuclei lie at different levels. Major (principal) cells include ciliated, goblet, and basal (intercalated) 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 in a basal direction.
- Langerhans cells: intraepithelial macrophages. Contain specific Birbeck granules (tennis racket-shaped). Capture antigens and present information to immune cells.
- Non-ciliated cells: analogues of intestinal M-cells involved in transepithelial antigen transport.
In the distal regions (small and terminal bronchioles), the cellular profile changes. Goblet cells disappear and are replaced by:
- Brush cells: sensory elements with microvilli acting as chemoreceptors.
- Clara cells (bronchiolar exocrinocytes): large cells with a dome-shaped apical part. They detoxify xenobiotics, secrete a surfactant-like fluid to prevent wall adhesion, stimulate leukocytes, and act as stem cells for the regeneration of distal epithelium.
Morphological Gradient: From Main to Terminal Bronchi
As they extend deeper into the lung tissue, the caliber of the bronchi decreases. This is accompanied by a strict histological gradient: epithelial height decreases, the cartilage framework is reduced, glands disappear, and the relative proportion of muscle tissue increases manifold.
| Structure | Large bronchi | Medium bronchi | Small bronchi |
|---|---|---|---|
| Epithelium | Pseudostratified ciliated | Pseudostratified (reduced height) | Bilayered ciliated |
| Cartilage Framework | Large hyaline cartilage plates | Small islands (elastic or hyaline cartilage) | Completely absent |
| Glands | In the submucosa | Often displaced between cartilage islands | Absent |
| Muscular Layer | Thin continuous | Proportion increases | Maximally expressed |
| Wall Layers | 4 layers | 4 layers | 2 layers (mucosa, adventitia) |
Briefly: answers to topical questions
What types of cartilage tissue form the framework of various parts of the bronchial tree?
The framework of the bronchial tree is formed by hyaline and elastic cartilage tissue, the structure of which predictably changes as the caliber of the airways decreases.
- Trachea and main bronchi — hyaline cartilage is represented by incomplete rings posteriorly (in the trachea) or complete rings (in the main bronchi).
- Large bronchi — the fibrocartilaginous layer is represented by large separate hyaline cartilage plates.
- Medium bronchi — the framework breaks down into islands, with the cartilage being not only hyaline but predominantly elastic.
- Small bronchi and bronchioles — cartilage elements are completely absent in the wall.
What is the histological structure of respiratory bronchioles?
The histological structure of respiratory bronchioles is characterized by the presence of alveoli opening directly into the bronchiole wall.
The structure of the wall segments between alveoli includes:
- Epithelium — simple cuboidal, partially ciliated. Cellular composition is represented by Clara secretory cells (most frequent), ciliated, and brush cells.
- Underlying layers — formed by a thin connective tissue plate and scattered smooth myocytes.
The more distally a respiratory bronchiole is located (higher branching order), the more alveoli are found in its wall.
The "minus one" rule: upon transition from medium to small bronchi, cartilages and the submucosa with glands disappear (leaving 2 layers out of 4), and goblet cells are replaced by Clara cells.
Class and exam questions
Why does the lumen of a small bronchus appear star-shaped on a histological slide?
This is the result of the contraction of a robust muscular layer. Due to the complete absence of a stabilizing cartilaginous framework, the mucosa of a small bronchus easily gathers into deep folds.
Where are glands located in medium bronchi?
In medium bronchi, dense fibrous tissue between cartilages is poorly developed. Therefore, the terminal portions of glands are often displaced from the submucosa directly into the spaces between cartilage islands.
What are the main differences between a small bronchus and a terminal bronchiole?
In the terminal bronchiole, the epithelium becomes simple (in the small bronchus, it is bilayered). Also, in the bronchiole, the muscular elements break down into separate bundles again, and the luminal relief becomes only slightly tortuous.
What else to study on this topic
- Zonal structure of hyaline cartilage in the fibrocartilaginous layer of the trachea
- Histophysiology of Clara cells and detoxification mechanisms in the respiratory tract
- Immune function of the epithelium: Langerhans cells and Birbeck granules
- Morphological boundary and classification challenges of intrapulmonary bronchi
- Organization of nuclear rows in pseudostratified ciliated epithelium
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