Histology of Larynx, Trachea, Bronchus_Dea Natalia_130110110190_E3/Respi

LARYNX

The larynx is a short passageway for air between the pharynx and trachea. Its wall contains skeletal muscles and pieces of cartilage, all of which make the larynx specialized for sound production. The lining of the larynx superior to the vocal folds is nonkeratinized stratified squamous epithelium. The lining of the larynx inferior to the vocal folds is pseudostratified ciliated columnar epithelium consisting of ciliated columnar cells, goblet cells, and basal cells. The mucus produced by the goblet cells helps trap dust not removed in the upper passages. The cilia in the upper respiratory tract move mucus and trapped particles down toward the pharynx; the cilia in the lower respiratory tract move them up toward the pharynx.

The low-power micrograph shows the upper laryngeal vestibule (LV), which is surrounded by seromucous glands (G). The lateral walls of this region bulge as a pair of broad folds, the vestibular folds (VF). These also contain seromucous glands and areolar tissue with MALT, often with lymphoid nodules (L) and are largely covered by respiratory epithelium, with regions near the epiglottis having stratified squamous epithelium. Below each large vestibular fold is a narrow space or ventricle (V), below which is another pair of lateral folds, the vocal folds or cords (VC). These are covered by stratified squamous epithelium and project more sharply into the lumen, defining the rim of the opening into the larynx itself. Each contains a large striated vocalis muscle (VM) and nearer the surface a small ligament, which is cut transversely and therefore difficult to see here.

TRACHEA

The layers of the tracheal wall, from deep to superficial, are the:1. mucosa

epithelial layer of pseudo-stratified ciliated columnar epithelium consists of ciliated columnar cells and goblet cells that reach the luminal surface, plus basal cells that do not; it provides the same protection against dust as the membrane lining the nasal cavity and larynx

underlying layer of lamina propria that contains elastic and reticular fibers2. submucosa

consists of areolar connective tissue that contains seromucous glands and their ducts3. hyaline cartilage

the 16–20 incomplete, horizontal rings of hyaline cartilage resemble the letter C, are stacked one above another, and are connected together by dense connective tissue

the open part of each C-shaped cartilage ring faces posteriorly toward the esophagus and is spanned by a fibromuscular membrane. Within this membrane are transverse smooth muscle fibers, called the trachealis muscle, and elastic connective tissue that allow the diameter of the trachea to change subtly during inhalation and exhalation, which is important in maintaining efficient airflow; the solid C-shaped cartilage rings provide a semirigid support so that the tracheal wall does not collapse inward (especially during inhalation) and obstruct the air passageway

4. adventitia consists of areolar connective tissue that joins the trachea to surrounding tissues

wall of the trachea is lined by typical respiratory epithelium (E) underlain by connective tissue (CT) and seromucous glands (G) in the lamina propria. The submucosa contains C-shaped rings of hyaline cartilage (C) covered by perichondrium (P). The watery mucous fluid produced by goblet cells and by the glands forms a layer that permits the ciliary movement to propel foreign particles continuously out of the respiratory system in the mucociliary escalator. The openings in the cartilage rings are on the posterior surface, against the esophagus, and contain smooth muscle and elastic tissue. These allow distention of the tracheal lumen when large pieces of food pass through the esophagus. The trachealis muscle in the opening of the C also contracts during the cough reflex to narrow the tracheal lumen and produce stronger expulsion of air and dislodged mucus in the air passages

Histology of Larynx, Trachea, Bronchus_Dea Natalia_130110110190_E3/Respi

BRONCHUS

as the branching becomes more extensive in the bronchial tree, several structural changes may be noted.1. the mucous membrane in the bronchial tree changes from pseudostratified ciliated columnar epithelium in the primary

bronchi, secondary bronchi, and tertiary bronchi to ciliated simple columnar epithelium with some goblet cells in larger bronchioles, to mostly ciliated simple cuboidal epithelium with no goblet cells in smaller bronchioles, to mostly nonciliated simple cuboidal epithelium in terminal bronchioles. (In regions where simple nonciliated cuboidal epithelium is present, inhaled particles are removed by macrophages.)

2. plates of cartilage gradually replace the incomplete rings of cartilage in primary bronchi and finally disappear in the distal bronchioles.

3. as the amount of cartilage decreases, the amount of smooth muscle increases. Smooth muscle encircles the lumen in spiral bands. Because there is no supporting cartilage, however, muscle spasms can close off the airways. This is what happens during an asthma attack, which can be a life-threatening situation.

In a cross-section of a large bronchus the lining of respiratory epithelium (E) and the mucosa are folded due to contraction of its smooth muscle (SM). At this stage in the bronchial tree, the wall is also surrounded by many pieces of hyaline cartilage (C) and contains many seromucous glands (G) in the submucosa which drain into the lumen. In the connective tissue surrounding the bronchi can be seen arteries and veins (V), which are also branching as smaller and smaller vessels in the approach to the respiratory bronchioles. All bronchi are surrounded by distinctive lung tissue (LT) showing the many empty spaces of pulmonary alveoli.

(a): A higher power view of the bronchus shows the epithelium (E) of mainly pseudostratified ciliated columnar cells with a few goblet cells. The lamina propria (LP) contains the distinct layer of smooth muscle (SM) surrounding the entire bronchus. The submucosa is the site of the supporting cartilage (C) and the adventitia includes blood vessels (V) and nerves (N). Lung tissue (LT) directly surrounds the adventitia of bronchi.

(b): This micrograph shows the epithelium of a smaller bronchus, in which the epithelium is primarily of columnar cells with cilia (arrows), with fewer goblet cells. The lamina propria has both smooth muscle (SM) and small serous glands