Histology of Nasal Cavity and Larynx

What are the four layers of the tracheal wall (from inside out)?

1. Mucosa 2. Submucosa 3. Cartilaginous layer 4. Adventitia

What epithelium lines the mucosa of the trachea?

Pseudostratified ciliated columnar epithelium with goblet cells (i.e. respiratory epithelium).

What are the main cell types in the tracheal epithelium?

1. Ciliated columnar cells 2. Goblet cells 3. Basal cells 4. Brush cells (with microvilli) 5. Small granule (Kulchitsky) cells — neuroendocrine cells.

What is the function of the ciliated and goblet cells in the trachea?

Goblet cells secrete mucus that traps particles; Cilia move mucus upwards toward the pharynx (mucociliary clearance).

What is found beneath the mucosa in the trachea?

The lamina propria, rich in elastic fibers and containing seromucous glands.

What is the composition and function of the submucosa?

Loose connective tissue with seromucous glands, blood vessels, lymphatics, and nerves; provides moisture and traps particulates.

What kind of cartilage supports the trachea, and how is it arranged?

Hyaline cartilage arranged in C-shaped rings, open posteriorly.

What muscle completes the posterior opening of the cartilage rings?

Trachealis muscle – a band of smooth muscle that bridges the gap and regulates airway diameter.

What is the outermost layer of the trachea and what does it contain?

Adventitia – loose connective tissue that blends with surrounding structures and contains blood vessels and nerves.

How does the wall structure of bronchi compare to the trachea?

The bronchi have the same four layers as the trachea (mucosa, submucosa, cartilage, adventitia), but show gradual structural changes as they branch and become smaller.

What type of epithelium lines the bronchi?

Pseudostratified ciliated columnar epithelium with goblet cells — same as the trachea in primary bronchi, but the epithelium becomes shorter and less pseudostratified in smaller bronchi.

What happens to goblet cells and cilia as bronchi become smaller?

Goblet cells decrease in number, and cilia persist, although less prominent in smaller branches.

What cells are found in the bronchial epithelium?

1. Ciliated columnar cells 2. Goblet cells 3. Basal cells 4. Clara (club) cells (in smaller bronchi) 5. Small granule (neuroendocrine) cells.

What is found in the lamina propria of bronchi?

Loose connective tissue with elastic fibers, lymphocytes, and mucous glands. Also contains smooth muscle, which becomes more prominent as bronchi branch.

How is cartilage arranged in bronchi compared to the trachea?

In the bronchi, irregular plates of hyaline cartilage (not C-shaped rings) surround the bronchial wall, decreasing in size and amount as the bronchi branch further.

What happens to the cartilage and glands as bronchi become smaller?

Both cartilage plates and seromucous glands decrease in amount and eventually disappear by the level of the bronchioles.

How is the smooth muscle arranged in the bronchi?

Forms a continuous circular layer beneath the epithelium and lamina propria — thicker than in the trachea and important in regulating airway resistance.

What marks the transition from bronchi to bronchioles?

1. Loss of cartilage and glands 2. Epithelium becomes simple ciliated columnar or cuboidal 3. Appearance of Clara (club) cells 4. Thicker smooth muscle relative to the wall.

What is the larynx and where is it located?

The larynx is an organ in the neck involved in breathing, phonation (voice production), and protecting the trachea against food aspiration. It lies below the pharynx and above the trachea.

What are the functional regions of the larynx in terms of epithelial lining?

1. Stratified squamous epithelium: lines areas exposed to mechanical stress — true vocal cords, upper epiglottis, and laryngeal side of the epiglottis. 2. Pseudostratified ciliated columnar epithelium (respiratory epithelium): lines the rest of the larynx (false vocal cords, ventricles, and subglottic region).

What are the layers of the true vocal cords?

Divided into three functional components: 1. Cover: epithelium, basement membrane zone, superficial lamina propria (Reinke’s space) 2. Transition: intermediate and deep layers of lamina propria (together form vocal ligament) 3. Body: vocalis muscle (a part of the thyroarytenoid muscle).

What type of epithelium covers the true vocal cords?

Stratified squamous non-keratinized epithelium — protects against mechanical trauma during phonation.

What is Reinke’s space?

The superficial layer of the lamina propria of the vocal cords — a loose, gelatinous layer that allows vibration during phonation. Its viscoelasticity is critical for voice production.

What type of epithelium lines the false vocal cords (vestibular folds)?

Pseudostratified ciliated columnar epithelium (respiratory epithelium) — contains glands, lymphatic tissue, and adipocytes.

What muscle makes up the body of the vocal cords?

Thyroarytenoid muscle, particularly the vocalis portion, a skeletal muscle responsible for changing pitch.

What type of glands are found in the larynx? Where are they most abundant?

Mixed seromucous glands, primarily found in the lamina propria of the false vocal cords, ventricles, and subglottic region.

What happens to the laryngeal structures with age?

1. Infants: No vocal ligament until ~4 years old; only 1 lamina propria layer. 2. Puberty: Vocal cords thicken and lengthen (especially in males due to testosterone), forming 3-layered lamina propria. 3. Adulthood: Increased elastin in lamina propria. 4. Old age: Atrophy of the vocalis muscle in both sexes.

What is the epiglottis made of and how is it lined?

1. Core: Elastic cartilage 2. Lining: Stratified squamous epithelium on anterior (lingual) surface and part of posterior (laryngeal) surface; Pseudostratified ciliated columnar epithelium at the base; May contain taste buds.

What are bronchioles and how do they differ from bronchi?

Bronchioles are smaller airways (<1 mm diameter) that arise from the tertiary (segmental) bronchi. They lack cartilage and glands, unlike bronchi, and are surrounded by prominent smooth muscle.

What type of epithelium lines bronchioles?

1. Larger bronchioles: ciliated simple columnar epithelium with goblet cells. 2. Smaller bronchioles: ciliated simple cuboidal epithelium with club (Clara) cells.

What are Clara (club) cells and what do they do?

Non-ciliated, dome-shaped epithelial cells that: 1. Secrete surfactant-like lipoproteins to reduce surface tension. 2. Detoxify harmful substances. 3. Act as stem cells for regeneration. 4. Secrete antimicrobial peptides (e.g., lysozyme, CC16 protein).

What replaces goblet cells in the terminal bronchioles?

Club (Clara) cells gradually replace goblet cells in smaller bronchioles and terminal bronchioles.

How is the smooth muscle arranged in bronchioles?

Smooth muscle forms a continuous circular layer that helps regulate airway diameter — particularly important in conditions like asthma.

Do bronchioles have cartilage or submucosal glands?

No — bronchioles lack both cartilage and glands.

What is the epithelium of the terminal bronchioles?

Simple cuboidal epithelium, composed mostly of ciliated cells and club cells.

What are respiratory bronchioles and how are they different from terminal bronchioles?

Respiratory bronchioles are the first part of the airway involved in gas exchange. They have discontinuous walls, with some regions having alveoli. Epithelium is simple cuboidal in non-alveolated areas and simple squamous in alveolated portions.

What comes after respiratory bronchioles in the respiratory tract?

Respiratory bronchioles lead into alveolar ducts, then into alveolar sacs, and finally into alveoli — the primary site of gas exchange.

What are alveoli and what is their main function?

Alveoli are small sac-like structures where gas exchange occurs between air and blood. They are lined by simple squamous epithelium and surrounded by a dense capillary network.

What are the two main types of alveolar epithelial cells?

1. Type I pneumocytes (type I alveolar cells): • Flattened, squamous cells • Cover ~95% of alveolar surface • Form part of the blood-air barrier 2. Type II pneumocytes (type II alveolar cells): • Cuboidal cells with lamellar bodies • Secrete pulmonary surfactant to reduce surface tension • Serve as stem cells for both type I and type II pneumocytes.

What is the blood-air barrier composed of?

1. Alveolar epithelium (type I pneumocyte) 2. Fused basal lamina of alveolar epithelium and capillary endothelium 3. Capillary endothelium.

What is the function of pulmonary surfactant?

Surfactant reduces surface tension within alveoli, preventing alveolar collapse during expiration. It is essential for normal lung function, especially in neonates.

What other important cells are found in alveoli?

• Alveolar macrophages (dust cells): phagocytose inhaled particles and microbes • Endothelial cells: line alveolar capillaries • Fibroblasts: produce collagen and elastin for alveolar support.

What structural adaptations support alveolar gas exchange?

• Extremely thin walls • Large surface area • Rich capillary network • Presence of pores of Kohn: allow collateral ventilation between alveoli.

What are alveolar ducts and alveolar sacs?

• Alveolar ducts: linear airways lined with alveoli along their walls • Alveolar sacs: clusters of alveoli opening into a common space — the final portion of the airway.

What is the histological difference between terminal and respiratory portions of the lungs?

• Terminal portions (terminal bronchioles): no alveoli, no gas exchange • Respiratory portions (respiratory bronchioles onward): have alveoli, participate in gas exchange.