Respiratory System

465 Nose

Section, almost parallel to the nasal ridge, through the soft tissue of the outer nose. It contains the following elements:

1 Outer surface, skin, multilayered keratinizing squamous epithelium—epidermis

2 Septum cartilage (cartilago septi nasi)

3 Lower nasal cartilage (cartilago alaris major)

4 Nasal apex

5 Sebaceous gland

6 Hair follicle

7 Dense connective tissue

Stain: iron hematoxylin-picric acid; magnification: × 5

466 Nasal Cavity and Nasal Sinuses

Frontal section through one half of the visceral cranium. It shows the nasal cavity and the nasal sinuses.

1 Anterior cranial fossa

2 Crista galli

3 Nasal septum with respiratory mucosa

4 Nasal cavity

5 Inferior nasal concha

6 Inferior nasal meatus

7 Middle nasal concha

8 Middle nasal meatus

9 Ethmoidal cells

10Orbital cavity

11Maxillary sinus

12Maxilla, upper jaw bone

13Soft palate

14Oral cavity

15Ethmoid bone, lamina perpendicularis

16Ethmoid bone, lamina orbitalis

17Maxillary bone

18Levator palpebrae superioris muscle and superior rectus muscle

19Superior oblique muscle

20Medial rectus muscle

21Retrobulbar fat (corpus adiposum orbitae)

The nasal conchae enlarge the surface of the respiratory region. They are covered with a ciliated multilayered columnar epithelium, which contains numerous goblet cells (cf. Fig. 111, 112). The voluminous corpus cavernosum in the mucosa of the nasal cavity stands out. It reaches a considerable width, especially in the lower and middle concha.

Stain: iron hematoxylin; magnification: × 6

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467 Larynx

Cross-section of the throat at the cricoid cartilage.

1	Cricoid cartilage	6	Posterior cricoarytenoid muscle
2	Inferior horn of the thyroid cartilage	7	Oblique part of the cricothyroid muscle
3	Esophagus	8	Sternothyroid muscle
4 Thyroid gland		9	Laryngeal mucosa
5	Adipose tissue	10 Cricothyroid muscle, straight part

The mucosa (tunica mucosa respiratoria) of the airways is covered with a multilayered columnar ciliated epithelium that contains mucin-producing goblet cells (cf. Figs. 111, 112). Exceptions: regio cutanea of the nasal vestibule, olfactory region of the upper nasal concha and the upper nasal septum, the mucosa of the vocal cords and the mucosa of the small bronchia.

Stain: azan; magnification: × 12

Respiratory System

468 Larynx

Frontal section through the plica ventricularis 1 , vocal cord 2 and laryngeal ventricle 3 of the larynx from an infant.

The connective tissue of the mucosa is loosely structured at the beginning of the larynx and in the laryngeal vestibule. However, it is rigidly attached at the vocal fold edge 4 . A multilayered nonkeratinizing squamous epithelium covers the mucosa of the laryngeal cavity and continues into the laryngeal vestibule. There, the cover changes to a ciliated multilayered stratified epithelium. A multilayered nonkeratinizing squamous epithelium interrupts the otherwise continuous respiratory epithelium at the edge of the vocal cord. This 4–5-nm wide tissue is called labium vocale. It allows the firm closing of the glottis.

The vocal fold 2 protrudes into the lumen of the larynx. It delimits the phonation space. The labium vocale includes the plica vocalis 2 , vocal ligament 4 and vocal muscle 5 .

1 Ventricular fold (plica ventricularis)

2 Vocal chord (plica vocalis)

3 Laryngeal ventricle

4 Vocal ligament

5 Thyroarytenoid muscle with vocal muscle

6 Elastic cone

7 Vestibular fold (plica vestibularis)

8 Seromucous tubuloalveolar glands, laryngeal glands Stain: azan; magnification: × 8

469 Larynx

The infraglottic cavity is situated under the vocal cords. It continues underneath the ring muscle into the lumen of the trachea. The infraglottic part is covered by a multilayered ciliated epithelium. This epithelium lines the entire lower airways all the way to the small bronchioles.

Ciliated epithelium of the infraglottic tissue from a 76-year-old man.

Scanning electron microscopy; magnification: × 2,500

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Respiratory System

470 Trachea

This cross-section of the tracheal wall shows the following layers: tunica mucosa respiratoria with a multilayered ciliated epithelium 1 and seromucous tracheal glands 2 in the lamina propria mucosae 4 . The tubular invagination is a secretory duct 3 . Many secretory ducts widen to funnel-like bays when they end on the surface epithelium. The airways do not have a submucosal layer as a cushion to dampen lateral movement. The mucosa is usually tightly attached to its base. This keeps the airways open. The following layer represents the hyaline tracheal cartilage 5 . It is covered by a strong perichondrium 6 on the side of the mucosa. The very strong perichondral connective tissue layer is stained orange. The tunica adventitia 7 (stained red) is shown at the lower edge of the figure.

1 Respiratory epithelium of the tunica mucosa

2 Tracheal glands

3 Secretory duct

4 Lamina propria mucosae

5 Tracheal cartilage

6 Perichondrium

7 Tunica adventitia

Stain: alum hematoxylin-eosin; magnification: × 20

471 Trachea

Vertical section through the tracheal wall.

The surface of the tunica mucosa consists of a typical respiratory epithelium. It is a ciliated multilayered columnar epithelium, which contains goblet cells (cf. Fig. 112). The goblet cells can be recognized because they appear lighter in this figure. The respiratory epithelium also contains cells, which are covered with microvilli, as well as sensory cells and APUD cells. It requires electron microscopy to show these elements. The epithelium is layered over a strong basal membrane. The epithelial layer is followed by the wide, highly vascularized lamina propria mucosae 2 , which contains collagen fibers, longitudinally oriented elastic fiber meshwork and many seromucous tracheal glands 3 Occasionally, there are also lymph follicles. Note the blood vessels in the lamina propria mucosae. The tracheal glands release their secretory product (mucin) directly onto the epithelial surface, thus covering the entire epithelium, including kinocilia, with a film of mucus. The lower parts of the figure show the hyaline ring cartilage of the trachea 5 with the perichondrium 4 . Note the blood vessels 6 in the lamina propria mucosae 2 .

1 Ciliated, multilayered stratified columnar epithelium with goblet cells (cf. Figs. 111, 112) 2 Lamina propria mucosae with blood vessels and tracheal glands

3 Tracheal glands

4 Perichondrium

5 Tracheal hyaline cartilage

6 Blood vessels

Stain: Masson-Goldner trichrome; magnification: × 200

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Respiratory System

472 Lung

Section of a human lung, including a small bronchus 1 . The mucosa, which lines the bronchus, is folded. This is an artifact. It is caused by the contraction of the smooth muscles during tissue fixation (star-shaped clearance). The mucosa in this figure is covered only by a ciliated, single-layered columnar epithelium. This epithelium contains sporadic goblet cells. The lamina propria (stained blue) is followed by a thin layer of circular muscle cells 2 , which are sheathed by elastic fibers. The bronchial glands 3 are situated outside the tunica muscularis 2 in the peribronchial connective tissue. The seromucous glands release a thin or not so thin mucous film of the mucosa surface. Parts of the bronchial cartilage 4 are visible at the upper edge of the figure on the right and in the lower part of the figure to the left of the bronchus. The left part of the figure shows lung alveoli 5 and alveolar ducts 6 .

1 Bronchus with folded mucosa

2 Tunica muscularis

3 Bronchial glands

4 Cartilage

5 Alveoli

6 Alveolar ducts

Stain: azan; magnification: × 40

473 Lung

Perspective view of the lung tissue from an adult human. It shows alveolar ducts 1 and alveoli, which facilitate the gas exchange 2 . The alveoli have diameters between 0.05 and 0.25 mm. Six to 12 loops of capillaries weave around each alveolus. The interalveolar septum is the common wall between adjacent alveoli. The interalveolar partitions are called alveolar septa. Rings of smooth muscles, which appear knob-like in cross-sections, partition the peripheral septa. This ring of smooth muscles is covered with cuboid epithelium. The upper right of the figure shows a pulmonary vessel (cf. Figs 474– 476).

1 Alveolar duct

2 Alveoli,

3 Alveolar sac

Stain: hematoxylin-eosin; magnification: × 80

474 Lung

Respiratory bronchioles divide into wide alveolar ducts 1 . Alveoli and alveolar sacs are arranged alongside the alveolar ducts and are continuous with them. They open into the long alveolar duct. The walls of the alveolar ducts widen at the end. There are muscle cells in that end region (see. Figs. 473, 475–477). A thin connective tissue septum (alveolar wall) is shared between the epithelia of two adjacent alveoli. The connective tissue consists of a network of collagen and elastic fibers. The pulmonary capillaries are part of this meshwork.

1 Alveolar duct

2 Alveoli

Stain: alum hematoxylin-eosin; magnification: × 200

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475 Lung

This is a semi-thin section of lung tissue from a rat. The rich capillarization of the alveolar septa is clearly visible. There are still erythrocytes in the lung capillaries 2 . Note the extremely thin blood-air barrier. It consists of nonfenestrated capillary endothelium and the continuous epithelial layer of the pulmonary alveoli (alveolar epithelium). Two basal membranes separate both layers (see Fig. 478). The alveolar epithelium consists of two different cell types, the flat alveolar epithelial cells or type I pneumocytes, and the large type II pneumocytes. Type I pneumocytes contribute about 93% of the alveolar surface cells. The type II pneumocytes produce a thin surfactant lipoid covering. Due to their numerous inclusion bodies (lysosomes), type II alveolar epithelial cells are also called granulated pneumocytes.

1 Alveoli

2 Capillaries with erythrocytes

3 Alveolar duct

Semi-thin section; stain: methylene blue-azure II; magnification: × 400

Respiratory System

476 Lung

Vibratome section of a feline lung. Adjacent alveoli are separated only by interalveolar septa (cf. Figs. 473, 475). The viewer looks through the septa onto the cut surfaces of the section. On both sides, the septa are covered by a flat alveolar epithelium, which consists of type I and type II pneumocytes. A dense capillary network pervades the septa. The septa also contain elastic, reticular and collagenous fibers as well as apart from fibrocytes, leukocytes, macrophages, mast cells and nerve fibers (not visible in this figure). The surface of the alveolar epithelium is recognizable in a few places only.

Scanning electron microscopy; magnification: × 560

477 Lung

This razor section through the lung of a rat shows the branches of a bronchiolus 1 . The terminal bronchioli 2 constitute the ends of the conductive bronchial tree. The continuations of the terminal bronchioli are the respiratory bronchioles, followed by the alveolar ducts 3 and the alveoli 4 . The section shows the epithelial lining of many alveoli and the interalveolar septa (cf. Figs. 475, 476, 478).

1 Bronchiolus

2 Terminal bronchioli

3 Alveolar duct

4 Alveoli

Scanning electron microscopy; magnification: × 90

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478 Lung

Section of an interalveolar septum (compressed). The alveolar epithelial cells

1 (type I pneumocytes) and the thin endothelium 2 of the capillaries form the diffusion barrier between the alveolar air and the erythrocytes in the capillaries.

The basal membranes of the alveolar epithelial cells and the endothelial cells fuse and form a single basal membrane 3 . The type I alveolar epithelial cells cover the interalveolar septa. There are fibrocytes and fibers between alveolar epithelium and the endothelium of the capillaries. However, this figure does not show them. Type II alveolar epithelial cells (type II pneumocytes) are also not present here.

The cytoplasmic processes of adjacent endothelial cells partially overlap like shingles. The endothelium of the capillaries is not fenestrated. Note the numerous pinocytotic vesicles. Intercellular junctions seal the intercellular spaces. The alveolar epithelium spreads out in a thin layer (“anuclear layer” of light microscopy). The cytoplasm contains few organelles.

1 Type I alveolar epithelial cells

2 Capillary endothelium

3 Combined basal membranes

4 Alveolus

5 Capillary lumen

Electron microscopy; magnification: × 30 000

Respiratory System

479 Fetal Lung

The epithelial bronchi of the entodermal pulmonary germ layer grow by dichotomous (equal) division to form the branched bronchial tree. This bronchial tree keeps on branching in the mesenchyme 1 . The mesenchyme in the vicinity of the bronchi is conspicuously dense. Its building unit consists of a tube of columnar epithelium 2 .

At this developmental stage, the first cell aggregation that intimates the forming lung organ resembles a branched tubuloacinar gland.

1 Mesenchymal interstitial connective tissue

2 Epithelial bronchi

Stain: hemalum-eosin; magnification: × 5

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