Aughey E., Frye F.L. Comparative veterinary histology with clinical correlates

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6.25

6.28

6.27

6.26

Cardiovascular System

6.27 Haemangiosarcoma in a

horse. The tumour cells are spindle-

shaped with large, often

hyperchromatic, nuclei and are

arranged into loosely interlacing

bundles that form irregular, blood-

filled channels and spaces. H & E.

×125.

6.25, 6.26 Myocardial degeneration and fibrosis in a 12-year-old dog with cardiomyopathy. In 6.25, the

collagenous tissue which replaces muscle bundle is stained red. Sirius Red. ×45. In 6.26, degenerative muscle

fibres are seen stained strongly orange in the centre. The striations of the muscle fibres are also demonstrated

with this stain. Masson’s trichrome. ×180.

6.28 Early arteriosclerotic

mineralization in a large pulmonary

artery (iguana). The mineral

deposition is highlighted in red.

Alcian blue/PAS. ×125.

The respiratory system has two functions: conduc-

tion and respiration. Conduction is carried out via a

continuous system of tubes carrying air from the nos-

trils into the nasal cavity through the nasopharynx

and larynx to the trachea and bronchi. The air is

warmed, moistened and filtered in these passages

before reaching the organs responsible for respira-

tion: the lung parenchyma, the respiratory bronchi-

oles, alveolar sacs and alveoli. It is between the alve-

oli and the capillaries that gas exchange takes place.

The paranormal sinuses are cavities found in

skull bones. The mucoendosteum lining these cav-

ities is continuous with the mucous membrane of

the nasal cavity.

7. RESPIRATORY SYSTEM

7.1 Skin. Nostril (horse). (1) Sinus hair. (2) Lamina propria.

(3) Sebaceous glands. (4) Sweat glands. H & E. ×20.

7.1

Conduction of air

The skin around the nostrils has long tactile hairs

and numerous sebaceous and sweat glands (7.1).

Respiratory epithelium lines all but the finer

divisions of the respiratory tract and consists of

pseudostratified columnar ciliated cells and mucus-

secreting goblet cells. The lamina propria is con-

tinuous with the perichondrium or periosteum

where appropriate, is very vascular, contains both

collagen and elastic fibres, and warms the inspired

air. Seromucous glands secrete into the lumen

through the epithelium (7.2–7.4).

7.2 Respiratory epithelium (horse).

(1) Pseudostratified columnar ciliated

epithelium with goblet cells.

(2) Lamina propria. (3) Seromucous

glands. (4) Smooth muscle. H & E.

×62.5.

7.2

7.3 Respiratory epithelium (horse).

(1) Pseudostratified columnar ciliated

epithelium with goblet cells.

(2) Lamina propria. (3) Blood vessels.

H & E. ×160.

7.3

7.4 Respiratory epithelium (horse).

The cilia project from the surface of

the epithelial cell as fine strands;

active mucus-secreting cells lie

between the ciliated cells (arrowed).

Scanning electron micrograph. ×1500.

7.4

Respiratory System

Cilia beat towards the pharynx, an area com-

mon both to the digestive and to the respiratory

system. The auditory tube connects the pharynx

to the middle ear and is common to the digestive,

respiratory and auditory systems (7.5). The gut-

tural pouch of equids is a diverticulum of the audi-

tory tube. The digestive surface is covered by a

stratified squamous epithelium that is continuous

with the oral cavity, and the respiratory surface by

respiratory epithelium continuous with the nasal

cavities (7.6 and 7.7). The epiglottis is a flap-like

structure projecting into the pharynx. A plate of

elastic cartilage provides internal support. The

upper digestive surface mucous membrane is cov-

ered by a non-keratinizing stratified squamous

epithelium (7.8) and the lower respiratory surface

by respiratory epithelium (7.9). Taste buds may be

present on the laryngeal aspect.

Comparative Veterinary Histology with Clinical Correlates

7.5 Auditory tube (horse). (1) Respiratory epithelium. (2) Lamina propria.

(3) Seromucous glands. H & E. ×100.

7.5

7.6 Guttural pouch (horse). (1) Respiratory epithelium. (2) Lamina propria.

(3) Seromucous glands. H & E. ×250.

7.6

7.7 Guttural pouch (horse). (1)

Respiratory epithelium; the goblet

cells are individually stained blue.

(2) Lamina propria. (3) Seromucous

glands. Alcian blue/PAS. ×200.

7.7

7.8 Epiglottis (horse). (1) Stratified

squamous epithelium. (2) Lamina

propria. (3) Seromucous glands.

H & E. ×200.

7.8

7.9 Epiglottis (horse). (1) Stratified

columnar epithelium with mucus-

secreting cells stained blue.

(2) Lamina propria. (3) Seromucous

glands. Alcian blue/PAS. ×125.

7.9

Respiratory System

The larynx is lined by respiratory epithelium. The

lamina propria is continuous with the perichon-

drium of the laryngeal cartilages. The vocal cords

are covered by stratified squamous epithelium (7.10

and 7.11). The trachea extends from the larynx to

the bifurcation of the extrapulmonary bronchi.

These tubes have the same structure: a lining of res-

piratory epithelium rising on a lamina propria of

loose connective tissue with elastic fibres and mixed

seromucous glands opening into the lumen.

Incomplete rings of hyaline cartilage keep the lumen

patent and smooth muscle fibres bridge the gap at

the dorsal aspect of the trachea (7.12). In the

bronchus the hyaline cartilage has a plate-like

arrangement. The smooth muscle forms a spiral and

appears as discontinuous blocks in transverse sec-

tions (7.13 and 7.14). A fibrous adventitial coat cov-

ers the trachea and the extrapulmonary bronchi.

Comparative Veterinary Histology with Clinical Correlates

7.10 Larynx (vocal cord; dog). (1) Stratified squamous epithelium. (2) Lamina

propria. (3) Simple tubular glands. H & E. ×62.5.

7.10

7.11 Larynx (vocal cord; dog). (1) Stratified squamous epithelium

covering the vocal cord. (2) Connective tissue core of the lamina propria.

(3) Parasympathetic ganglion. H & E. ×160.

7.11

7.12 Trachea (sheep). (1) Respiratory

epithelium. (2) Lamina propria.

(3) Seromucous glands. (4) Hyaline

cartilage. Gomori’s trichrome. ×25.

7.12

7.14

7.13

Respiratory System

7.13 Bronchus (sheep).

(1) Respiratory epithelium.

(2) Lamina propria. (3) Smooth

muscle. (4) Simple tubular glands

open into the lumen through the

epithelium. (5) Perichondrium.

(6) Hyaline cartilage. H & E. ×25.

7.14 Bronchus (ox). (1) Respiratory

epithelium; the goblet cells are

stained specifically. (2) Lamina

propria. (3) Smooth muscle.

(4) Simple tubular glands.

Gomori/aldehyde fuchsin. ×200.

7.18 Adult lung (sheep). (1) Elastic connective tissue

with an outer layer of mesothelium of the visceral pleura.

(2) Alveoli. (3) Blood vessels. H & E. ×62.5.

7.18

7.15 Bovine fetal (160-day) lung. Blood vessels

(arrowed). (1) Pleural mesothelium. (2) Mesenchyme.

(3) Duct system. H & E. ×5.

7.15

7.16 Bovine fetal (160-day) lung.

(1) Large duct lined by a simple

columnar epithelium. (2) Smooth

muscle. (3) Vascular mesenchyme.

(4) Small ducts. H & E. ×25.

7.16

7.17 Fetal lung at term (cow). (1) Respiratory bronchiole

lined by simple columnar epithelium. (2) Alveoli. H & E.

×62.5.

7.17

Respiration

The bronchi, both extrapulmonary and intrapul-

monary, bring air to the lungs and branch out

within the lungs into the bronchioles, which culmi-

nate in clusters of minute sacs: the alveoli. In the

fetal lung the duct system is developed whereas the

respiratory part develops slowly (7.15–7.17).

Expansion begins with the first respiratory move-

ments after birth. Thereafter, the lung expands in

tandem with the growth of the animal.

Each lung is covered by elastic connective tissue

with an outer layer of mesothelium, the visceral

pleura (7.18). Connective tissue septa divide the

lung into lobes and lobules and the intrapulmonary

bronchi have the same structure as the extrapul-

monary bronchi (7.19–7.23). The epithelium of

Comparative Veterinary Histology with Clinical Correlates

7.19

7.19 Intrapulmonary bronchus. Lung (cow).

(1) Lumen lined by respiratory epithelium.

(2) Lamina propria. (3) Smooth muscle.

(4) Hyaline cartilage. (5) Blood vessel.

(6) Alveoli. Gomori/aldehyde fuchsin. ×52.

7.20 Intrapulmonary bronchus. Lung (sheep).

(1) Lumen lined by respiratory epithelium.

(2) Lamina propria. (3) Smooth muscle.

(4) Hyaline cartilage. (5) Simple tubular

glands. (6) Blood vessel. (7) Bronchiole.

(8) Alveolar duct. (9) Alveoli. H & E. ×62.5.

7.20

Respiratory System

7.21 Intrapulmonary bronchus. Lung (sheep).

(1) Respiratory epithelium. (2) Lamina propria.

(3) Smooth muscle. (4) Hyaline cartilage. H & E. ×200.

7.21

7.22 Intrapulmonary bronchus. Lung (cow). A

parasympathetic ganglion is present surrounded

by vascular connective tissue of the lamina

propria.Gomori/aldehyde fuchsin. ×160.

7.22

7.23 Intrapulmonary bronchus (dog). (1) The

intrapulmonary bronchus is lined by respiratory

epithelium (arrowed). (2) Alveoli. Scanning electron

micrograph. ×500.

7.23

7.24 Bronchiole. Lung (sheep). (1) The bronchiole is lined by cuboidal/low

columnar epithelium with ciliated and non-ciliated cells (arrowed). (2) Smooth

muscle. (3) Alveolar duct. (4) Alveoli. The free cells in the lumen are

macrophages. H & E. ×125.

7.24

the bronchioles is columnar or even cuboidal and

ciliated. In the smaller bronchioles the epithelium

is thinner, the lamina propria is elastic, the smooth

muscle forms a complete ring and there is no

adventitia (7.24 and 7.25).

Non-ciliated bronchiolar cells (Clara cells) are

tall, dome-shaped and protrude into the bronchio-

lar lumen. They replace the mucus-secreting gob-

let cells at this level. Both ciliated cells and Clara

cells are present in the terminal and respiratory

bronchioles (in the dog and cat they are lined by the

latter exclusively). Clara cells divide to form other

Clara or ciliated cells and have an important role in

the repair of damaged epithelium. Their secretion

also keeps the small airways patent.

Respiratory bronchioles are lined with a low

columnar or cuboidal epithelium with ciliated and

bronchiolar cells, an elastic lamina propria and a

smooth muscle layer. This opens into the alveolar

duct lined by squamous epithelium, interrupted by

atria and alveoli along its length (7.26). Alveoli are

the functional exchange part of the lung. The septa

are very thin, with both elastic and collagen fibres,

and contain one of the most extensive capillary net-

works in the body. Cells of the immune system,

derived from blood monocytes, are also present

and migrate through the alveolar epithelium into

the air space, where they phagocytose particulate

matter and micro-organisms to become dust cells

(alveolar phagocytes). The respiratory membrane

where gas exchange takes place consists of capil-

lary endothelial cells, alveolar epithelial cells and a

fused basement membrane (7.27). The squamous

alveolar cell, the lining cell responsible for gas

exchange, is a type I pneumocyte. The great alve-

olar cell secretes surfactant to reduce surface ten-

sion. It is a type II pneumocyte, is cuboidal and

projects into the lumen.

Comparative Veterinary Histology with Clinical Correlates