Page 274 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

256 Veterinary Histology of Domestic Mammals and Birds

tubules. In most species their diameter is around 0.5 mm
· numerous laterodorsal secondary bronchi,
VetBooks.ir · 7–10 mediodorsal secondary bronchi, (1–1.5 mm in the chicken).
Parabronchi have several distinctive features (Figures
· 4–7 lateroventral secondary bronchi and
11.28 and 11.29):
· 4 medioventral secondary bronchi.

Third-order bronchi, or parabronchi, are also referred to · they anastomose with one another,
as ‘air pipes’. Parabronchi arising from mediodorsal sec- · chambers called atria open from their walls into
ondary bronchi within the dorsal palaeopulmo meet and infundibula and air capillaries,
anastomose with their ventral counterparts in the inte- · they contain gas exchange units,
rior of the lung (refer to anatomy texts). The parabronchi · their diameter is uniform within species and
are arranged in a parallel hexagonal array of elongated · they manifest as hexagonal tubes.

Table 11.2 Histological features of the bronchial tree and gas exchange system of domestic mammals.

Segment Epithelium Glands Connective tissue and muscle Special features
Bronchi Respiratory Tubulo-acinar Hyaline cartilage plates and Cartilage supported
epithelium with seromucous fragments, with helically and by elastic fibre
cilia, microvilli, glands circularly oriented smooth network
goblet cells, neuro- muscle bundles underlain by
endocrine cells dense interstitial connective
tissue
Bronchioles Transforms from Absent No cartilage, helically and Connective tissue
pseudostratified circularly oriented smooth supported by elastic
into simple muscle fibres, loose interstitial fibre network
columnar or connective tissue
cuboidal partly
ciliated epithelium,
Clara cells, neuro-
endocrine cells
Terminal Simple cuboidal Absent Smooth muscle cells and Particularly well
bronchioles epithelium, Clara connective tissue, elastic fibre developed in
cells, neuro- network ruminants, pigs and
endocrine cells horses
Respiratory Simple cuboidal Absent Smooth muscle cells and Number of alveoli
bronchioles epithelium, Clara connective tissue, reinforced increases with
cells, Types I and II by elastic fibre network each generation
alveolar epithelial (I–III), abundant
cells in carnivores,
occasionally seen in
the horse, rare in the
pig and in ruminants
Alveolar Simple squamous Absent Elastic fibre network, muscle Up to 10 alveolar
ducts epithelium cells in inter-alveolar septa ducts are formed
(alveolar (acts as sphincter) from the branching
epithelium), Types of each respiratory
I and II alveolar bronchiole,
epithelial cells, branching of alveolar
macrophages ducts gives rise to
alveolar sacs
Alveoli Simple squamous Absent Loose connective tissue Blood–air barrier
epithelium with prominent elastic fibre
(alveolar network
epithelium), Types
I and II alveolar
epithelial cells,
macrophages

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