VAN 211: Veterinary Histology and Embryology (2+2)

CONDUCTING DIVISION

• Trachea divides into two chief bronchi one for each lung, entering at the hilus, the bronchus divides into smaller bronchi which gives rise to several orders of bronchioles.
•  The general structure consists of a mucosa, muscularis, a fibro-elastic membrane which encloses a cartilage and loose collagenous peribronchial layer.
• Gradual changes in structure occur, as the main bronchus, divides into smaller bronchi, several order of bronchioles till the segment called terminal bronchiole is reached.
• These changes consist of the following.
  • The mucosa is lined by pseudo-stratified columnar cilated epithelium, in the larger bronchi.
  • The epithelium gradually decreases in height and the goblet cells become fewer.
  • The epithelium becomes simple columnar ciliated with no goblet cells in the terminal bronchiole.
  • The glands in the submucosa progressively decrease and finally disappear.
  • There is a relative increase of smooth muscle till the terminal bronchiole.
  • The hyaline cartilage does not occur in the form of rings as in trachea but as isolated plates in bronchi, which also disappear, in bronchioles.

Bronchi

• Mucosa: Pseudostratified columnar ciliated epithelium with numerous goblet cells. In submucosa mucous glands are present. Plain muscle lies inner to cartilage plates and forms a continuous cuticular layer (unlike in the trachea). Hyaline cartilage occurs only in the form of isolated plates.
• Bronchioles (small unit) Mucosa simple columnar ciliated epithelium with Goblet cells. No glands occur in the submucosa. Smooth muscle forms a continuous cirular layer. Cartilage plates are absent.
• Terminal Bronchioles: The mucosa is lined by simple columnar ciliated epithelium. No goblet cells are present and no glands occur below the mucosa. A relatively thicker smooth muscle is present, in the wall but cartilage is absent. The terminal bronchiole divides into two or more respiratory bronchioles.