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

RESPIRATORY SYSTEM

• The epithelium of respiratory tree is derived from endoderm. The primordium of this system is the laryngo-tracheal groove, which runs lengthways in the floor of the foregut just behind the last pair of pharyngeal grooves.
• On the ventral surface, the endoderm projects as a ridge. This ridge is the primordium for caudal part of larynx, trachea in front and caudally the bronchi and lungs.
• A lateral groove appears on each side at the junction of the ridge and the oesophagus, which becomes deeper and thus separates the oesophagus from the trachea and lung bud.

The ridge extends forwards up to the level of the fourth branchial arches. The mesenchyme of the fourth and fifth arches gives rise to the thyroid, cricoid, epiglottic and arytenoid cartilages and laryngeal muscles. The primitive opening of the pharynx into the trachea forms the glottis.

The trachea elongates and bifurcates into two bronchial buds on the right side and one on the left. The end of each main tube continues as stem bronchus.

As the bronchial buds elongate and branch, the tubular system in each pulmonary lobe becomes bush-like with dorsal, ventral, lateral and medial rami.

The splanchnic mesoderm within which the early respiratory tree has developed is later named as the mediastinum and the mesenchyme furnishes the connective tissue and cartilage plates of the tracheal and bronchial wall, where the blood vessels and nerve fibres grow.

A series of anomaly in connection with the development of this system is a fistula between the trachea and oesophagus due to incomplete separation of the laryngo-tracheal groove from the gut.