University:
Stanford University
Course:
MED 101 | Human Anatomy
Academic year:

2023
Views:

291

Pages:

1
Author:

larbi43100

Development of the respiratory system 0 .Introduction The entire respiratory system develops from the respiratory diverticulum which derives from the primitive foregut at the fourth week of development. This endodermal diverticulum is the origin of the epithelium which lines the lumen of the respiratory tract and the pulmonary alveoli. The other constituents of the airways, such as pulmonary parenchyma, derive from the surrounding mesenchyme and vessels. The development of the respiratory system extends throughout pregnancy and continues even beyond birth. 1. AIRWAY TRAINING 1.1. The respiratory diverticulum The respiratory diverticulum appears at the 4th week in the form of a groove from the ventral side of the pharyngeal intestine to its caudal part. This gutter, open posteriorly, separates from the primitive intestine which gave rise to it due to the proliferation of two zones of mesenchyme which are inserted between the two structures and progress in a caudal direction. The outline then forms a rectilinear tube, blind at its caudal end, which remains in communication with the cavity of the pharyngeal intestine through its cephalic end. This communication orifice becomes the laryngeal orifice, separated from the base of the tongue by the epiglottis. 1.2. Bronchial buds As soon as it becomes individualized, the respiratory diverticulum divides at its caudal end into two buds, the right and left bronchial buds which will be at the origin of the main bronchi while the straight segment will be at the origin of the trachea. During the 5th week the bronchial buds will divide in turn but asymmetrically: on the left, the bronchial bud divides into two secondary bronchial buds which correspond to the future left lobar bronchi while on the right the bronchial bud is at the origin of three buds which correspond to the future right lobar bronchi. From then on, the lobar buds will be the site of successive divisions into two branches (dichotomous mode) which will give rise, between the 5th and 17th week, to bronchial elements whose caliber decreases as the divisions progress. This mechanism results in the formation of the entire airway from the lobar bronchi to the bronchioles (17th order divisions), although with frequent individual variations. Incidentally, the last divisions can occur beyond birth. 1.3. The walls of the airways The walls of the airways result from the differentiation of the mesenchyme in contact with the tips of the buds due to a reciprocal endoderm/mesenchyme induction mechanism. This differentiation is at the origin of the constituents of the walls: connective, muscular and cartilaginous tissues which will take on different aspects depending on the bronchial caliber.