The Maturation of Airway Structure and Function

Objectives

After completing this article, readers should be able to:

1. Explain the physiologic reasons that immature infants are at a greater risk of mechanical ventilation-induced airway damage than are more mature infants.

2. Describe the age-related differences in airway function during perinatal development.

3. Delineate how differential expression of smooth muscle myosin heavy-chain isoforms affects airway smooth muscle physiology and airway mechanics.

4. Explain how expression of smooth muscle proteins contributes to the functional maturation of airway smooth muscle.

Introduction

Airway formation is one of the earliest events in the development of the respiratory system. However, continued maturation of the airway continues well into postnatal life. With advances in the medical care of increasingly preterm infants, an understanding of developmental airway physiology becomes imperative. This article reviews maturation of the airway and its components as well as their contributions to airway function during development.

Airway Embryology

Airway development in the human begins during the fourth week of gestation when the respiratory diverticulum, or lung bud, branches from the embryonic foregut. The esophagotracheal septum forms and separates the foregut into the esophagus dorsally and the trachea ventrally. Thus, the developing airway is of foregut endodermal origin. Elongation of the respiratory diverticulum forms the trachea; branching forms the main stem bronchi. Growth and elongation continues in the caudal direction under the influence of airway secretions and physical forces. By the end of the 16th week of gestation, branching of the conducting airways is complete.

The airway matures initially in the trachea and proceeds distally. Tracheal cartilage begins to form during the seventh week of gestation. Maturation of the distal airway cartilage is not complete until after birth. Epithelial differentiation begins in the trachea during the 10th week of gestation. Together with phasic contractions of the fetal airway smooth muscle, which are present following the 23rd gestational week, lung fluid …