Martha L. Warnock, M.D. and Leslie H. Zimmerman, M.D.
University of California, San Francisco
Chapter 1: Development and Anatomy
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Lung development: The lungs develop as a ventral protrusion of the foregut early in gestation. By 16 weeks, the major branches of the bronchial tree are developed. Gas-exchanging units number 20 million at birth, increase to 300 million at age 8, and continue to increase in size until the chest cage is fully developed. Thus, most alveoli develop after birth . Embryologically, the parathyroid glands and the thymus develop from the pharyngeal pouches and then migrate into the thorax. The thyroid develops at the base of the tongue and migrates caudally. Remnants of the thyroglossal duct and the pharyngeal pouches can remain and become cystic later on. Anomalies of migration can lead to ectopic glandular tissue .
Anatomy: Airways--The trachea is supported by a series of C-shaped cartilages (below) with the membranous, acartilaginous portion located posteriorly. Anomalies include ring cartilages and accessory tracheal bronchi, which may cause neonatal tracheal stenosis . The airways branch dichotomously about 20-24 times before reaching the pleura. After 12-16 branches, the bronchi lose their cartilages and become bronchioles .
The trachea and bronchi (defined as having cartilage in their walls) are lined by pseudostratified, ciliated columnar epithelium (below). The term "pseudostratified" refers to the location of nuclei at different levels, giving the appearance of stratification. However, all cells touch the basement membrane; thus true stratification, as in stratified squamous epithelium, is not normally seen. Epithelial lining cells consist of ciliated cells, goblet cells (<20% normally), an occasional Kultchitsky cell (neuroendocrine cells not visible with H&E), and reserve cells that have the potential to give rise to any of the differentiated cells. In the photo, note the cilia and an underlying dark pink line, the terminal bar. These two features can be used to define benign respiratory epithelium in cytologic preparations.
Epithelium rests on a thin basement membrane of type IV collagen. The subepithelial tissue is composed of loose connective tissue, including some fat cells, seromucous glands, lymphatics, bronchial arteries and veins, smooth muscle, and occasional lymphoid aggregates. Seromucous glands, composed of equal numbers of foamy mucus-secreting cells and blue-staining serous cells, normally occupy about 1/3 of the distance between basement membrane and perichondrium (normal gland to wall or Reid ratio). Chronic irritation of the respiratory mucosa produces hyperplasia of these glands (increased Reid ratio) and an increased ratio of mucus-secreting to serous cells .
The cilia on the surface of columnar respiratory cells (above) have an important function. They beat in a watery sol phase and propel mucus in the gel phase toward the mouth. Normal ciliary ultrastructure consists of a central pair of microtubules and 9 circumferentially-arranged doublets with dynein arms connected to the central pair by radial spokes. The outer dynein arm is more prominent than the inner one. Certain genetic or reactive abnormalities are associated with impaired ciliary function (ciliary dyskinesia syndromes), and may predispose patients to sinusitis, bronchiectasis, and male infertility (immotile sperm) .