What are the main types of cells in the respiratory system?

The cells of the respiratory mucosa may be categorized by considering those tissues that make up the conducting portion (nose, nasal cavity, pharynx, larynx, trachea, bronchi, and terminal bronchioles) and the respiratory portion (respiratory bronchioles, alveoli).

The respiratory mucosa lines the conducting portion. This mucosa consists of epithelium and an underlying tissue called the lamina propria which supports the epithelium. The cells that make up part of the respiratory epithelium depend on the location within the respiratory tract. The nasal cavity, the upper part of the pharynx (throat), the trachea, and the bronchi are lined with pseudostratified ciliated columnar epithelium. Columnar cells are hexagonal cells; "pseudostratified" means that all cells touch the basal lamina, or basement membrane. These cells all contain cilia, or organelles that extend from the surface of the cell. These cilia move mucus upward into the pharynx. Mucus is secreted by goblet cells within the epithelium. Parts of the pharynx (oropharynx) have stratified squamous epithelium (cells are in layers, but all cells do not contact the basement membrane); these cells are flat.

Parts of the conducting airway contain chondrocytes as part of the supporting connective tissue known as cartilage. This particular type of cartilage has chondrocytes within open areas or lacunae surrounded by the matrix (protein fibers and ground substance). The mucosa of the trachea is surrounded by a layer of connective tissue called the submucosa. The trachea also has smooth muscle. Bronchi also have mucosa, cartilage, and smooth muscle. The smaller bronchi which are located within the lung are lined by pseudo-stratified columnar epithelium, and have smooth muscle that completely encircles the lumen (opening of the tube) and goblet cells. For the respiratory airway, bronchioles lack goblet cells, and have either columnar epithelium or cuboidal epithelium. The alveoli consist of simple squamous epithelium (Type I cells), septal cells (Type II cells) that produce surfactants that prevent alveoli from collapsing, and alveolar macrophages that scavenge any particles which may have escaped the mucus and cilia. Elastic and smooth muscle fibers maintain the shape of the alveoli.

References

Martini, FH. 2004. Fundamentals of anatomy and physiology, 6th ed. San Francisco: Benjamin-Cummings. Ch. 4, 5, 23.

Ham, AW. 1953. Histology, 2nd ed. Philadelphia: Lippincott. Ch. 24.