Lung Morphology and Particle Deposition

LUNG MORPHOLOGY AND PARTICLE DEPOSITION

The cellular composition of the lung as well as its geometry influence particle deposition. A few of the more important cellular characteristics are described in this technical note.

The largest airway, the trachea, is lined by a ciliated epithelium covered by mucus and serous secretions produced by the cells and glands of the epithelium. These secretions may trap and/or dissolve some inhaled particles. The beating of the cilia tends to drive the secretions upward toward the mouth where they are ingested or expectorated. Any particles deposited in this portion of the lung will also be expelled in this manner.

As the airways divide and become smaller in diameter, the composition of the epithelium also changes becoming thinner with less ciliated cells and increasing numbers of Clara or non-ciliated bronchiolar cells. Here clearance via the mucociliary escalator is less likely than in the trachea. However, the Clara cells do produce secretions and have metabolic capabilities. Thus they can detoxify locally absorbed chemicals.

As airways become even smaller in diameter (respiratory and terminal bronchioles), they open into the alveolar region. The alveolar region is lined primarily by very thin Type I epithelial cells. The other less frequent epithelial cell that occurs with more frequency is the pulmonary macrophage. Macrophages are specifically recruited (by migration or cell division) to the alveolar region when there is a local particle burden. They engulf the particles and either migrate into the lung interstitium or move up the airways to be caught and removed by the mucociliary escalator.

It is especially important to understand the anatomy of the alveolar region of the lung, as this is where the primary function of the lung occurs - that of gas exchange. Only a layer of surfactant along with the thin Type I cell, a thin interstitium containing fibroblasts and endothelial cells separate air from blood. A particle overload in this region could result in decreased oxygenation of the blood. Particle overload would occur when more particles were deposited than could be engulfed and removed by macrophages and the mucociliary escalator.

This brief descriptive note has not taken inherent particle toxicity into account as a factor in altered particle deposition. One must be aware that the chemical composition of the particle itself may have an effect not only upon deposition, but also upon clearance and final morphological changes in the lung. For example, quartz particles produce granulomas in the alveolar region while fly ash particles do not. Ultrafine particles defined as having a diameter of less than 100 nm have recently been found to have the potential to cause more lung damage than previously thought. 200 nm diameter titanium dioxide particles are considered harmless in humans but 20 nm diameter titanium dioxide particles cause severe inflammation indicative or morphological damage. Particle toxicity will be a subject for a future note.

Reference: Toxic Responses of the Respiratory System, Witschi, H. and Last, J.A. in Casarett & Doull's Toxicology - The Basic Science of Poisons, Fifth Edition, McGraw-Hill, 1996.

by Susan G. Shami, Sc.D.