A morphometric study of nasal-pharyngeal growth for particle deposition in the rat

Animal studies frequently are used in assessing potential human health effects from exposure to inhaled toxicants. Such studies also are used to investigate sensitive subpopulations such as children. Among other factors that influence the degree to which animal models are predictive of human effects in the delivered dose of the toxicant to the various regions of the respiratory tract. Because the rat is an obligatory nose breather, an understanding of the rat nasal-pharyngeal airway geometry is needed to relate exposures to delivered doses. In this study, the growth and development of the rat nasal-pharyngeal airway was studied at one-week intervals in male Fischer-344 rats from one to five weeks. Casts of an adult (60 day) and an aging (441 day) rat were included for comparison. Replica casts of the nasal-pharyngeal airway were made by injecting silicone rubber through the trachea, and sections in anterior-posterior positions were made for morphometric study. A simple structure of the nasal-pharyngeal airway was found in the young rats. While the percentage of the airway composed of turbinates was similar at all ages, the surface area of the turbinates increased 7.7-fold between 7 and 60 days. Because of the simpler structure and smaller surface area in the young rat, extrathoracic clearance is probably less efficient, resulting in a higher delivered dose to the lung of a young rat than to that of an adult rat exposed to the same toxicant concentration.