Ultrastructure of the tracheobronchial epithelium of the normal and formaldehyde-exposed guinea-pig

ABSTRACT

Thirty adult male guinea pigs were used in this study, fifteen were taken as control, while the other fifteen were exposed to formaldehyde.The tracheobronchial epithelium of all animals were examined by means of light and electron microscopy. The tracheobronchial epithelium of the control animals was found to consist of ciliated, goblet, basal and intermediate cells. The general structure of the epithelium is suggested to provide a barrier restricting the transfer of exogenous material from the airway lumen to the blood stream. Moreover, the actively motile cilia of the ciliated cells are responsible for transport of mucus with included foreign materials to the throat where the mucus normally swallowed. The goblet cells continuously secrete mucus which formed a physical barrier between the inhaled pollutants and underlying epithelium. The results of this work suggested that, both motile cilia and secretion of correct amount of mucus with suitable vesicoelasticity are responsible for maintenance of mucociliary clearance. Also the intermediate cells were observed to be differentiated into goblet cells. Following formaldehyde exposure, profound general and special epithelial and submucosal changes, ranging from moderate to severe were observed within the tracheobronchial epithelium. Early after short periods of formaldehyde exposure, the structural changes in the bronchial epithelium were observed to be less severe than that of the tracheal epithelium. However, later on, with prolongation of the period of formaldehyde exposure, both trachea and bronchi were severely affected. The general epithelial changes included, 1] ulceration of the tracheobronchial epithelium which is emphasized to expose and sensitize the airway receptors to inhaled irritants which are responsible for the pathogenesis of occupational asthma. 2] proliferative changes in the form of simple hyperplasia, basal and goblet cell hyperplasia and squamous metaplasia have occurred and suggested the possibility of formaldehyde carcinogenicity 3] thickening of the basement membrane which is considered to be a protective mechanism against inhaled formaldehyde. On the other hand, the special epithelial changes were evidenced by the appearance of a great number of ciliary abnormalities and apocrine massive mucus secretion of the goblet cells with subsequent impairment of the mucociliary clearance. Both fibrosis and airway obstruction are considered to be the latest sequlae of long term injury produced by formaldehyde exposure as indicated by the appearance of fibroblasts and smooth muscle fibers within the submucosa of tracheobronchial epithelium