Mast cells in normal and pathological specimens of the equine lung.

In lungs from 22 horses, the number and type of mast cells in histologically normal specimens were compared with those in specimens with various histological lesions. Tissues fixed in Carnoy's solution were superior to those fixed in formalin for the identification of mast cells. Fixation of lung specimens from eight horses in Carnoy's solution and formalin allowed the identification of two subpopulations of mast cells: formalin-sensitive, which predominated and formalin-resistant. Specimens with mild chronic tracheitis and mild and moderate chronic bronchitis generally had slight increases in mast cell numbers in comparison to histologically normal specimens. An increase in mast cell numbers was also observed around small and large bronchioles with mild, moderate and severe bronchiolitis and in fibrosed pulmonary pleurae in comparison to specimens with no histological lesions. The most striking increase in mast cell numbers was seen in fibrosed alveolar septa; there were approximately four times as many mast cells in these specimens as in specimens with normal histology.