Mast cell density and subtypes in the skin of dogs with atopic dermatitis

Skin biopsies from seven dogs with atopic dermatitis and 13 dogs with no clinical or histological evidence of skin diseases were examined. The study of the atopic dogs included 11 biopsy samples of nonlesional skin and 15 samples of lesional skin. One section of each tissue sample was stained with haematoxylin and eosin and another with toluidine blue to demonstrate the sulphated acid glycosaminoglycans in mast cell (MC) granules. To investigate MC subtypes, the MC-specific proteases tryptase and chymase were examined by a double enzyme-immunohistochemical staining technique. With the double labelling technique a significantly lower mast cell density was demonstrated in lesional (P = 0.0023) and nonlesional (P = 0.0004) skin samples of the atopic dogs than in the skin of control dogs. In the dermis of control dogs, a median mast cell density of 31.2 MC/mm2 was detected with the toluidine blue staining method and of 27.5 MC/mm2 with the double labelling technique. In lesional dermis of atopic dogs 29.8 MC/mm2 were seen with toluidine blue while only 12.4 MC/mm2 were stained with the double labelling method (P = 0.0027). A similar difference was observed in nonlesional dermis samples, in which a mast cell density of 23.3 MC/mm2 was detected with toluidine blue but only 6.4 MC/mm2 with the double labelling method (P = 0.0127). The data provide evidence that mast cell granule heterogeneity exists in the dog and suggests that degranulation occurs selectively, depending on the pathological condition of the canine skin. Further investigations on the pathophysiological role of mast cell subtypes may help to elucidate the pathogenesis of atopic dermatitis.