Mast cell distribution, epidermal thickness and hair follicle density in normal canine skin: possible explanations for the predilection sites of atopic dermatitis?

Mast cell counts, epidermal thickness and hair follicle density were quantified in toluidine blue stained sections of normal skin from 20 different body regions in 10 dogs and compared to the predilection sites of canine atopic dermatitis. Mast cell distribution varied significantly from site to site (P < 0.0001) and counts in the superficial dermis were significantly higher than the deeper dermis (P < 0.05). Mast cell counts were highest in the medial and lateral pinna (mean 10.4-11.3 per high power field, HPF) and in the ventral interdigital skin of the hind and fore feet (mean 9.2-9.5 per HPF). Counts in these regions were at least 150% higher than all the other sites (means ranging between 2.9 and 6.0 per HPF). Variations in mast cell counts, epidermal thickness or hair follicle density did not adequately explain the predilection sites of canine atopic dermatitis. However, the results provide some evidence that cutaneous mast cell distribution may be involved in the frequent occurrence of ear and foot pruritus in this disease.