A 48-year-old man with a 3-month history of oral erosions and scattered cutaneous blisters on the chest undergoes skin biopsy. Histology shows suprabasal acantholysis with a 'tombstone' appearance of basal cells. Serum testing is positive for anti-desmoglein 3 antibodies. Which feature most reliably distinguishes pemphigus vulgaris from pemphigus foliaceus?