Bullae are simply large vesicles. They may arise from cleft formation in or below the epidermis, or following coalescence of adjacent foci of intercellular epidermal edema. Comedone P,S Dilated hair follicle containing a pigmented impaction of lipid and keratinaceous debris (a "blackhead). Often seen in keratinization defects; can be seen with demodicosis, and are a frequent finding on the abdomen of dogs with hyperadrenocorticism.