Although ingrown toenail (IT) is a common problem, the morphologic alterations presented in this condition have rarely been presented in literature. We have studied 14 cases from 12 different patients and studied the biopsies of all of them, not only with routine techniques but also with histochemical stains (Masson trichrome, periodic acid-Schiff, and Gram) and with immunohistochemical techniques mindbomb homolog-1 (MIB-1), p63, D240, and AE1-AE3). The patients were clinically classified into 4 stages (1, 2a, 2b, and 3). The main changes evidenced were ingrowing of the squamous epithelium (either from the matrix or mostly from the hyponichium) and chronic plasma-cell rich inflammation with prominent Vascularization. The intensity of these changes varied depending on the stage: whereas they were mild in stages I and 2a, they were prominent in stages 2b and 3. In advanced stages, the epithelial nests showed certain dyskeratosis. The expression of MIB-1 was, nevertheless, only basal and parabasal. p63 was intensively expressed by the epithelial nests. We therefore conclude that (a) IT shows epithelial ingrowing with accompanying granulation tissue; (b) according to the clinical stage of the disease, this pattern can be more or less complex; (c) in advanced stages of IT, the pattern can mimic a well-differentiated squamous cell carcinoma; (d) MIB-1 is not helpful in distinguishing IT from squamous cell carcinoma; (e) the high expression of p63 by the ingrowing epithelium should not be the source of a misdiagnosis of malignancy.
