Trichogenic trichoblastoma. An unusual neoplasm of hair germ.

Tumors of hair germ are a very rare group of primary cutaneous neoplasms that have been divided into two groups: epithelial and mesenchymal. Epithelial neoplasms of hair germ are further subdivided into those that are purely epithelial (trichoblastomas) and into epithelial neoplasms with mesenchymal components that may cause inductive changes toward the earliest phase of hair follicle differentiation (trichoblastic fibroma) or a more advanced and complete hair follicle formation (trichogenic trichoblastoma). We herein report an ulcerated trichogenic trichoblastoma whose epithelial and mesenchymal components recapitulated the formation of numerous primitive dermal papillae of the hair bulb because there were numerous strands of small polygonal epithelial cells that delimited areas of a dense and abundantly cellular connective tissue in a hair papilla-like pattern.