A neurofibroma is a benign nerve-sheath tumor in the peripheral nervous system. In 90% of cases, they are found as stand-alone tumors (solitary neurofibroma, solitary nerve sheath tumor or sporadic neurofibroma), while the remainder are found in persons with neurofibromatosis type I (NF1), an autosomal-dominant genetically inherited disease. They can result in a range of symptoms from physical disfiguration and pain to cognitive disability. Neurofibromas arise from nonmyelinating-type Schwann cells that exhibit biallelic inactivation of the NF1 gene that codes for the protein neurofibromin. This protein is responsible for regulating the RAS-mediated cell growth signaling pathway. In contrast to schwannomas, another type of tumor arising from Schwann cells, neurofibromas incorporate many additional types of cells and structural elements in addition to Schwann cells, making it difficult to identify and understand all the mechanisms through which they originate and develop. Types Neurofibromas have been subdivided into two broad categories: dermal and plexiform. Dermal neurofibromas are associated with a single peripheral nerve, while plexiform neurofibromas are associated with multiple nerve bundles. According to the World Health Organization classification system, dermal and plexiform neurofibromas are grade I tumors. Plexiform neurofibroma are more difficult to treat and can transform into malignant tumors. Dermal neurofibroma do not become malignant. Dermal neurofibroma Anatomy Dermal neurofibromas (sometimes referred to as cutaneous neurofibromas) originate in nerves in the skin. Three kinds are distinguished: Discrete cutaneous neurofibromas: Sessile or pedunculated masses on the skin, which are fleshy and non-tender, and can vary in size. Discrete subcutaneous neurofibromas: Lie below and look like bumps on the skin, which can sometimes be tender. Deep nodular neurofibromas: Involving tissues and organs underneath the dermis, but otherwise resembling cutaneous and subcutaneous neurofibromas. Age of onset Dermal neurofibromas typically arise in the teenage years and are often associated with the onset of puberty. In people with Neurofibromatosis Type I, they tend to continue to increase in number and size throughout adulthood, although limits exist as to how big they get, and cases progress at highly variable rates. Medical complications Dermal neurofibromas can lead to stinging, itching, pain, and disfigurement. No evidence of malignant transformation has been found. Plexiform neurofibroma Anatomy Plexiform neurofibromas can grow from nerves in the skin or from more internal nerve bundles, and can be very large. Internal plexiform neurofibromas are very difficult to remove completely because they extend through multiple layers of tissue and the attempt would damage healthy tissue