During their histogenesis, cutaneous neural tumors either recapitulate or retain to a variable extent the architectural arrangement and cytologic components of the normal peripheral nerve. Therefore, familiarity with the normal histology of the peripheral nerve is essential for the correct characterization of neural tumors.1 In short, the structural organization of the peripheral nerve follows a hierarchical arrangement of different compartments. The basic unit is the nerve fiber, which is composed of an axon (neurite) and the surrounding Schwann cells.2 Axons are cytoplasmic extensions of neurons that are located in the central nervous system (CNS) or in the sympathetic ganglionic chain. Axons become myelinated by multiple periaxonal rotations of the surrounding Schwann cells, thus creating a concentric layer of cytoplasmic membranes. Unmyelinated nerve fibers develop by invagination of the axons into the cytoplasm of the Schwann cells. In the next architectural compartment, several nerve fibers form nerve fascicles that are surrounded by a sheath, called a perineurium. The space within the perineurium is referred to as the endoneurium.2 The perineurium is composed of a specialized cell type, the perineurial cell, that is in continuity with the pia-arachnoidal lining of the CNS. Besides the nerve fibers, the endoneurium also contains fibroblasts, capillaries, and mast cells. In the final organizational compartment, numerous nerve fascicles are grouped into nerve bundles that are held together by a protective sheath referred to as the epineurium.2 The epineurium is made up of fibroblasts, collagen, and adipose tissue, but fine vascular structures also penetrate its wall. The sensory nerves are connected to various types of sensory corpuscles (eg, pacinian, meissnerian, Vater-Pacini, and Krause) that are differentiated to specific sensory perceptions.
Cutaneous neural tumors represent either primary tumors of the peripheral nerves or ectopic-heterotopic tissues from the CNS or sympathetic ganglionic chain. Further classification is usually based on their assumed histogenetic differentiation. Table 33-1 contains one of the currently used classifications of cutaneous neural tumors.
TABLE 33-1Classification of Cutaneous Neural Tumors ||Download (.pdf) TABLE 33-1 Classification of Cutaneous Neural Tumors
|PERIPHERAL NERVE SHEATH TUMORS ||NEURAL HETEROTOPIAS |
(reactive or hamartomatous)
True nerve sheath neoplasms
Soft tissue type
Nerve sheath myxoma
Granular cell tumor
Malignant peripheral nerve sheath tumor
Neuroendocrine carcinoma of the skin
(Merkel cell carcinoma)
Primary primitive neuroectodermal tumor
Pigmented neuroectodermal tumor of infancy
REACTIVE OR HAMARTOMATOUS LESIONS OF PERIPHERAL NERVE
Synonyms: Spontaneous neuroma, solitary circumscribed neuroma, palisaded encapsulated neuroma, true neuroma, amputation neuroma, traumatic neuroma, supernumerary digit.