Schwannian crystalline-like inclusions bodies (Fardeau-Engel bodies) revisited in peripheral neuropathies

In 1969, Fardeau and Engel described polygonal organelles containing crystalline-like structures located in the Schwann cell cytoplasm of unmyelinated fibers. Such inclusions were reported in various conditions, mainly with axonal lesions. They were also reported in a few cases of peripheral neuropathy associated with primary mitochondriopathy. Although they are surrounded by a double membrane, their mitochondrial origin is not definitely proven. Their significance remains obscure but they deserve to be better known as Fardeau-Engel bodies, so as not to be mistaken with mitochondrial crystalline inclusions, which are frequently observed in patients with ragged-red fibers in muscle.