SEMA3A and myeloid sarcoma: Positive regulators of OPC differentiation in MS are found in acute areas of inflammation and in early stages of lesion formation [36], but they are absent in a chronic inflammatory environment [37], where inhibitors of oligodendrocyte differentiation, such as TGF-β (Transforming growth factor beta) and semaphorin 3A (SEMA3A), are present [34].