The degree of cellular differentiation and maturation has a significant impact on prognosis and risk group assignment. Based on light microscopy features, neuroblastomas are classified into: undifferentiated (shown here), poorly-differentiated, and differentiating subtypes. In undifferentiated (Classic, Grade III-IV, Schwannian stroma-poor) subtype cellular differentiation is either absent or limited to less than 5% of the tumor. The tumor is composed of a uniform population of undifferentiated neuroblasts without clearly identifiable neuropil or rosettes. The neuroblasts are arranged in sheets or lobules which are incompletely separated by thin fibrovascular septa. With S-100 protein immunostain, there are no or very few Schwannian cells in these septa. The neuroblasts have large round to ovoid nuclei with salt and pepper chromatin, one or more prominent nucleoli, scanty cytoplasm, and ill-defined cytoplasmic borders.