Poorly-differentiated neuroblastoma is composed of typical neuroblasts and 5% or less of the tumor shows differentiation toward mature ganglion cells. It is the most commonly seen pattern in neuroblastoma group. The tumor shows neuropil and Homer-Wright rosettes consisting of an arrangement of tumor cells around a central area (not a lumen, canal, or a blood vessel) filled with neurofibrillary processes. Lobules of neuroblasts are completely or incompletely separated by delicate fibrovascular septa which usually contain S-100 protein positive Schwann cells or Schwannian blasts sometimes mimicking paragangliomas or pheochromocytomas. Calcification is usually present in the form of basophilic aggregates, round concretions, or in a “chicken wire” fashion enveloping individual tumor cells.