The blastemal component of a Wilms tumor has the prototypical appearance of a small round blue cell tumor. It is highly cellular and consists of undifferentiated small round-to-oval cells with hyperchromatic nuclei and scant cytoplasm. The cells may be arranged in diffuse sheets, serpentine, or nodular patterns. Mitotic activity is generally brisk. Apoptosis (illustrated here) and foci of necrosis may be seen within blastema.