A 3-year-old boy presents with an abdominal mass detected by his mother during bathing. Imaging shows a unilateral intrarenal mass that does not cross the midline. Histopathology reveals the triphasic pattern characteristic of Wilms tumor. The structure marked **A** in the diagram represents the blastemal component (small round blue cells). Which of the following best describes the embryologic origin and clinical significance of this blastemal component?