Figure 1. Wilms tumor shows remarkable mimicry of the nephrogenic process. The left panel shows the nephrogenic zone of an 18-week gestation human fetal kidney. There is orderly differentiation of the primitive blastema into tubular (epithelial) components in response to reciprocal inductive signals from the branching ureteric bud. The right panel shows a classic triphasic Wilms tumor in which all three cell types (primitive blastema, epithelia and stroma) are seen. In this particular tumor, epithelial differentiation is proceeding as far as an attempt to make pseudoglomeruli.

Figure 2. Intralobar nephrogenic rest (ILNR). ILNRs are regions of residual primitive nephrogenic tissue thought to arise at an early stage of nephrogenesis. As in this case (white arrow), they usually lie immediately adjacent to the associated Wilms tumor (black arrow), the latter often showing prominent stromal differentiation. ILNRs usually mimic the entire spectrum of renal histogenesis, are usually single and are thought to have a high tumorigenic potential. (Photograph courtesy of Dr Gordan Vujanic, University Hospital of Wales, Cardiff.)

Figure 3. Perilobar nephrogenic rest (PLNR). A PLNR (white arrow) is a well-circumscribed area of incompletely differentiated fetal renal tissue. These are usually found at the periphery of the renal lobule, are often multiple and show restricted differentiation potential, mimicking late stages of nephrogenesis with mainly tubular differentiation. The associated Wilms tumors (black arrow) usually have restricted, mainly epithelial differentiation. (Photograph courtesy of Dr Gordan Vujanic, University Hospital of Wales, Cardiff.)

Figure 4. Age distribution of Wilms tumor in relation to bilaterality and type of associated nephrogenic rest (a) unilateral unicentric tumors; (b) bilateral tumors; (c) unilateral, multicentric tumors. (Data courtesy of Professor Norman Breslow and the National Wilms Tumor Study Group, North America.)

Figure 5. Structure of the WT1 gene, showing the position of the two alternative splice sites, comprising the whole of exon 5 and the additional amino acids, KTS, in the linker between the third and fourth zinc-fingers. The position of two commonly mutated arginine residues in Denys–Drash syndrome is indicated (R³⁶⁶ and R³⁹⁴) – these residues are critical for DNA binding by these zinc-fingers. The N-terminal transcriptional regulatory domains defined by in vitro studies are indicated, the dotted line indicates the possible usage of an alternative initiation codon.