The cells of the blastema (now called scleroblasts) were arranged side-by-side forming in single row and had begun the synthesis of the lepidotrichial matrix in the region directed toward the basal membrane of the epithelium (Figures 6, 7 and 8).
In embryonic life the kidney develops from the metanephric blastema. If remnants of this tissue remain within the renal parenchyma in post natal life, they often develop into Wilms' tumour or rarely into metanephric adenoma.6 Thus Wilms' tumour and metanephric adenoma are histogenetically related and MA is considered to be the benign counterpart of Wilms' tumour in adults.4,6,14 Since they arise from the same precursor tissue, metanephric adenoma and Wilms' tumour may sometimes co-exist and adequate histological sampling is essential to differentiate between the two tumours.4,5 Histologically metanephric adenoma is also very similar to the metanephric hamartomatous element of nephroblastomatosis.5
The aetiology is unclear, although it is hypothesised that the problem lies with the ureteric bud crossing over to the contralateral metanephric blastema, thereby inducing differentiation of a renal promordium.4
Using the zebrafish caudal fin regeneration model, we have previously shown that TCDD-induced AHR activation blocks the tissue regeneration process via activation of the Wnt/[beta]-catenin signaling pathway, leading to improper formation of the wound epithelium and blastema (Mathew et al.
Jeffery, "Evolution of the chordate regeneration blastema: differential gene expression and conserved role of notch signaling during siphon regeneration in the ascidian Ciona," Developmental Biology, vol.
The tumor consisted in tumor epithelial component (abortive tubules and glomeruli) surrounded by metanephric blastema and tumor immature spindled cell stroma without any anaplasia or emboli of tumor cells.
Crossed renal ectopia is a rare entity detected incidentally in 20 to 30% of cases  and results from the aberrant migration and crossing ofthe midline by the metanephric blastema and the ureteral bud , usually occurring during the fourth to eighth week of gestation .