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 .
In rats, the metanephric kidney develops at embryonic day 12, and this is followed by the ingrowth of the UB into the metanephric blastema
, inducing the metanephric mesenchymal stem cells (MMSCs) at the bud tips to condense around the UB tips.
Wilms' tumour also called as nephroblastoma is a malignant renal neoplasm of childhood that arises from remnant of immature kidney that is from abnormal proliferation of metanephric blastema
without differentiation into glomeruli and tubules.
It's thought to come from very specialised cells in the embryo known as metanephric blastema
. These cells are involved in the development of the child's kidneys while they're in the womb.
(3,4) From a pathophysiology standpoint, horseshoe kidney occurs during the second and sixth week of gestation when the inferior portion of the metanephric blastema
fuses to form an isthmus, commonly in the lower renal pole (90%) and anteriorly to the aorta and vena cava.
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
These genes code for a variety of transcription and growth factors required for the regulation and orchestration of interactions between the ureteric bud and the metanephric blastema
and its predecessor's tissues, the mesonephric and pronephric ducts.
The ureteral bud arises from the dorsal aspect of the distal mesonephric duct, which extends in a dorsocranial fashion to meet and induce differentiation of the metanephric blastema
. This forms the adult definitive kidney.
Over bending and rotation of the caudal end of the embryo prevents the ureteric bud from merging with the ipsilateral metanephric blastema
and thus is attracted towards the now more closer contralateral side.
This is thought to result from the abnormal development and migration of the ureteric bud and metanephric blastema
during the fourth week to eighth week of gestation.
Moreover, the ingrowth of the branching ureteric buds into the metanephric blastema
results in the characteristic lobulated appearance of the definitive kidney.