The inadequate stimulation of metanephrogenic blastema
by the ureteric bud results in renal maldevelopment.
The bud grows laterally and invades the centre of the metanephrogenic blastema
, the primordial renal tissue.
The ureteral bud also grows cranially towards the metanephrogenic blastema
to stimulate renal development.
(2) There are several theories to explain the occurrence of supernumerary kidney: a) bifurcation of ureteric bud with independent penetration into metanephrogenic blastema
that develop and divide in two kidneys; b) two independent ureteral buds that penetrate the metanephrogenic blastema
that divides into two and c) fragmentation of metanephrogenic blastema
due to linear infarcts.
This structure then divides to two or three before reaching the metanephrogenic blastema
.[3-5] In such cases, it may be difficult to decide whether the left kidney represents a trifid pelvis, purely a peduncular arrangement or elongation of the major calyces in association with a rudimentary pelvis.