Final answer:
The definitive kidney or metanephros is formed from the interaction of the ureteric bud and the metanephric mesenchyme. The ureteric bud becomes the ureters and part of the bladder, while the metanephric mesenchyme develops into nephrons. The kidneys and ureters are retroperitoneal with the ureters connecting to the bladder in a way that prevents urine backflow.
Step-by-step explanation:
The Formation of the Definitive Kidney (Metanephros)
The definitive kidney, or metanephros, is the fully mature third and final stage of kidney development in the human embryo. It begins forming by the fifth week of gestation. The metanephros arises from two distinct embryonic structures: the ureteric bud, which becomes the ureters and part of the bladder, and the metanephric mesenchyme, which turns into the functional kidney units known as nephrons.
The ureteric bud is an outgrowth of the mesonephric duct that interacts with the metanephric blastema or mesenchyme. This interaction induces the formation of the nephrons, starting from the renal vesicles to eventually form the glomeruli, Bowman's capsules, proximal and distal convoluted tubules, loop of Henle, and connecting tubules. Concurrently, the branching of the ureteric bud creates the collecting duct system and the renal pelvis, which narrows into the ureters. The ureters then develop and connect to the bladder to permit the flow of urine.
The kidneys and ureters are completely retroperitoneal, while the bladder is covered by the peritoneum only at the dome. The ureters propel urine towards the bladder via peristalsis and penetrate the bladder wall obliquely to prevent backflow, forming a physiological valve. This process is crucial for normal urinary function and preventing conditions like vesicoureteral reflux.