Collectively, these blastema form the metanephric system, which has two components: The ureteric bud from the mesonephric duct makes contact with a caudal region of intermediate mesoderm – the metanephric blastema (Fig 2). It appears in the 5th week of development and becomes functional around the 12th week. The metanephros forms the definitive kidney. They act as a primitive excretory system in the embryo, with most tubules regressing by the end of the 2nd month.Īdditionally, the mesonephric duct sprouts the ureteric bud caudally, which induces the development of the definitive kidney. These tubules receive a tuft of capillaries from the dorsal aorta – allowing for the filtration of blood – and they drain into the mesonephric duct (a continuation of the pronephric duct). First, the presence of the pronephric duct induces nearby intermediate mesoderm in the thoracolumbar region to form mesonephric tubules. The mesonephros develops caudally (inferiorly) to the pronephros. This early system is non-functional and regresses completely by the end of week 4. These tubules join into the pronephric duct, which is a duct that extends from the cervical region to the cloaca (distal end) of the embryo.
In total, 6-10 pairs of nephrotomes are formed. Segmented divisions of intermediate mesoderm form tubules, known as nephrotomes. Its development begins in the cervical region of the embryo.
The pronephros appears in the 4th week of development. They are all derived from the urogenital ridge. In the embryo, the kidneys develop from three overlapping sequential systems the pronephros, the mesonephros, and the metanephros.
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