Development of the Urinary System
The urinary system consists of the kidneys, ureters, bladder and urethra. They are derived from the urogenital ridge, a region of intermediate mesoderm that gives rise to both the embryonic kidney and the gonad.
The definitive kidney becomes functional in the 12th week of development where urine is passed into the amniotic fluid, swallowed, and recycled.
In this article, we will look at the embryology of the urinary system and its clinical correlations.
Development of the Kidneys
The kidneys are primarily responsible for filtering and excreting waste products from the blood. They also have a role in water and electrolyte balance.
In the embryo, the kidneys develop from three overlapping sequential systems – the pronephros, the mesonephros and lastly, the metanephros.
The pronephros is the primary kidney system, and appears in the 4th week of development.
Its development begins in the cervical region of the embryo. Segmented divisions of mesoderm form tubules, known as nephrotomes. In total, 6-10 pairs of nephrotomes are formed.
These tubules join into the pronephric duct (a duct that extends from the cervical region to the cloaca of the embryo). This early system is non-functional and regresses completely by the end of week 4.
The mesonephros develops caudally (inferiorly) to the pronephros. First, the presence of the pronephric duct induces nearby intermediate mesoderm in the thoracolumbar region to form mesonephric tubules.
These tubules receive a tuft of capillaries from the aorta – allowing for the filtration of blood – and they drain into the mesonephric duct (a continuation of the pronephric duct). They act as a primitive excretory system in the embryo, with most tubules regressing by the end of the 2nd month.
Additionally, the mesonephric duct sprouts the ureteric bud caudally, which induces the development of the definitive kidney.
The metanephros forms the definitive kidney, and appears in the 5th week of development. It becomes functional around the 12th week.
The ureteric bud from the mesonephric duct makes contact with a caudal region of intermediate mesoderm – the metanephric blastema. Collectively, they form the metanephric system, which has two components:
- Collecting system – derived from the ureteric bud.
- It dilates to create the renal pyramid, major and minor calyces and collecting tubules.
- Excretory system – derived from the metanephric blastema.
- Each collecting tubule from the collecting system is covered by a metanephric tissue cap which gives rise to the excretory tubules.
- The proximal end of the excretory tubule forms the Bowman’s capsule around a glomerulus, creating the nephron. The excretory tubule elongates to form the proximal convoluted tubule, loop of Henle and distal convoluted tubule.
The definitive kidney initially develops in the pelvic region before ascending into the abdomen. In the pelvis, the kidney receives its blood supply from a pelvic branch of the abdominal aorta and as it ascends, new arteries from the abdominal aorta form to supply the kidney. The pelvic vessels usually regress, but can persist as accessory renal arteries.
Clinical Relevance: Horseshoe Kidney
A horseshoe kidney (also known as a cake kidney or fused kidney) is where the two developing kidneys fuse into a single horseshoe-shaped structure.
This occurs if the kidneys become too close together during their ascent from the pelvis to the abdomen – they become fused and consequently ‘stuck’ underneath the inferior mesenteric artery.
This type of kidney is still drained by two ureters, and is usually asymptomatic, although it can be prone to obstruction.
Development of the Bladder and Urethra
The bladder and urethra of the urinary system are ultimately derived from the cloaca – a hindgut structure that is a common chamber for the gastrointestinal and urinary systems.
In the 4th-7th weeks of development, the cloaca is divided is divided into two parts by the uro-rectal septum:
- Urogenital sinus (anterior)
- The upper part of the urogenital sinus forms the bladder.
- The pelvic part forms the entire urethra in females, most of the urethra in males, and some of the female reproductive tract
- The phallic part forms part of the female reproductive tract, and the spongy urethra in males.
- Anal canal (posterior)
The bladder is initially drained by the allantois. However, this is obliterated during development and becomes a fibrous cord – the urachus. A remnant of the urachas can be found in adults; the median umbilical ligament, which connects the apex of the bladder to the umbilicus.
As the bladder develops from the urogenital sinus, it absorbs the caudal parts of the mesonephric ducts (also known as the Wolffian ducts). The uterers, which have formed as outgrowths of the mesonephric ducts, enter the bladder separately. The final structure varies between sexes: