Next Lesson - The Nephron
Core
The urinary and reproductive systems develop together. Both begin from the intermediate mesoderm. This is a strip of tissue on each side of the midline of the early embryo, part of the layer of cells called the mesoderm. The strip is called the urogenital ridge. In this article, weeks are counted from fertilisation unless the text says obstetric or gestational weeks, which are counted from the first day of the last period (about two weeks earlier).
The development of the kidneys in the embryo involves the formation of three separate systems, with development of the pronephros, mesonephros and metanephros overlapping in time. They are all derived from the urogenital ridge, the strip of intermediate mesoderm that also gives rise to the reproductive system. The part of this strip that forms the urinary system is known as the nephrogenic cord, and the part that gives rise to the reproductive system is the gonadal ridge.
The pronephros begins development around the start of the fourth week after fertilisation (approximately the sixth obstetric gestational week) in the cervical region of the embryo.
Divisions of intermediate mesoderm form between 6 and 10 pairs of tubules named nephrotomes. These nephrotomes join the pronephric duct, which is a duct that extends from the cervical region to the cloaca (the common chamber at the tail end of the embryo that the urinary, reproductive and digestive tracts open into; see later).
The pronephros is never functional, and regresses completely by the end of the fourth week after fertilisation (approximately the sixth obstetric gestational week).
The mesonephros is the second embryonic kidney system to develop. It starts to form caudally (inferiorly), below the pronephros. The pronephric duct induces the development of the mesonephros from the nearby mesoderm in the thoracolumbar region (the middle part of the trunk, where the chest meets the lower back).
The mesonephros is made of tubules (small tubes). These tubules receive a blood supply from the aorta, which allows blood to be filtered. They drain into the mesonephric duct, a continuation of the pronephric duct. In males, this mesonephric duct goes on to form the epididymis, vas deferens and seminal vesicles (parts of the male internal genitalia, the reproductive organs inside the body) under the influence of testosterone.
The mesonephros acts as the excretory system for the embryo for weeks four to eight. Around the fifth week after fertilisation, the ureteric bud develops from the caudal (lower) end of the mesonephric duct, prompting the development of the metanephros (the definitive kidney).
The metanephros is the definitive kidney, meaning this is the same kidney that exists into adulthood.
It appears in around the 5th week of development and becomes functional at around 12 weeks of gestation (about 10 weeks after fertilisation). Its formation is prompted when the ureteric bud comes into contact with an area of intermediate mesoderm called the metanephric mesenchyme (the tissue that will form the nephrons).
The metanephros has two components:
Collecting System
The ureteric bud begins as a structure branching off the mesonephric duct. It forms near the lower end of the mesonephric duct, close to where the duct joins the cloaca (the common outlet of the embryo; see later).
The collecting system is derived from the ureteric bud, which dilates to form the ureter, renal pelvis, calyces (cup-shaped spaces that collect urine from the kidney tissue) and the collecting tubules.
Excretory System
The excretory system is formed from the intermediate mesoderm.
It develops to form the nephron: the Bowman’s capsule (the cup around the glomerulus, where filtering starts), proximal convoluted tubule, loop of Henle (the hairpin-shaped section) and distal convoluted tubule.
Anatomical Migration
The metanephros develops at the caudal end of the embryo. This means that to get to the anatomical position in an adult (the lumbar region) the kidney needs to migrate superiorly. As it does this, it forms multiple new branches of the aorta to provide its blood supply. The testes behave differently: they keep their original arterial supply as they descend, so the testicular artery is long compared with the renal arteries.
Diagram - The Three Stages of Embryological Kidney Development
SimpleMed original by Dr. Maddie Swannack
Development of the Bladder and Urethra
The bladder and urethra are primarily formed from the cloaca, the structure that is the common outflow for the urinary, reproductive and gastrointestinal tracts.
Between the fourth and seventh weeks of development, the cloaca divides into two portions separated by the uro-rectal septum (a wedge of tissue that grows down between them). The septum also divides the cloacal membrane (the thin plate of tissue that seals the cloaca from the outside) into a urogenital membrane at the front and an anal membrane at the back. Later these membranes break down (rupture), allowing the hindgut (the end part of the embryonic gut) to open to the outside.
The posterior portion of the cloaca becomes the anal canal.
The anterior portion becomes the urogenital sinus. This further divides into three:
- The superior portion forms the bladder.
- The pelvic portion forms the urethra in females, and the prostatic and membranous portions of the urethra in males (the part within the prostate and the short part that passes through the pelvic floor). It also contributes to the female reproductive system.
- The caudal portion forms the spongy (penile) urethra in males, and contributes to the female reproductive system.
Initially, the bladder is drained by the allantois, a structure inside the connecting stalk (the tissue that attaches the embryo to the placenta and later forms the umbilical cord) that links the bladder to the yolk sac. The allantois later becomes the urachus, a cord that connects the top of the bladder to the umbilicus (navel). The urachus eventually turns into fibrous tissue to form the median umbilical ligament, which can be found in adults.
As the bladder grows, it absorbs the lower (caudal) ends of the mesonephric ducts at its base. This gives rise to the bladder trigone, a triangle of tissue on the inside of the bladder. The two upper corners of the trigone are the openings of the ureters (the ureteric orifices). The lower corner is the opening of the urethra (the internal urethral orifice).
The rest of the mesonephric duct develops differently in males and females. In males, because of testosterone produced by the testes, the mesonephric ducts persist and go on to form the epididymis, vas deferens and seminal vesicles. In females, without testosterone from the testes, the ducts degenerate. Small remnants may remain, such as the Gartner duct.
Abnormalities in Development of the Urinary System
As with the development of any system, development can go wrong in several ways. The examples below are not a complete list.
Renal Agenesis: failure of a kidney to form. If the ureteric bud fails to interact properly with the surrounding mesoderm, the kidney tissue will not develop. If this happens on both sides, it is fatal, usually soon after birth. The fetus makes no urine, so there is too little amniotic fluid (oligohydramnios). The lungs then cannot develop properly (pulmonary hypoplasia) and the baby cannot breathe. This chain of events is called the Potter sequence.
Migration Errors: the metanephros forms in the pelvic region, and sometimes it fails to migrate up to the lumbar region. This leaves a pelvic kidney. Most pelvic kidneys cause no symptoms, but they are more prone to urinary tract infections and kidney stones.
Horseshoe Kidney: a congenital abnormality in which the lower (inferior) poles of both kidneys are fused, forming one U-shaped kidney.
Duplication Defects: if the ureteric bud splits early in development, a kidney may have more than one ureter attached to it.
Accessory Renal Arteries: when the kidneys ascend into the lumbar region, they normally form new arteries and degenerate their old ones. In this condition, the old ones do not degenerate, meaning the kidney has multiple arteries supplying it.
Persistent Cloaca: a defect in the formation of the uro-rectal septum, so that the cloaca does not divide. A persistent cloaca is a common outflow tract shared by two or more of the urinary system, the reproductive system or the gastrointestinal tract.
Edited by: Dr. Marcus Judge
Reviewed by: Adrian Judge
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