Wikipedia:Osmosis/Renal Agenesis
Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH
Alright, so renal agenesis—genesis is the origin or formation of something, and the prefix a means not, and renal refers to the kidneys, so renal agenesis is when the kidneys don’t form. Since there are two kidneys, renal agenesis can refer to just one kidney not developing, called unilateral renal agenesis, or URA, or neither kidney developing, called bilateral renal agenesis, or BRA.
Alright so during fetal development, first off you’ve got this structure called the mesonephric duct which is involved in development of urinary and reproductive organs, and during the 5th week of gestation, a little guy called the ureteric bud starts pushing its way into another structure called the metanephric blastema, and together, these two little embryologic structures go on to develop into a kidney. At about the 7th week, nephrogenesis, or formation of the kidneys, starts under the influence of that ureteric bud.
By about 20 weeks, the ureteric bud has formed the ureters, renal calyces, collecting ducts, and collecting tubules, while the metanephric blastema develops into the nephron itself, which includes the epithelial cells and the podocytes of Bowman’s capsule.
In the third trimester and throughout infancy, the kidneys continue to grow and mature.
With renal agenesis, the ureteric bud fails to induce development of the metanephric blastema, and so either one or both kidneys don’t develop. Although not completely known, it’s thought that this is a result of a combination of genetic as well as in utero environmental factors such as toxins and infections.
Newborns with unilateral renal agenesis are usually asymptomatic if the other kidney’s otherwise healthy. Now that that one kidney’s doing all the filtering, though, over time unilateral renal agenesis can lead to hypertrophy, or growth of the kidney, which later in life can increase the risk of hypertension as well as renal failure.
Alright so let’s move to bilateral agenesis, which is where there are no kidneys. Normally, as the kidneys start to function around week 16, they start to filter the fetal blood, which means some fetal urine gets produced, some of which contributes to the amniotic fluid that the fetus floats around in.
With bilateral renal agenesis—and no kidneys—urine isn’t produced and so there’s less amniotic fluid, and so they develop oligohydramnios, where oligo- means low or less and -hydramnios refers to the amount of amniotic fluid.
A couple of things happen as a result of this, first that amniotic fluid is crucial for the development of the fetal lungs, by both helping the airways physically stretch out as well as contributing amino acids like proline, which helps with the formation of mesenchyme and collagen in the lung. So with less amniotic fluid, the lungs can’t develop all the way, called pulmonary hypoplasia. Not only that though, with less amniotic fluid, there’s less space in the amniotic sac, and so the fetus is literally compressed into a smaller space, which causes developmental abnormalities like a flattened face, widely separated eyes with epicanthal folds, low-set ears, as well as limb abnormalities like clubbed feet. Taken together, this set of consequences from oligohydramnios is called Potter sequence, or sometimes Potter syndrome.
It’s worth mentioning that bilateral renal agenesis isn’t the only thing that can cause Potter sequence, it can also be seen with other causes of oligohydramnios like a blockage in the urinary tract or occasionally prolonged rupture of membranes.
In general, bilateral renal agenesis is incompatible with life outside the womb, and is therefore usually fatal within the first few days after birth, although depending on lung development and overall health, some newborns might be put on dialysis until they are strong enough to have a kidney transplant.
Okay so as a super short review—unilateral renal agenesis is where only one kidney develops, and is usually asymptomatic, and bilateral renal agenesis is where neither kidney develops, which results in oligohydramnios and the characteristics features of Potter sequence.
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Sources
[edit]First Aid USMLE Step 1 (text)
Pathoma (text/video)
http://jasn.asnjournals.org/content/20/7/1465/F1.expansion
http://emedicine.medscape.com/article/983477-overview#a5
http://emedicine.medscape.com/article/1005696-overview#a5
https://en.wikipedia.org/wiki/Potter_sequence