Multicystic dysplastic kidney or MCDK is a congenital disease where one or both kidneys don’t form quite right, specifically causing them to not drain urine properly, which results in urine building up in the kidneys and forming multiple fluid-filled sacs called cysts.
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, the 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.
Although not completely known, it’s thought that MCDK is a result of some sort of abnormal induction of the metanephric blastema by the ureteric bud.
This failure might be the fault of the mesonephric duct not forming right, or the ureteric bud not forming right, or both.
Regardless of the cause of failure, the ureteric bud is supposed to go on to form the ureters as well as the rest of the tubules that branch out to collect urine.
So as blood starts coming in to be filtered, and urine starts getting produced, a failure to properly develop into these urine-collecting tubules means that the urine has nowhere to go, and so it builds up in the kidneys and forms these fluid-filled cysts that are composed of abnormal connective tissue—especially cartilage—that actually replaces normal kidney tissue and decreases the kidney’s ability to function.