Medullary sponge kidney, also known as cacchi-ricci disease is a congenital disease where the medullary part of the kidney, which is deeper inside the kidney, gets loaded with fluid-filled cysts which gives the kidney the appearance of a sponge.
Now, the kidney can be divided into the cortex which is the outer layer and the medulla which is the layer below that.
Kidneys contain millions of tiny nephrons which filter the blood - each of which is like a little tube receiving blood on one end in the cortex.
From there, the nephron dips into the medulla, and then goes back out into the cortex, and finally dips back into the medulla a second time to connect to the collecting ducts which gather up all of the urine. Not every nephron has this exact structure, but a lot of them do.
From there, the urine drains through the papilla which is an inverted cone shaped pyramid, that, like a shower head, pours urine into the calyces, which comes from the latin -calix which means large cup, kinda like a Roman chalice.
From there it enters the renal pelvis which funnels the urine into the ureter, then to the bladder, and then finally leaves the body out the urethra.
Now, medullary sponge kidney has to do with the development of the kidney. 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.