Kidney Development and Anatomy

Kidney Development and Anatomy

• Mesoderm forms by the 3rd week of development.
• Three successive kidney pairs develop: pronephros (non-functional and disappears), mesonephros (partial function, forms Wolffian and Müllerian ducts), and metanephros (permanent kidney).
• Wolffian ducts form male genitalia; Müllerian ducts form uterus and vagina.
• Kidney size: typically 9-12 cm long, 5-7 cm wide, and 2-3 cm thick. Length and width should be within 2 cm of each other.
• Kidney layers (outer to inner): fibrous renal capsule, perirenal fat, renal fascia (Gerota's fascia), pararenal fat.
• Renal fascia surrounds kidney, fat, and adrenal glands.
• At the kidney hilum (VAU): vein is anterior, artery is in between vein and ureter, and ureter exits posteriorly.
• Normal renal cortex thickness is greater than 1 cm (10 mm or more).
• Renal sinus contains inner hyperechoic fat, calyces, renal pelvis, connective tissue, vessels, and lymphatics.
• Number of minor calyces equals the number of medullary pyramids.
• Renal pelvis is a funnel-shaped structure connecting major calyces to ureter.
• Kidney vasculature: main renal artery branches to segmental (5), interlobar, arcuate, and interlobular arteries.
• Junctional parenchymal defect: hyperechoic break in cortex, typically at upper pole of right kidney, due to partial fusion of ranunculi (embryonic kidney).
• Column of Bertin: prominent cortical tissue between medullary pyramids; may appear as a mass. Echogenicity is similar to cortex.

Kidney Function and Disease

• Kidney functions: filtering waste for urine, maintaining water/electrolyte/acid balance, hormone release.
• Stages of Chronic Kidney Disease (CKD) are based on glomerular filtration rate (GFR): Normal GFR > 90, end-stage GFR < 15.
• Renal Imaging (RI) evaluates transplant rejection, hydronephrosis, and renal disease. RI equation: (Peak systolic - End diastolic) / Peak systolic; normal RI < 0.7; RI of 1 = diastolic absent.
• Unilateral renal agenesis can be associated with VACTERL and MURCS syndromes.
• Complete duplex kidney: two ureters; incomplete: one ureter.
• Collecting system duplication: usually longer than normal; complete central cortical break within hyper echoic sinus is seen.
• Ectopic ureter: ureter draining upper pole inserts into ectopic bladder location. Common complication: ureterocele.
• Crossed fused renal ectopia: developing kidneys fuse in pelvis and one ascends to its normal position, dragging other across the midline, absence of other kidney, or both ureters connect to both sides of the bladder.
• Horseshoe kidney: lower poles commonly fuse anterior to aorta.
• Autosomal dominant polycystic kidney disease (ADPKD): most common inherited kidney disease; associated with cysts in liver and pancreas (and spleen in rarer cases). Associated with cerebral arterial aneurysms, especially berry aneurysms of the circle of Willis.
• Autosomal recessive polycystic kidney disease (ARPKD): more common in childhood and typically presents at birth; associated with pulmonary hypoplasia, hepatic fibrosis, and portal hypertension.
• Multi-cystic dysplastic kidneys (MCDK): most common abdominal mass in newborns, typically unilateral.
• Renal cysts (complex): hemorrhagic, infected, septate; malignancy suggested by multiple/thick septations, thick calcifications, or mural nodule/solid component.
• Peripelvic cysts are within the renal sinus; parapelvic cysts protrude into the sinus from parenchyma.
• Long-term dialysis increases risk of acquired cystic kidney disease and RCC.
• Von Hippel-Lindau disease: inherited disease presenting in the second and third decade with specific visual impairment and associated tumors (RCC, pheochromocytomas, islet cell tumors, renal and pancreatic cysts).
• Tuberous sclerosis: increased incidence of renal cysts and angiomyolipomas (AML).
• Oncocytomas: benign renal tumor with central scar and spoke-wheel pattern on angiogram.
• Angiomyolipomas (AML): mesenchymal tumor with fat cells, smooth muscle, and thick blood vessels. AML frequently located in the right.
• Renal cell carcinoma (RCC): most common solid renal mass in adults; associated with hematuria, acquired cystic renal disease, Von Hippel Lindau syndrome, tuberous sclerosis, and ADPKD. Males more commonly affected, especially after age 50.
• Wilm's tumor (Nephroblastoma): most common childhood renal tumor; associated with Beckwith-Wiedemann syndrome.
• Mesoblastic nephroma: most common neonatal and infant renal tumor.
• Transitional cell carcinoma (TCC): most common bladder neoplasm.
• Metastatic renal tumors: common sources include lung and breast.
• Hydronephrosis causes: intrinsic (calculi, UPJ obstruction, tumors); extrinsic (neoplasm, trauma, BPH). Hydronephrosis more common on the right side. Most common obstruction location: ureterovesical junction.
• Nephrocalcinosis: renal parenchymal calcium deposits, usually bilateral and diffuse, cortical or medullary, or both.
• Staghorn calculi: large calculi that fill the pelvis and calyces of the kidney.
• Renal infarction: initially difficult to see sonographically; later appears hypoechoic then echogenic.
• Emphysematous pyelonephritis: gas within the kidney, causing reverb or comet-tail artifacts; associated with anaerobic bacteria, often in diabetic patients.
• Xanthogranulomatous pyelonephritis (XGP): chronic pyelonephritis from chronic infections and obstructions. Common cause: Candida.
• Renal tuberculosis: associated with pulmonary infection. Infectious diseases show nonspecific masses, diffusely heterogeneous appearance, and difficult-to-define borders.
• Acute kidney injury (AKI) mechanisms: prerenal (inadequate perfusion), intrinsic (renal damage), and postrenal (obstruction).
• Chronic kidney disease (CKD)/End-stage renal disease: irreversible, nephrons reduced in function, GFR and resorptive capability decrease. Sonographic findings not specific: small, echogenic, shrunken kidneys.
• Hydronephrosis indicates postrenal failure; abnormal RI indicates intrinsic renal disease.

Renal Transplant and Renal Vascular

• Leading cause of kidney transplantation: diabetes.
• Left kidney favored for transplantation due to longer renal vein.
• Cadaveric transplant survival: 7-10 years; living donor: 15-20 years.
• Arterial anastomosis: external or internal iliac artery.
• Immediate post-transplant poor function: acute tubular necrosis (ATN).
• Acute transplant rejection: enlarged kidney, decreased echogenicity, loss of corticomedullary boundary, and increased flow resistance (RI).
• Post-transplant complications: fluid collections (hematomas, urinomas), renal artery kinking/thrombosis, renal vein thrombosis.
• Transplant dysfunction RI: >0.8.
• Renal artery stenosis: most commonly located at origin or first 2 cm; symptoms: uncontrolled or sudden onset hypertension.
• Decreased renal size (<9cm of length) suggests renal artery stenosis.
• Direct evaluation of renal artery stenosis: renal artery velocities and renal artery/aorta ratio (RAR) >3.5.
• Indirect evaluation: intrarenal waveform evaluation (parvus tardus).
• Renal artery thrombosis: causes prerenal failure, acute flank pain, hematuria, and sudden rise in blood pressure.
• Renal artery thrombosis findings: focal hypoechoic areas of infarct, absence of intrarenal arterial flow; renal enlargement.
• Causes of renal vein thrombosis: nephrotic syndrome, hypercoagulability disorders, malignant tumors, extrinsic compression, trauma, transplant rejection.
• Renal vein thrombosis findings: dilated thrombosed renal vein, absence of intrarenal venous flow, enlarged hypoechoic kidney, high resistance renal artery waveform (increased RI).

Explore the fascinating journey of kidney development and anatomy, starting from the mesoderm formation in the third week of gestation. Learn about the three kidney pairs—pronephros, mesonephros, and metanephros—and their respective roles in the human body. This quiz covers essential aspects including kidney structure, layers, and the relationships between relevant anatomical features.