“Urogenital System”

Urinary Tract
The embryologic development of the kidneys encompasses three phases. These are the pronephros, mesonephros, and metanephros. They go through overlapping cycles of growth and regression, resulting in the formation of the definitive kidneys and urogenital system
from the latter two stages.
At the end of the third week, paired collections of primitive tubules form and empty into paired excretory ducts that lead to the cloaca.This pronephros never functions in humans and involutes by the early fifth week.
While the pronephros is already involuting, a pair of structures forms just caudal to the pronephros. The tubules of this mesonephros actually function. They reach their full maturation by the end of the seventh week. Development, maturation, and involution all proceed in
a craniocaudal progression, with considerable overlapping The intermediate mesoderm from which the mesonephros originates is divided by a longitudinal cleft, and a medially placed mesonephric duct is formed. A few days later, this duct communicates with the cloaca. Arteries that originate from the iliac arteries and the distal aorta supply the mesonephros itself. Drainage of the tubules is via the mesonephric duct (previously called the Wolffian duct). The latter partially involutes while playing a major role in the development of the genital system in the male.
As the mesonephros and its duct regress, the metanephros starts to develop, culminating in the development of the definitive kidney and ureter. This process starts with the formation of a small bud from the mesonephric duct, just proximal to its emptying into the cloaca (Fig. below).
image
Sagittal schematic of a 5-week embryo shows the allantois and hindgut emptying into the cloaca. The developing urorectal septum (URS) has started to partition the cloaca into separate GI and urinary compartments. The mesonephros (MES) and its duct are seen dorsally and superiorly. A short diverticulum from the mesonephric duct (developing ureter) is capped by the metanephric blastema (MET).

As this bud elongates, it develops a bulbous end (ampulla) and a slender neck. The ampullary end eventually becomes the renal pelvis, and the slender neck becomes the ureter proper. Specialized cells at the ampullary end interact with the surrounding mesoderm (metanephric blastema) to form the nephrons and connective tissue of the kidney.
The ureteric bud undergoes simultaneous lengthening and division. The developing metanephros grows into the space vacated by the regressing mesonephros. The branching of the ureteric bud continues through many generations, leading to the development of the renal pelvis, calyces, papillae, cribriform plates, and even the collecting tubules. This branching also leads to the lobular contours of the fetal kidney that may persist into adulthood. The fetal kidney also undergoes a gradual fusion of calyces, but not papillae, that continues into the postpartum period, resulting in compound calyces.
The development of the metanephros is accompanied by a change in its position and orientation. When the ureteral bud first contacts the metanephric blastema, it is at the level of the lower lumbar spine or upper sacrum. Originally, the paired metanephros almost touch each other in the midline. During the rapid growth of the caudal end of the fetus, they grow into the space occupied by the involuting  esonephros. Straightening of the fetal spinal curvature adds to the apparent migration of the kidney By the end of the third month, the kidney is located at the level of the second or third lumbar vertebra, and at birth, it has ascended to the T12-L1 level. Rotation of the kidney along its longitudinal axis accompanies this ascent. The original renal pelvis lies along the anterior aspect of the developing kidney, eventually reaching an anteromedial location by the ninth week. Rarely, the kidney overrotates, leading the blood vessels to wrap around its posterior surface. The blood supply to the developing kidney also undergoes change as it migrates cephalad. Lateral sacral branches from the distal aorta supply the metanephros. As the kidney migrates superiorly, lateral stem (end) arteries from more rostral levels of the aorta progressively supply it until the definitive renal arteries develop. Persistence of the more caudal arteries results in multiple and/or aberrant renal arteries

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