question: Considering that, developmentally, the ureter grows from the bladder and joins the kidneys later on in development. How does the body manage to get the ureter to join when the two organs are so distant?
comment: The kindey forms from a structure called the mesonephric duct. This duct ends in a blind end just near the cloaca which at the early stages is just the front part of the very basic gut tube at the bottom. The cloaca will develop into the bladder as the embryo grows. The early kidney or mesonephric duct at this stage starts out very low in the embryo so the distance between the duct and the cloaca is very small. The top parts of the duct degenerate during development until the very bottom part is all that remains. The kidney then moves up to its adult position, nestling under the adrenal glands. The adrenal glands dont grow out of the kidney, but are often pictured as if they are one object. Think of it more like a hat the kidney moves up into wearing. The ureter elongates as the kidneys move up. This makes the ureter long despite the fact that in development the distance between the "mesonephric duct" and the "cloaca" is very small. This explains it in greater detail. URL_0
passages: One example of this, the most well described of the developmental METs, is kidney ontogenesis. The mammalian kidney is primarily formed by two early structures: the ureteric bud and the nephrogenic mesenchyme, which form the collecting duct and nephrons respectively (see kidney development for more details). During kidney ontogenesis, a reciprocal induction of the ureteric bud epithelium and nephrogenic mesenchyme occurs. As the ureteric bud grows out of the Wolffian duct, the nephrogenic mesenchyme induces the ureteric bud to branch. Concurrently, the ureteric bud induces the nephrogenic mesenchyme to condense around the bud and undergo MET to form the renal epithelium, which ultimately forms the nephron. Growth factors, integrins, cell adhesion molecules, and protooncogenes, such as "c-ret", "c-ros", and "c-met", mediate the reciprocal induction in metanephrons and consequent MET.
   In humans, all of the branches of the ureteric bud and the nephronic units have been formed by 32 to 36 weeks of gestation. However, these structures are not yet mature, and will continue to mature after birth. Once matured, humans have an estimated two million nephrons (approximately 1,000,000 per kidney) but this number is highly variable ranging widely from approximately 300,000 to over 2 million per kidney.
  
  Section::::Migration.
   In the majority of vertebrates, the mesonephros persists into the adult, albeit usually fused with the more advanced metanephros; only in amniotes is the mesonephros restricted to the embryo. The kidneys of fish and amphibians are typically narrow, elongated organs, occupying a significant portion of the trunk. The collecting ducts from each cluster of nephrons usually drain into an "archinephric duct", which is homologous with the vas deferens of amniotes. However, the situation is not always so simple; in cartilaginous fish and some amphibians, there is also a shorter duct, similar to the amniote ureter, which drains the posterior (metanephric) parts of the kidney, and joins with the archinephric duct at the bladder or cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult.
   Section::::Structure.:Development.
  
  The mammalian kidney develops from intermediate mesoderm. Kidney development, also called "nephrogenesis", proceeds through a series of three successive developmental phases: the pronephros, mesonephros, and metanephros. The metanephros are primordia of the permanent kidney.
  
  Section::::Function.
   The organ is active in adult forms of some primitive fish, like lampreys or hagfish. It is present at the embryo of more advanced fish and at the larval stage of amphibians where it plays an essential role in osmoregulation. In human beings, it is rudimentary, appears at the end of the third week (day 20) and replaced by mesonephros after 3.5 weeks. Despite this transient appearance in mammals, the pronephros is essential for the development of the adult kidneys. The duct of the mesonephros forms the Wolffian duct and ureter of the adult kidney. The embryonic kidney and its derivatives also produces the inductive signals that trigger formation of the adult kidney.
   After inducing the metanephric mesenchyme the lower portions of the nephric duct will migrate caudally (downward) and connect with the bladder, thereby forming the ureters. The ureters will carry urine from the kidneys to the bladder for excretion from the fetus into the amniotic sac. As the fetus develops, the torso elongates and the kidneys rotate and migrate upwards within the abdomen which causes the length of the ureters to increase.
  
  Section::::External links.
  
  BULLET::::- University of Colorado
  
  BULLET::::- Overview - Edinburgh
  
  BULLET::::- University of Michigan
  
  BULLET::::- UNSW Embryology
  
  BULLET::::- University of Montreal: 3D animation male urogenital development
   The development of the kidney proceeds through a series of successive phases, each marked by the development of a more advanced kidney: the archinephros, pronephros, mesonephros, and metanephros. The pronephros is the most immature form of kidney, while the metanephros is most developed. The metanephros persists as the definitive adult kidney.
  
  Section::::Phases.:Archinephros.
  
  The archinephros is considered as hypothetical or primitive kidney.
  
  Section::::Phases.:Pronephros.
  
  The pronephros develops in the cervical region of the embryo.
   The mesonephros persists and form the permanent kidneys in fish and amphibians, but in reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the permanent kidney (metanephros) develops beginning during the sixth or seventh week, so that by the beginning of the fifth month only the ducts and a few of the tubules of the mesonephros remain.
  
  Section::::Wolffian duct.:Development in male.
  
  In the male the Wolffian duct persists, and forms for example the epididymis, the ductus deferens, the ejaculatory duct, seminal vesicle and efferent ducts.
   The mesonephros as a whole produces urine from the 6th through the 10th week of development. Despite the similarity in structure, function, and terminology, however, the mesonephric nephrons do not form any part of the mature kidney or nephrons. As the more caudal nephrons form, the more cranial nephrons are already degenerating. In females the mesonephros degenerates entirely, though vestigial structures such as Gartner's ducts, the epoophoron, and paroophoron are common. In males, a few of the more caudal tubules will survive and give rise to the efferent ductules of the testis, the epididymis, vas deferens, seminal vesicle, as well as vestigial structures such as the appendix testis, appendix epididymis, and paradidymis.
   The rudiments of the permanent kidneys make their appearance about the end of the first or the beginning of the second month. Each kidney originates as a ureteric bud from the caudal end of the Wolffian duct, which, in turn, originates from intermediate mesoderm. The ureteric bud starts close to where the Wolffian duct opens into the cloaca, and grows dorsalward and rostralward along the posterior abdominal wall, where its blind extremity expands and subsequently divides into several buds, which form the rudiments of the renal pelvis and renal calyces; by continued growth and subdivision it gives rise to the collecting duct system of the kidney. The other, more superficial, portion of the diverticulum, on the other hand, becomes the ureter.
   During early development (approximately day 22 in humans), the pronephric duct forms from the intermediate mesoderm, ventral to the anterior somites. The cells of the pronephric duct migrate caudally whilst inducing adjacent mesenchyme to form the tubules of the initial kidney-like structure called the pronephros. This process is regulated by Pax2/8 markers. The pronephros is active in adult forms of some primitive fish and acts as the primary excretory system in amphibious larvae and embryonic forms of more advanced fish. In mammals however, the pronephric tubules and the anterior portion of the pronephric duct degenerates in 3.5 weeks to be succeeded by the mesonephros, the embryonic kidney.
   In lungfish and tetrapods, the renal portal vein is joined by a vein traveling upwards from the abdominal vein, which can bring venous blood from the hind limbs and ventral body wall into the renal portal system, or alternatively, enable blood from the tail and groin to pass into the hepatic portal system, already served by blood from the gut, via the hepatic portal vein, and from the hind legs and ventral body wall, via the abdominal vein. In fishes and salamanders, the renal portal vein branches and enters a capillary network very similar to the ones in the nephric portal system. In frogs and amniotes, metarterioles appear, with capillary networks connected to them, and sphincter muscles around the entrances to the capillaries. In birds, the system is very complex, with sphincters around the metarterioles themselves. In fishes and salamanders, due to the lack of metarterioles, all the blood passes through the capillaries. However, in frogs and amniotes, most of the blood usually passes through the metarterioles instead, although it can still be diverted through the capillary networks if need be. The system is completely abandoned in mammals, with the result that all the blood from the tail (if present), groin, pelvic area, and hind legs is forced to pass directly into the posterior vena cava, and the only blood passing out of the kidney through the renal vein is that that entered it through the renal artery. The subcardinals become the part of the posterior vena cava between the hepatic vein and the renal veins, and other portions become the internal gonadal (spermatic or ovarian) veins, and the suprarenal veins. The posterior cardinal veins become the veins of the pelvis, tail, and hind legs, and the part of the azygos vein that is closest to the heart.
   Early kidney structures include the pronephros and mesonephros, whose complexity, size and duration can vary greatly between vertebrate species. The adult kidney, also referred to as the "metanephric kidney", forms at the posterior end of the intermediate mesoderm after the degeneration of previous, less complex kidney structures.
  
  Section::::Derived Organs and Tissues.:Kidneys.:Pronephros.
   Kidneys are normally located in the retroperitoneal space between the T12 and L3 vertebrae after ascending from the pelvis during development to rest underneath the adrenal glands. In patients with this condition, the horseshoe kidney ascent is commonly arrested by the inferior mesenteric artery due to the central fusion of the kidneys. However, this is present in only 40% of cases, and, in 20% of cases, the fused kidney remains in the pelvis. Its ascension may also be restricted by its own renal artery. Additionally, during normal development, the kidneys undergo a 90 degree medial rotation while ascending. However, due to the renal fusion, this rotation is impaired resulting in abnormal placement of the ureters. This in turn can lead to urinary stasis and drainage issues. Furthermore, approximately 70% of kidneys in normal individuals are supplied by a single renal artery with the remaining 30% having embryonic collateral or accessory arteries. With horseshoe kidneys, the majority are supplied by derivatives of the abdominal aorta or common illiac arteries depending on the final position of the kidneys.
   Ureteral development begins in the human fetus around the 4th week of embryonic development. A ureteric bud, arising from the mesonephric (or Wolffian) duct, gives rise to the ureter, as well as other parts of the collective system. In the case of a duplicated ureter, the ureteric bud either splits or arises twice. In most cases, the kidney is divided into two parts, an upper and lower lobe, with some overlap due to intermingling of collecting tubules. However, in some cases the division is so complete as to give rise to two separate parts, each with its own renal pelvis and ureter.
   Section::::Derived Organs and Tissues.
  
  As development proceeds, the intermediate mesoderm differentiates sequentially along the anterior-posterior axis into three successive stages of the early mammalian and avian urogenital system, named pronephros, mesonephros and metanephros respectively (anamniote embryos form only a pronephros and mesonephros). The intermediate mesoderm will eventually develop into the kidney and parts of both male and female reproductive systems.
  
  Section::::Derived Organs and Tissues.:Kidneys.
   During the embryologic development of the kidney, the ureteropelvic junction is the last part of the ureter to become patent.
  
  Section::::Function.
  
  The ureters are a component of the urinary system. Urine, produced by the kidneys, travels along the ureters to the bladder.
  
  Section::::Clinical significance.
  
  Cancer of the ureters is known as ureteral cancer.
  
  Failure of the ureteropelvic junction to become patent during development is the most frequent cause of bilateral hydronephrosis, particularly in male neonates. Pyeloplasty, which involves excision of the stenotic section and creation of a new junction, is the most common and effective treatment for this problem.
   Renal agenesis
  
  Renal agenesis is a medical condition in which one (unilateral) or both (bilateral) fetal kidneys fail to develop.
  
  Unilateral and bilateral renal agenesis in humans, mice and zebra fish has been linked to mutations in the gene GREB1L. It has also been associated with mutations in the genes "RET" or "UPK3A". in humans and mice respectively.
  
  Section::::Type.
  
  Section::::Type.:Bilateral.
   Section::::Causes.
  
  Bladder outlet obstruction is classified based on where along the urinary tract the obstruction occurs, including upper urinary tract obstruction and lower urinary tract obstruction. Depending on the location of the obstruction, one or both sides of the urinary tract will be involved. In approximately 50% of cases of congenital hydronephrosis, there is no known cause. In many cases, obstruction along the urinary tract in utero leads to some form of CAKUT mentioned above.
  
  Section::::Causes.:Upper urinary tract obstruction.
  
  Upper urinary tract obstruction includes the renal pelvis and upper ureters.
  
  Section::::Causes.:Upper urinary tract obstruction.:Ureteropelvic junction obstruction.
   In cartilaginous fishes, the part of the archinephric duct closest to the testis is coiled up to form an epididymis. Below this are a number of small glands secreting components of the seminal fluid. The final portion of the duct also receives ducts from the kidneys in most species.
  
  In amniotes, however, the archinephric duct has become a true vas deferens, and is used only for conducting sperm, never urine. As in cartilaginous fish, the upper part of the duct forms the epididymis. In many species, the vas deferens ends in a small sac for storing sperm.
   Section::::Structure.:Development.
  
  Section::::Structure.:Development.:Mesonephric vesicle.
  
  Formation of each mesonephric nephron begins when a bit of the "intermediate mesoderm" adjacent to the mesonephric duct differentiates to form a "mesonephric vesicle".
  
  Section::::Structure.:Development.:Mesonephric tubules.
  
  This vesicle then elongates to form the "mesonephric tubule", attaching to the mesonephric duct on one side. Meanwhile, an artery from the dorsal aorta begins extending towards the mesonephric tubule. When these two structures contact each other, they form the glomerulus and the Bowman's capsule surrounding it. The mesonephric tubule is also known as the "Wolffian tubules" (or "Kobelt's tubules").
   The GAS1 gene plays a significant role in Kidney development. Conserved DNA binding motif, which is located in the Gas1 promoter, is directly bind by the WT1, and then the Gas1 mRNA is activated to transcript to the NPCs. The WT1 has been confirmed as a necessary part for expressing Gas1 in kidneys in vivo. Loss of function of GAS1 in vivo results in hypoplastic kidneys with reduced nephron mass due to premature depletion of NPCs. In humans, fetal period is the most significant time point for inducting a new nephrons, no matter what kind of mammals, once the NPCS disappeared, there is no possibility for inducing the new nephrons. 
   Kidney development
  
  Kidney development, or nephrogenesis, describes the embryologic origins of the kidney, a major organ in the urinary system. This article covers a 3 part developmental process that is observed in most reptiles, birds and mammals, including humans. Nephrogenesis is often considered in the broader context of the development of the urinary and reproductive organs.
  
  Section::::Phases.
   The permanent kidney of amniotes, the metanephros, develops during the 10th week in human embryos and is formed by the reciprocal interactions of the metanephrogenic blastema (or metaneophrogenic mesenchyme) and the ureteric bud. Gonadal derived neurotrophic factor (GDNF) secreted by the metanephrogenic blastema activates the receptor tyrosine kinase RET, via the co-receptor GFRα1 and triggers outgrowth of Ret positive cells from the nephric duct towards the GDNF signal, promoting ureteric bud outgrowth and invasion. Once the bud invades the metanephrogenic blastema, a permissive signal in the form of Wnt proteins is activated and stimulates the condensation of metanephric mesenchymal cells around the ureteric bud tips, beginning the polarisation of the blastema to generate the epithelial cells of parts of the nephron: the proximal tubules, loops of Henle and the distal convoluted tubules. The ureteric bud secretes FGF2 (fibroblast growth factor 2) and BMP7 (bone morphogenic protein 7) to prevent apoptosis in the kidney mesenchyme. Condensing mesenchyme then secretes paracrine factors that mediate branching of the ureteric bud to give rise to the ureter and collecting duct of the adult kidney.
   The kidneys of fish are typically narrow, elongated organs, occupying a significant portion of the trunk. They are similar to the mesonephros of higher vertebrates (reptiles, birds and mammals). The kidneys contain clusters of nephrons, serviced by collecting ducts which usually drain into a mesonephric duct. However, the situation is not always so simple. In cartilaginous fish there is also a shorter duct which drains the posterior (metanephric) parts of the kidney, and joins with the mesonephric duct at the bladder or cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult. Hagfish and lamprey kidneys are unusually simple. They consist of a row of nephrons, each emptying directly into the mesonephric duct.
  