Kidney development
Encyclopedia
Kidney development, or nephrogenesis, describes the embryologic
origins of the kidney
, a major organ in the urinary system
. It is often considered in the broader context of the development of the urinary and reproductive organs
.
During approximately day 22 of human gestation
, the paired pronephroi appear towards the cranial end of the intermediate mesoderm
. In this region, epithelial
cells arrange themselves in a series of tubules called nephrotome
s and join laterally with the pronephric duct
. This duct is fully contained within the embryo and thus cannot excrete filtered material outside the embryo; therefore the pronephros is considered nonfunctional in mammals.
, ending in a capillary tuft analogous to the glomerulus of the definitive nephron
. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct
. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca
. The mammalian mesonephros is similar to the kidneys of aquatic amphibian
s and fish
es.
, near its attachment to the cloaca. This bud, also called the metanephrogenic diverticulum, grows posteriorly and towards the head of the embryo. The elongated stalk of the ureteric bud, called the metanephric duct, later forms the ureter
. As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form the collecting duct system
of the kidney. It also forms the major and minor calyces and the renal pelvis
.
The portion of undifferentiated intermediate mesoderm in contact with the tips of the branching ureteric bud is known as the metanephrogenic blastema. Signals released from the ureteric bud induce the differentiation of the metanephrogenic blastema into the renal tubules. As the renal tubules grow, they come into contact and join with connecting tubule
s of the collecting duct system, forming a continuous passage for flow from the renal tubule to the collecting duct. Simultaneously, precursors of vascular endothelial cells begin to take their position at the tips of the renal tubules. These cells differentiate into the cells of the definitive glomerulus.
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 one million nephrons (approximately 500,000 per kidney) or more.
, 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.
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
origins of the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
, a major organ in the urinary system
Urinary system
The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder and the urethra.-Kidney:...
. It is often considered in the broader context of the development of the urinary and reproductive organs
Development of the urinary and reproductive organs
The development of the urinary and reproductive organs as a part of the prenatal development, concerns the urinary system and sex organs. The latter is a part of the stages of sexual differentiation....
.
Phases
The development of the kidney proceeds through a series of successive phases, each marked by the development of a more advanced kidney: the 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.Pronephros
It develops in the cervical region of the embryo.During approximately day 22 of human gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
, the paired pronephroi appear towards the cranial end of the intermediate mesoderm
Intermediate mesoderm
Intermediate mesenchyme or intermediate mesoderm is a type of mesoderm that is located between the paraxial mesoderm and the lateral plate.It develops into the part of the urogenital system * forms of urogenital system...
. In this region, epithelial
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
cells arrange themselves in a series of tubules called nephrotome
Nephrotome
The nephrotome is a section of the mesoderm. It gives rise to the pronephros and eventually to the rest of the kidney.A nephrotome is an obsolete term used to describe rounded mesodermal portions of the forming pronephros. In old texts the pronephros is described as forming through the fusion of...
s and join laterally with the pronephric duct
Pronephric duct
-Development:The development of the pronephric duct is a part of the development of the urinary and reproductive organs.In the outer part of the intermediate mesoderm, immediately under the ectoderm, in the region from the fifth cervical segment to the third thoracic segment, a series of short...
. This duct is fully contained within the embryo and thus cannot excrete filtered material outside the embryo; therefore the pronephros is considered nonfunctional in mammals.
Mesonephros
The development of the pronephric duct proceeds in a cranial-to-caudal direction. As it elongates caudally, the pronephric duct induces nearby intermediate mesoderm in the thoracolumbar area to become epithelial tubules called mesonephric tubules. Each mesonephric tubule receives a blood supply from a branch of the aortaAorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
, ending in a capillary tuft analogous to the glomerulus of the definitive nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct
Wolffian duct
The mesonephric duct is a paired organ found in mammals including humans during embryogenesis....
. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
. The mammalian mesonephros is similar to the kidneys of aquatic amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
es.
Metanephros
During the fifth week of gestation, the mesonephric duct develops an outpouching, the ureteric budUreteric bud
The ureteric bud is a protrusion from the mesonephric duct during the development of the urinary and reproductive organs. It later develops into the adult kidney, except for the nephrons, which, in contrast, originate from the metanephric blastema....
, near its attachment to the cloaca. This bud, also called the metanephrogenic diverticulum, grows posteriorly and towards the head of the embryo. The elongated stalk of the ureteric bud, called the metanephric duct, later forms the ureter
Ureter
In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....
. As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form the collecting duct system
Collecting duct system
The collecting duct system of the kidney consists of a series of tubules and ducts that connect the nephrons to the ureter. It participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and antidiuretic hormone.There are several...
of the kidney. It also forms the major and minor calyces and the renal pelvis
Renal pelvis
The renal pelvis or pyelum is the funnel-like dilated proximal part of the ureter in the kidney.In humans, the renal pelvis is the point of convergence of two or three major calyces...
.
The portion of undifferentiated intermediate mesoderm in contact with the tips of the branching ureteric bud is known as the metanephrogenic blastema. Signals released from the ureteric bud induce the differentiation of the metanephrogenic blastema into the renal tubules. As the renal tubules grow, they come into contact and join with connecting tubule
Connecting tubule
In the kidney, the collecting tubule is a tubular segment of the renal collecting duct system that connects the distal convoluted tubule to the cortical collecting duct.-Classification:...
s of the collecting duct system, forming a continuous passage for flow from the renal tubule to the collecting duct. Simultaneously, precursors of vascular endothelial cells begin to take their position at the tips of the renal tubules. These cells differentiate into the cells of the definitive glomerulus.
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 one million nephrons (approximately 500,000 per kidney) or more.
Migration
After inducing the metanephric mesenchyme the lower portions of the nephric duct will migrate caudally (downward) and connect with the bladderUrinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
, thereby forming the ureters. The ureters will carry urine from the kidneys to the bladder for excretion from the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
into the amniotic sac
Amniotic sac
The amniotic sac is the sac in which the fetus develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the Chorion,...
. As the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
develops, the torso
Torso
Trunk or torso is an anatomical term for the central part of the many animal bodies from which extend the neck and limbs. The trunk includes the thorax and abdomen.-Major organs:...
elongates and the kidneys rotate and migrate upwards within the abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
which causes the length of the ureters to increase.