Kidney development
Kidney development,ornephrogenesis,describes theembryologicorigins of thekidney,a major organ in theurinary 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 thedevelopment of the urinary and reproductive organs.
Phases
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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.[1]The pronephros is the most immature form of kidney, while the metanephros is most developed. The metanephros persists as the definitive adult kidney.[citation needed]
Archinephros
[edit]Thearchinephrosis considered as hypothetical or primitive kidney.
Pronephros
[edit]The pronephros develops in the cervical region of the embryo. During approximately day 22 of humangestation,the paired pronephri appears towards the cranial end of theintermediate mesoderm.In this region,epithelialcells arrange themselves in a series of tubules callednephrotomesand join laterally with thepronephric 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 humans.
Mesonephros
[edit]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 calledmesonephric tubules.Each mesonephric tubule receives a blood supply from a branch of theaorta,ending in a capillary tuft analogous to theglomerulusof the definitivenephron.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 orWolffian duct.The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to thecloaca.The mammalian mesonephros is similar to the kidneys ofaquatic amphibiansandfishes.
Metanephros
[edit]During the fifth week of gestation, the mesonephric duct develops an outpouching, theureteric bud,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 themetanephric duct,later forms theureter.As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form thecollecting duct systemof the kidney. It also forms the major and minorcalycesand therenal 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 therenal tubules.As the renal tubules grow, they come into contact and join withconnecting tubulesof 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 definitiveglomerulus.
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 200,000 to over 2.5 million per kidney.[2]
Migration
[edit]After inducing the metanephric mesenchyme the lower portions of the nephric duct will migrate caudally (downward) and connect with thebladder,thereby forming theureters.The ureters will carry urine from thekidneysto the bladder for excretion from thefetusinto theamniotic sac.As thefetusdevelops, thetorsoelongates and thekidneysrotate and migrate upwards within theabdomenwhich causes the length of theuretersto increase.
References
[edit]- ^Bruce M. Carlson (2004).Human Embryology and Developmental Biology(3rd ed.). Saint Louis: Mosby.ISBN0-323-03649-X.
- ^Bertram JF, Douglas-Denton RN, Diouf B, Hughson MD, Hoy WE (2011). "Human nephron number: implications for health and disease".Pediatr Nephrol.26(9): 1529–33.doi:10.1007/s00467-011-1843-8.PMID21604189.S2CID23205927.