Pronephros is the most basic of the three excretory organs that develop in

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, corresponding to the first stage of

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

development. It is succeeded by the

mesonephros The mesonephros () is one of three excretory system, excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included ...

, which in fish and amphibians remains as the adult kidney. In

amniotes Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibious stem tetrapod ancestors during the ...

, the mesonephros is the embryonic kidney and a more complex metanephros acts as the adult kidney. Once a more advanced kidney forms, the previous version typically degenerates by apoptosis or becomes part of the male reproductive system. The pronephros develops from the intermediate mesoderm, as do the later kidneys. It is a paired organ, consisting of a single giant nephron that processes blood filtrate produced from glomeruli or glomera- large embryonic glomeruli. The filtrate is deposited into the coelom. It then passes through thin ciliated tubules into the pronephric nephron where it is processed for solute recovery. The organ is active in adult forms of some primitive

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, like

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

s or

hagfish Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...

. It is present at the embryo of more advanced fish and at the

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

l stage of

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s where it plays an essential role in osmoregulation. In

human being Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s, it is rudimentary, appears at the end of the third week (day 20) and replaced by

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

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.

Development

The pronephros is the first in a sequence of kidneys that form in vertebrate embryos. The pronephric

primordium A primordium (; : primordia; synonym: anlage), in embryology, is an Organ (anatomy), organ or tissue in its earliest recognizable stage of development. Cell (biology), Cells of the primordium are called primordial cells. A primordium is the simp ...

develops from the

intermediate mesoderm Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develops in ...

, lying between the paraxial (somitic) mesoderm and the lateral plate. In many organisms (e.g. amphibians) this primodium forms anteriorly then migrates posteriorly to fuse with the cloaca, while in others it forms along the length of the intermediate mesoderm.

Drainage

In both amphibians and zebrafish, the pronephros has a single nephron attached to a nephric duct, which in turn is linked to the cloaca. Although these kidneys have a simple anatomical organization with only a single nephron, the nephrons have a segmental and functional complexity that is very similar to that in more complex kidneys such as mesonephroi and metanephroi.

Filtration

One unique feature of pronephroi is the arrangement by which the glomerular filtrate is generated and collected by the nephron. In pronephroi the glomerulus (or glomus if it extends over multiple body segments) projects into the coelom rather than into the proximal tip of the nephron. The glomerular filtrate flows directly into the coelom, or a dorsal compartment of the coelom known as the nephrocoel. In jawless fishes, the pronephric glomus projects into the pericardial cavity. Fluids are swept from the filtration cavity into the nephron through ciliated funnels known as nephrostomes. These thin epithelial tubes are densely packed with cilia and have a distinct morphology to the other tubular epithelia of the kidney.

Relationship to nephrotomes

Older anatomical texts describe the pronephros as condensing from nephrotomes, but modern visualization techniques have shown that this represents a histological artifact.

In amphibians, fishes, and mammals

Once the more complex mesonephros forms the pronephros undergoes apoptosis in amphibians. In fishes, the nephron degenerates but the organ remains and becomes a component of the immune system. In mammals, a functional pronephros, in the context of an organ performing waste excretion or osmoregulation, does not develop. However, a kidney primordium that runs along the intermediate mesoderm does form and links up to the cloaca. This duct is known as the pronephric duct, mesonephric duct or

Wolffian duct The mesonephric duct, also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct, is a paired organ that develops in the early stages of embryonic development in humans and other mammals. It is an important structure that p ...

. While this transient primordium never forms functional nephrons, the duct derived from it is essential to the development of the more complex later kidneys.

References

External links

Development of the pronephros in zebrafish
{{Authority control Embryology of urogenital system Kidney