What Is Pronephros? Key Concepts & Stages for Students
It is the first stage of kidney development. The most basic of the three excretory organs that emerge invertebrates, the pronephros, corresponds to the first stage of kidney development. A pronephric kidney can be described as a paired organ with a single enormous nephron that filters blood filtrate produced by glomeruli or glomerate—large embryonic glomeruli. The pronephros is the first kidney to develop in the embryo of more evolved vertebrates. It is quickly replaced by the mesonephros, which lacks nephrostomes and only pulls fluid from the glomerulus after three and a half weeks in humans.
Nephrons - The Functional Unit of a Kidney
Each human kidney contains around 1,000,000 nephrons. The most rudimentary nephrons can be found in rudimentary fish kidneys as the pronephros, amphibian larvae, and embryos of more evolved vertebrates.
The nephrons seen in amphibians' and most fish's kidneys as the mesonephros, as well as in the late embryonic development of more sophisticated vertebrates, are only marginally more advanced in structure.
Adult kidneys, or metanephros, of land vertebrates such as reptiles, birds, and mammals have the most mature nephrons.
In the mammalian kidney, each nephron is a long tubule, or exceedingly fine tube, measuring 30–55 mm that is about 1.2–2.2 inches in length. This tube is closed on one end, extended, and folded into a double-walled cuplike shape on the other.
The renal corpuscle capsule, also known as Bowman's capsule, encloses the glomerulus, a cluster of small blood arteries called capillaries.
Blood travels into and out of the glomerulus via tiny arteries known as arterioles, which enter and exit the glomerulus through the capsule's open end.
Fluid filters out the blood in the glomerulus via the inner wall of the capsule and into the nephron tubule in the renal corpuscle.
The secretion of certain compounds into the filtrate and the selective reabsorption of water and other ingredients from it change its composition as it moves through the tubule.
Urine is the last product, which is transported into the renal pelvis via the collecting tubules.
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Pronephric Kidney Development
A number of nephrons filter plasma, recover solutes still required by the organism and then drain into a common collecting duct in vertebrates' kidneys. The pronephros, the most primitive kind of vertebrate kidney, is functioning in the early larvae of amniotes such as amphibians and fish. The intermediate mesoderm, which lies between the paraxial (somitic) mesoderm and the lateral plate, gives rise to the pronephric primordium. This primordium forms anteriorly and migrates posteriorly to merge with the cloaca in many creatures (e.g., amphibians), whereas it forms throughout the length of the intermediate mesoderm in others.
Stages of Development
The embryonic archinephros gives rise to three types of adult-vertebrate kidneys- the pronephros in the anterior region, the mesonephros in the middle portion, and the metanephros in the hind region.
Functions
The arrangement by which the glomerular filtrate is formed and collected by the nephron is a distinctive feature of pronephroi. The glomerulus of the pronephric kidney, sometimes known as the glomus if it extends across many body segments, projects into the coelom rather than the proximal tip of the nephron.
The glomerular filtrate passes straight into the coelom, or the nephrocele, a dorsal compartment of the coelom. The pronephric glomus of jawless fishes projects into the pericardial cavity.
Fluids are swept from the filtration cavity into the nephron by nephrostomes, which are ciliated funnels. The cilia are densely packed in these narrow epithelial tubes, which have a different shape from the other tubular epithelia of the kidney.
Post filtration the drainage is also the main function.
Conclusion
In amphibians, the pronephros undergoes apoptosis after which the more complicated mesonephros arises. The nephron degenerates in fish, but the organ survives and forms part of the immune system. In mammals, a functional pronephros does not develop in the context of an organ that performs waste elimination or osmoregulation. A kidney primordium does, however, form along the intermediate mesoderm and connects to the cloaca. The pronephric duct, mesonephric duct, or Wolffian duct are all names for this duct. While the duct generated from this temporary primordium never generates functional nephrons, it is critical for the formation of the more sophisticated later kidneys.
FAQs on Pronephros Explained: Structure, Function, and Development
1. What is a pronephros?
The pronephros is the most basic and earliest of the three excretory organs that develop in vertebrate embryos. It appears first during the process of organogenesis. While it serves as the main kidney in the larvae of most fish and amphibians, in higher vertebrates like humans, it is a transient, non-functional structure that degenerates and is replaced by the more advanced mesonephros.
2. What is the primary function of the pronephros in the organisms where it is active?
In organisms where it is functional, such as in the larvae of amphibians and fish, the primary function of the pronephros is excretion and osmoregulation. It filters waste products from the coelomic fluid (body cavity fluid) and expels them from the body, helping to maintain the correct balance of water and salts. This function is crucial for survival during the early life stages of these animals.
3. Why is the pronephros sometimes called the “head kidney”?
The pronephros is often referred to as the “head kidney” due to its anatomical position. It develops in the most anterior (cranial or head) region of the embryo’s body cavity. This name is particularly common when discussing primitive fish like hagfish, where the pronephros persists into adulthood and is located furthest forward compared to the other kidney structures.
4. How does the pronephros differ from the mesonephros?
The pronephros and mesonephros represent two sequential stages of kidney development, with several key differences:
Developmental Timing: The pronephros is the first to form and is more primitive, while the mesonephros develops later and is more complex.
Structure: The pronephros typically consists of simple tubules that open into the body cavity via structures called nephrostomes. In contrast, the mesonephros has more advanced nephrons with internal glomeruli that directly filter blood.
Function in Humans: In human embryos, the pronephros is entirely non-functional and quickly degenerates. The mesonephros, however, is briefly functional, producing small amounts of urine during early embryonic life.
5. What is the developmental sequence of the three embryonic kidneys in vertebrates?
Vertebrate kidney development occurs in a sequence of three overlapping stages, from simplest to most complex:
Pronephros: The first and most primitive kidney, which is transient in most vertebrates.
Mesonephros: The second stage, which replaces the pronephros. It serves as the principal excretory organ in the embryos of higher vertebrates and as the permanent kidney in most fish and amphibians.
Metanephros: The final and most advanced stage, which develops into the permanent, functional adult kidney in reptiles, birds, and mammals, including humans.
6. What is the evolutionary significance of the pronephros?
The pronephros is evolutionarily significant because it represents the most ancestral form of the vertebrate kidney. Studying it provides insight into how more complex excretory systems evolved. Its structure is a classic example of the principle that “ontogeny recapitulates phylogeny,” where the developmental stages of an organism (ontogeny) reflect the evolutionary history of its species (phylogeny). The persistence of its duct (pronephric duct) to form later structures is also crucial for understanding developmental biology.
7. In which adult animals does the pronephros remain as the functional kidney?
While the pronephros is an embryonic or larval structure in most vertebrates, it persists as the functional kidney throughout the adult life of some of the most primitive vertebrates. The primary examples include adult hagfish (Myxine) and some species of lampreys. In these jawless fish, the pronephros carries out all necessary excretory and osmoregulatory functions.
