What is Pronephros?

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.

Stages

Functional Features

Archinephros

Archinephros is an ancestral vertebrate kidney that is retained by hagfish and some caecilians larvae and found in higher animal embryos. The archinephric kidney, or Wolffian, ducts are two tubes that go between the body cavity and the back and lead to the outside. The body cavity is connected to the Wolffian ducts by a series of tubules, one pair for each body segment. Each tubule is ciliated when it enters the body cavity, and at each of these openings, termed nephrostomes, a knot of capillaries forms.

Pronephros

A pronephros is made up of one or more nephrons, the proximal ends of which open into the coelom and the distal ends of which drain into a shared nephric duct that proceeds caudally to open into the cloaca. Each side of the body has one pronephros and one nephric duct in the pronephric kidney. The mesonephroi, or the second set of kidneys, develops as the larvae grow.

Mesonephros

The mesonephros is a permanent kidney that develops posterior to and replaces the pronephros of the embryonic and larval stages in amphibians and most fish. It's a paired organ made up of nephrons with capsules that filter blood from the glomerulus and tubules with cells that reabsorb water and nutrients while secreting nitrogenous waste. 

When the glomeruli in some marine fish are missing, urine develops entirely in the tubules, conserving water. The Wolffian duct, a remnant of the pronephros, empties the tubules into a lengthy tube. 

Metanephros

The mesonephros develops in the embryo in more evolved vertebrates, but the metanephros takes over after the 10th week in humans. 

In reptiles, birds, and mammals, the metanephros is a permanent kidney that develops during the 10th week in human embryos from the lower section of the Wolffian duct and replaces the embryonic tissue known as the mesonephros. It is a compact, paired organ with many nephrons; from the metanephros to the bladder, a ureter separate from the Wolffian duct leads.


Functions

  1. 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.

  2. 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.

  3. 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.

  4. 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 (Frequently Asked Questions)

Q.1) What is the Role of Pronephros Present in Humans?

Answer.) The primary stage of development of the embryonic kidney is pronephros, however, it has no role in human development and remains non-functional.

Q.2) Why is Pronephros Referred to as the "Head Kidney"?

Answer.) In Myxine and some primitive teleosts, the head kidney, so named because of its position in the anterior portion of the body, is still a working kidney. It has a small number of collecting tubules around 3-15 of them, each with a nephrostome for collecting waste from a single glomus.

Q.3) What is the difference between Pronephros and Mesonephros?

Answer.) In humans, the pronephros is the first nephric stage, and in most primitive vertebrates, the adult kidney. Mesonephros is the primary excretory organ during early embryonic life seen within 4 to 8 weeks and is formed by the creation of mesonephric tubules from the intermediate mesoderm.

Q.4) Is the Mesonephros Responsible for the Production of Urine?

Answer.) From the sixth through the tenth week of development, the entire mesonephros produces urine. The mesonephric nephrons, despite their identical structure, function, and name, are not a part of the adult kidney or nephrons.