Mesonephros Structure Function and Role in Embryonic Development
Mesonephros, permanent kidney of amphibians and most fish that develops posterior to and replaces the pronephros of the embryonic and larval stages. It is a paired organ composed of nephrons with capsules that filter blood from the glomerulus and tubules whose cells reabsorb water and nutrients and secrete nitrogenous wastes. Some marine fish lack glomeruli, so urine forms solely in the tubules, conserving water.
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The tubules empty into a long tube called the Wolffian duct, which is a remnant of the pronephros. The mesonephros develops in the embryo of more advanced vertebrates, but in humans, it is replaced by the metanephros after the 10th week.
Urinary System
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Vertebrates have experimented with three types of kidneys: the pronephros, or first type; the mesonephros, or intermediate kidney; and the metanephros, or permanent kidney. All are derived from cellular plates known as nephrotomes, which connect somites to the mesodermal sheets that surround the body cavity. The vestigial pronephros is composed up of many pairs of tubules which are connected to the independently formed excretory ducts which grow downward and enter the cloaca, which is the common outlet for genital products, urine, and intestinal wastes.The mesonephros is formed by 40 pairs of nephric (kidney) tubules that connect to the same excretory ducts, which are referred to as the mesonephric ducts. Until the 10th week, the two sets of mesonephric tubules function as functioning kidneys.
Each permanent kidney, or metanephros, develops further down the tail. Each mesonephric duct has a ureteric primordium near its hind end. The ureteric stem elongates and expands terminally, resulting in the formation of the renal pelvis and calices; continued bushlike branching results in the formation of collecting ducts. A mass of nephrostome tissue is invaded by the early ureteric bud. The branching collecting ducts gradually break up this tissue into tiny lumps, each of which develops into a long secretory tubule, or nephron, and connects to a nearby terminal twig of the duct system. In each kidney, the continued proliferation of ducts and nephric tissue results in the formation of over a million urine-producing tubules.
The blind caudal end of the endodermal hindgut absorbs the stem of each mesonephric duct, allowing the remainder of the duct and the ureter to enter the hindgut separately. A cloaca is an expanded region of the gut that is now a potential receptacle for faeces, urine, and reproductive products. It is then divided into a rectum in back and a urogenital sinus in front. In turn, the sinus will specialise into the urinary bladder and urethra. The prostate gland forms as a series of buds from the urethra near the bladder.
Wolffian Duct
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The Wolffian duct (also known as the mesonephric duct) is a pair of embryogenic tubules that drain the primitive kidney (mesonephros) to the cloaca. It also produces a lateral branch that forms the ureteric bud. The Wolffian duct develops into the trigone of the urinary bladder in both males and females.
Development
Female
In the absence of testosterone, the Wolffian duct regresses in females. Inclusions, on the other hand, may persist, resulting in the epoophoron and Skene glands. A Gartner duct or Gartner duct cyst may form where a remnant exists lateral to the vaginal wall.
Male
The Wolffian duct develops into rete testis, penile ducts, epididymis, ductus deferens, and adrenal glands when the ducts are exposed to testosterone during embryogenesis. The prostate develops independently of the urogenital sinus.
Key Points
In males, Wolffian ducts produce the epididymal tube, ductus deferens, ejaculatory duct, and seminal vesicle.
The ovary differentiates into the medulla of the ovary, and immature ova develop from cells in the yolk sac's dorsal endoderm.
The genital tubercle develops into the phallus, which then transforms into either the penis or the clitoris.
Key Terms
Wolffian ducts are those mesonephric ducts which connect the original kidney (or mesonephros) to the cloaca and serve as the anlage for some male reproductive organs.
Müllerian Ducts: They are the paired embryonic ducts which run down the lateral sides of the urogenital ridge and wind up in the primitive urogenital sinus at the Müllerian eminence. They are formed in the female and are responsible for making up the Fallopian tubes, uterus, cervix, and the upper two-thirds of the vagina, and they are not present in the male.
Development in Mesonephros
From day 24 of gestation, the mesonephros develops from the dorsolumbar segments of the nephrogenic cord. During the fourth week of pregnancy, cells of the mesonephric duct proliferate caudally and begin to form the mesonephric kidney (4 mm; 26th to 28th somite stage).
FAQs on Mesonephros in Embryonic Kidney Development
1. What is the mesonephros?
The mesonephros is a temporary embryonic kidney that functions during early vertebrate development. It develops from the intermediate mesoderm and acts as the main excretory organ in early embryos before being replaced by the metanephros (permanent kidney) in mammals. In some lower vertebrates, it persists as a functional kidney in adults.
2. What is the function of the mesonephros?
The primary function of the mesonephros is temporary filtration of blood and removal of nitrogenous waste during embryonic life. Its functions include:
- Formation of mesonephric tubules that filter blood
- Drainage of waste into the mesonephric (Wolffian) duct
- Contribution to male reproductive structures in males
3. When does the mesonephros develop in humans?
The mesonephros develops during the 4th week of human embryonic development. It becomes functional around weeks 6–10 and then gradually regresses as the metanephros forms and takes over kidney function. By the end of the first trimester, most of it has disappeared except for parts contributing to male reproductive organs.
4. What is the difference between pronephros, mesonephros, and metanephros?
The pronephros, mesonephros, and metanephros are three successive stages of kidney development in vertebrates.
- Pronephros: Earliest and nonfunctional in humans
- Mesonephros: Temporarily functional embryonic kidney
- Metanephros: Permanent kidney in reptiles, birds, and mammals
5. What are mesonephric tubules?
The mesonephric tubules are filtering units of the mesonephros that help remove waste from embryonic blood. Each tubule:
- Forms a cup-shaped structure around a glomerulus
- Connects laterally to the mesonephric duct
- Allows filtration and passage of waste fluid
6. What happens to the mesonephros in males and females?
In males, parts of the mesonephros persist and form reproductive ducts, while in females it mostly regresses. Specifically:
- In males: The mesonephric (Wolffian) duct develops into the epididymis, vas deferens, and seminal vesicles
- In females: The mesonephric structures largely degenerate
7. What is the mesonephric duct?
The mesonephric duct, also called the Wolffian duct, is a tube that drains the mesonephros and later forms male reproductive organs. During embryonic life, it:
- Receives filtrate from mesonephric tubules
- Empties into the cloaca
- Differentiates into male genital ducts under testosterone influence
8. Is the mesonephros functional in adult humans?
No, the mesonephros is not functional in adult humans because it is replaced by the permanent kidney, the metanephros. Most of the mesonephric tissue degenerates before birth, except for portions that contribute to male reproductive structures.
9. From which germ layer does the mesonephros develop?
The mesonephros develops from the intermediate mesoderm, a subdivision of the mesoderm germ layer. The intermediate mesoderm gives rise to:
- Pronephros
- Mesonephros
- Metanephros
- Parts of the reproductive system
10. Why is the mesonephros important in embryology?
The mesonephros is important in embryology because it serves as a temporary kidney and contributes to reproductive system development. Its significance includes:
- Providing early excretory function
- Forming the mesonephric duct system
- Giving rise to key male reproductive structures
