Mesonephros

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).