EVOLUTIONARY TRENDS IN URINOGENITAL SYSTEM - PRO, MESO AND METANEPHROS

In vertebrates Kidneys (organs of renal excretion) and gonads (organs of reproduction) have different functions. Yet, they are in close proximity and some of their ducts have relation in embryo development and function. Some excretory ducts carry genital products. Hence two systems are collectively- Urinogenital system.

Development

 The excretory organs are the kidneys which consist of units called nephrons. A nephron is an excretory tubule with a capillary network connected with it.  The nephrons are derived from the mesoderm. The part of the mesoderm that gives rise to the kidney is called nephrotome or the nephrogenic mesoderm  The nephrons develop from the nephrotome in a sequence starting from the anterior end. There is a pair o uniferous tubules/neprons in each trunk segment, and they are segmentally arranged.  The inner ends of the tubules are spherical and open primitively into the coelom by ciliated funnels called nephrostomes.  Their outer ends unite in to a longitudnel duct on each side leading behind into cloaca  The inner ends come to be closely associated with blood vessels forming the glomerulus as that the waste products are separated from the blood and passed out.

Archi nephros

The ancestral craniates (jawless vertebrates) are supposed to have had a complete set of segmentally arranged tubules as each side extending throughout the trunk region and connected at their outer ends with a longitudinal duct leading behind into cloaca. Such a type of kidney has been called archinephros or holonephros. This type of kidney does not exist in the adult of any modern vertebrates though the kidney of hag fish very nearly approaches it.

In modern forms the tubules arise in 3 stages in succession differing in position, time of development embryonic history and their relation to the blood capillaries. They are known as pronephros, mesonephros and metanephros.

Pronephros Mesonephros Metanephros (Head Kidney) (Mid kidney) (hind kidney) 1 Pronephric tubules develop Mesonephric tubules develop from the The metanephric tubules segmentally in the anterior most middle part of the nephrotome, develop from the posterior part of the nephrotome appear posterior to the pronephric part of the nephrotome, tubules, segmentally extending along behind the mesonephros in the the body cavity form of buds. The bud grows posteriorily pulling down the nephrotome along with it. 1 Pronephros consist of a few A greater number of tubules than the The adult metanephric kidney number of pronephric tubules pronephros is formed from displaced nephrogenic mesoderm/ nephrotome. 2. A number of tubules are formed The tubules multiply by branching The tubules are massed together initially but the number is and lose their segmental and encased forming a reduced during embryonic arrangement and their nephrostomes compact body. The tubules are development appear irregularly dispersed in the thrown into coils and there are peritoneal envelop and more compact no nephrostome than those of the pronephros 3 Each tubule opens into the body Each tubule at its inner end gives a blind The metanephric tubules are S cavity by the nephrostome. Close diverticulum (Bowman’s capsule). A shaped with renal/ malpigian to the nephrostomes, on the branch of the renal artery enters the blind corpuscle and glomerulus. medium side projections of the diverticulum converting it into a double dorsal aorta extend into the coelom. walled cup. Inside the cup the arteriole The tubule consists of proximal These projections are called forms a knot of capillaries called and distl convoluted parts glomerulii glomerulus. The glomerlus with connected by a U shaped loop of Bowman’s capsuleis known as the Henle. malpigian corpuscle.

As the tubules are formed their As the tubules are formed their outer ends Gives rise Metanephric duct or outer ends unites into the unites into the Mesonephric/ Wolffian ureter longitudnel duct/ Pronephric duct duct 4. External Glomeruli are not There is closer association between the There is closer association directly associated with the tubules. tubules and the blood capillaries and thus between the tubules and the blood The coelomic epithelium the glomerulus is internal. As a result in capillaries as a result in the waste seperates the waste prodcucts from the capillaries and these the waste products passes directly from products passes directly from products are discharged into the capillaries into the tubules capillaries into the tubules. coelom which is then collected by glomerulus is internal the nephrostome that opens into the tubule 5 At their outer ends the tubules unite The mesonephric tubules acquire connection The metanephric duct arises as an into the longitudinal duct on each side with the pronephric duct. As the pronephric outpushing from the hind and called the pronephric duct.The duct tuules degenerate this duct is now known as meosnephric duct/Wolffian duct. The of each side grows backward and leads the mesonephric duct or wolffian duct. end of the duct gives rise to the into the cloaca which opens to the collecting tubules within the kidney. exterior.

Examples: Examples: Examples: a) The pronephros is retained as a) Functional kidney of cyclostomes (Except Functional adult kidney of all an adult kidney only in myxinoids), fishes and amphibians reptiles, birds and mammals Hagfishes and in some teleosts b) Functional embryonic kidney in reptiles, b) Functional embryonic kidney birds and mammals in cyclostomes, dipoans fishea and apmphibians c) Non functional temporary structure in the embryos of reptiles, birds and mammals (In those veretebrates in which pronephros become adult kidneys are called head kidneys due to its anterior position behind the head Urinary bladder

Urinary bladder is not found in all vertebrates. The urinary bladder must have developed as an adaptation to terrestrial life. They actually serve as water reservoirs and also help in absorption of water. The urinary bladder arises in three ways in different classes of vertebrates.

a. In some fishes the bladder is formed from the fused ends of the mesonephric ducts or wolffian ducts b. In other fishes and in amphibians, the diverticulum from the cloaca forms the cloacal bladde. It is independent of the mesonephric duct. c. In snakes, crocodiles and birds the urinary bladder is absent. d. In the mammals and in a few reptiles the bladder is formed from the base of allantois. It is also called as allantoic bladder. The ureters open in to the posterior wall of the urinary bladder at its base. The urinary bladder opens to type exterior through a narrow duct called the urethra.

Reproductive organs

Gonads arise as ridges of the coelomic epithelium from the inner wall of the splanchnocoel; they extend longitudinally on either side between the dorsal mesentery nd mesonephros. As they develop, they bulge in to the coelom and eventually suspended by peritoneal folds, the mesovarium for the ovary and the mesorchium for the testis.

In cyclostomes there are no special ducts to carry the genital products to the exterior, they fall in to the coelom and find their wy to the urinogenital sexus by the paired abdominal pores. In other vertebrates there are ducts the vasa deferentia in the male and oviducts in the female by which the genital products find their way out. The vasa derentia are the Wolffian ducts and the oviducts are the Mullerian ducts.

The Mullerian ducts arise in 2 ways

1. In elasmobranches each pronephric ducts splits longitudinally and gives rise to 2 ducts – Wolffian ducts and Mullerian ducts. The Wolffian ductsreturns its connection with the mesonephros and serves as the ureter. The mullerian duct becomes the oviduct

2. In amphibians and other tetrapoda the mullerian ducts arises s a separate formation in the form of a groove by the side of the wolffian duct and the walls of the groove become rolled up as a tube.

EVOLUTION OF GENITAL DUCTS IN VERTEBRATES: FATE OF PRO/ MESO/ METANEPHRIC DUCTS

Pronephric duct Mesonephric duct Metanephric duct The pronephric duct The mesonephric duct is also called the Wolffian duct. The The metanephric duct arises as an outpushing is just a duct for mesonephric duct unlike the pronephric duct has a dual of mesonephric duct. It grows forward to meet exception and it function the kidney and functions as the ureter The does not transport In fishes and amphibian it not only carries the excretory ureters drain the kidneyof urine open ito the gametes. wastes in both sexes but also functions as the gonoduct in cloaca in reptiles and monotremes. In mammals males the ureters open into the urinary bladder The gametes in in males : It helps to transport sperms to exterior. This is In males hagfishes are sent because the interior uriniferous tubules of the mesonephros The mesonephric duct (wolffian duct) persists out through the are no longer excretory in function but take up a reproductive nd functions as the gonoduct establishing pores from the body function and they establish connection with the seminiferous connection with vasa efferentia cavity. tubule and are now called vasa efferentia The sperms pass by The anterior part of the Woffian duct becomes the vasa efferentia into the wolffian duct which serves as ciled in mammals as epididymis.The mullerian channel for both sperms and urine. duct a trophies In the teleosts the In Females: elasmobranches and amphibians the In females gametes are sent out mesonephric ducts serve only to carry excretory wastes. A The mullerian ducts are the oviducts and the through mesenteric separate mullerian duct takes its origin by splitting of the Wolffian duct become vestigial. filds that serve as pronephric duct in the elasmobranches.This junctions as the gonoducts ostium or the opening of the oviduct into the coelom, that receives eggs. The mullerian duct conducts the eggs out of the body to the exterior through the cloaca.