Urino genitial system of vertebrates

INTRODUCTION
The term urogenital refers to something that has both urinary and genital origins. The word urogenital is used because the urinary and reproductive systems in males merge.
These are grouped together because of their proximity to each other, their common embryological origin and the use of common pathways (ex. urethra).
Kidneys and urinary ducts form the urinary system.
The Urinary system performs two important homeostatic processes like excretion and osmoregulation. This system is intimately associated both anatomically, and in terms of embryonic origin with the genital system.
The genital system includes the gonads which generate gametes and the genital ducts that serve as passages for the gametes.
Though functionally different the two organ systems the urinary and the genital system are treated together as the urino- genital system, since both develop from the same segmental blocks of trunk mesoderm or adjacent tissues and share many of the ducts.
Thus although the two systems have nothing common functionally they are closely associated in their use of common ducts and are studied under the broad heading of urinogenital system.
The function of the excretory system is crucial in considering the possible environment of the ‘vertebrate life ’. Several main functions can be attributed to all vertebrate excretory systems:
Excretion of nitrogenous waste products.
Maintaining homeostasis with regard to ions (i.e. salt balance).
Regaining valuable substances (glucose, salts, amino acids, etc.)
Maintaining a physiological osmotic value (i.e. water balance).
The excretory system is formed by a series of paired, segmental nephrons that begin with a nephrostome opening into the coelomic cavity.
A pair of glomeruli per segment, supplied by branches from the aorta, projects into the coelomic cavity close to these nephrostomes.
At a later stage of development, the glomerulus/nephrostome area becomes separated from the rest of the coelomic cavity by an epithelial fold.
The nephrons connect to a duct that is formed by caudal growth of the most anterior nephric tubules. These paired urinary ducts open near the anal region.

1. Urinogenital system in vertebrates ( Vertebrate Kidneys and Ducts) Dr. P.B.Reddy M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM PG DEPARTMENT OF ZOOLOGY GOVERTNAMENT PG COLLEGE, RATLAM.M.P [email protected]

2. INTRODUCTION The term urogenital refers to something that has both urinary and genital origins. The word urogenital is used because the urinary and reproductive systems in males merge. These are grouped together because of their proximity to each other, their common embryological origin and the use of common pathways (ex. urethra ) . Kidneys and urinary ducts form the urinary system. The Urinary system performs two important homeostatic processes like excretion and osmoregulation. This system is intimately associated both anatomically, and in terms of embryonic origin with the genital system. The genital system includes the gonads which generate gametes and the genital ducts that serve as passages for the gametes. Though functionally different the two organ systems the urinary and the genital system are treated together as the urino- genital system, since both develop from the same segmental blocks of trunk mesoderm or adjacent tissues and share many of the ducts. Thus although the two systems have nothing common functionally they are closely associated in their use of common ducts and are studied under the broad heading of urinogenital system.

3. The function of the excretory system is crucial in considering the possible environment of the ‘vertebrate life ’. Several main functions can be attributed to all vertebrate excretory systems: Excretion of nitrogenous waste products. Maintaining homeostasis with regard to ions (i.e. salt balance). Regaining valuable substances (glucose, salts, amino acids, etc.) Maintaining a physiological osmotic value (i.e. water balance). The excretory system is formed by a series of paired, segmental nephrons that begin with a nephrostome opening into the coelomic cavity. A pair of glomeruli per segment, supplied by branches from the aorta, projects into the coelomic cavity close to these nephrostomes. At a later stage of development, the glomerulus/nephrostome area becomes separated from the rest of the coelomic cavity by an epithelial fold. The nephrons connect to a duct that is formed by caudal growth of the most anterior nephric tubules. These paired urinary ducts open near the anal region .

4. VERTEBRATE KIDNEYS Excretory organs in protochordates are very different from the higher vertebrates. Balanoglossus (Hemichordate) has a glomerulus in the proboscis region to excrete nitrogenous wastes from the blood. Herdmania (Urochordata) has a neural organ near the solid nerve ganglion located in between the two siphons. Amphioxus (Cephalochordata) possesses protonephridia that carry hundreds of flame cell-like solenocytes that excrete wastes in the atrial cavity and to the outside. Kidneys evolved in primitive fresh water vertebrates to excrete excess water that was continuously entering the body by osmosis. Later, kidneys also acquired the function of removing the waste materials from body. In invertebrates and also in hag fishes, body fluids are isotonic to sea water and hence they do not have problem of osmoregulation. Cartilaginous fishes and also coelacanths retain considerable amount of urea in the blood so that blood is isosmolal to sea water and osmotic problems are avoided. I t is believed that kidneys in all modern vertebrates evolved from a hypothetical kidney known as Archinephros or Holonephros, which extended from anterior to the posterior end of the body, with segmentally arranged glomeruli and nephrotomes.

5. Evolution of the vertebrate kidney Evolution of the vertebrates is viewed in terms of the external osmotic environment in which various classes evolved. Fresh water, marine and terrestrial habitats possessed different problems for the maintenance of internal water balance and the excretion of nitrogenous wastes. The evolution of the kidney in vertebrates illustrates how pronephric, mesonephric and metanephric kidney, represent successful evolutionary responses to these environmental pressures. So many variations in the evolution of the kidney are correlated with these environmental factors. Variations in the structure of the vertebrate kidney from fish to man are primarily in the nature of alterations in number, complexity, arrangement and location of the kidney tubules.

6. Fig.1. Embryonic development of nephric tubules in vertebrates. Embryo showing location of developing kidney (nephric ridge) and the appearance of segmental nephrotome in the posterior part of the intermediate mesoderm. Kidney removes waste products from the body, maintain balanced electrolyte levels, and regulate blood pressure. Embryological origin: The kidney in all vertebrate is originated from the intermediate nephrogenic mesoderm. The kidney as a whole is made up of two elements, the kidney duct and the kidney tubules. They are all derived from the urogenital or Nepghric ridge. Function: Regulation of extracellular fluid volume. The kidneys work to ensure an adequate quantity of plasma to keep blood flowing to vital organs. Regulation of osmolarity. Regulation of ion concentrations. Regulation of pH. Excretion of wastes and toxins. Production of hormones.

7. Objectives To study and describe the urinogenital system of the proto chordates . To describe the basic plan of urinogenital system in vertebrates. To give a comparative account of the urinogenital system of all vertebrates. To describe the origin and embryonic development of kidneys, gonads and their associated ducts, discuss the morphological and physiological adaptations of the urinogenital system of vertebrates.

8. Vertebrate Kidneys and Ducts Basic Structure: All vertebrates have kidneys like the human kidneys, they are made of many nephrons. However, there are many differences in the structure and function of various vertebrate kidneys that adapt them to the environment in which the animals live. In vertebrates, there is a pair of compact kidneys, lying dorsal to the coelom in trunk region, one on either side of vertebral column. A kidney is made of a large number of uriniferous tubules or nephrons. Their number, complexity and arrangement is different in different groups of vertebrates. The uriniferous tubules arise in an embryo from a special part of the mesoderm, called mesomere or nephrotome. Primitively the uriniferous tubules develop from the nephrotome in a sequence commencing from the anterior end. They are segmental in arrangement with one pair of uriniferous tubules for each trunk segment.

9. Kidney development , or nephrogenesis Phases .Archinephros Pronephros Mesonephros Metanephros 2. Migration The development of the kidney proceeds through a series of successive phases, each marked by the development of a more advanced kidney: the archinephros, pronephros, mesonephros, and metanephros. The pronephros is the most immature form of kidney, while the metanephros is most developed. The metanephros persists as the definitive adult kidney. All three are fundamentally alike, differing principally in their relationship to the blood system, in degree of complexity, and in efficiency. They are all derived from the urogenital ridge.

10. Archinephros The ancestral vertebrates had a pair of kidneys running through the entire length of the coelom. Each had segmentally arranged tubules; one pair per body segment. Has been retained by the larvae of hagfish and some caecilians. It also occurs in the embryos of higher animals as the simplest kind of excretory organ. This is consisted of a pair of archinephric ducts located on the dorsal side of the body cavity and is extending the length of the coelom. Each duct is joined by a series of segmentally arranged tubules, one pair of tubules to a segment. At its other end, the tubule is opened into the coelom by a ciliated, funnel-shaped, peritoneal opening called the nephrostome. Each tubule is ciliated where it opens into the body cavity, and a knot of capillaries ( external glomerulus) occurs at each of these openings, or nephrostomes. Tissue fluid discharge occurs through glomerulus> coelom> nephrostomes> tubules > archinephricduct > cloaca> outside.

11. Structure of uriniferous tubule: (i) A ciliated peritoneal funnel near the proximal end of the tubule which opens into the splanchnocoel by a nephrostome or coelomostome (often the peritoneal funnel itself is called a nephrostome). (ii) A convoluted ciliated tubule opening into a longitudinal collecting duct, and a Malpighian body or renal corpuscle. The Malpighian body has a double-walled Bowman’s capsule, enclosing a network of inter-arterial blood capillaries, called glomerulus where filtration of blood takes place. Bowman’s capsule and glomerulus together form the Malpighian body or renal corpuscle. An afferent arteriole brings blood into the glomerulus and an efferent arteriole takes blood away from it. Then the efferent arteriole breaks up into capillaries along the entire course of the uriniferous tubule and finally the blood goes to a renal vein. Encapsulated glomerulus is internal and is common. The glomerulus without a capsule suspended freely in the coelomic cavity is called external glomerulus which is found in embryos and larvae. Malpighian bodies with glomeruli are lacking in some fishes, embryos and larvae and their kidneys are called aglomerular. In an adult the uriniferous tubules are elongated and coiled, so that their segmental arrangement is lost and they become enclosed in a connective tissue capsule to form a kidney.

12. 1. Pronephros: It is the most primitive and present in few cyclostomes and the embryonic development of all vertebrates. It develops in the cervical region of the embryo from mesenchymal buds of pronephric primordia, or nephrotomes. It consist of a varying number of anteriorly located pronephric tubules together with a pair of archinephric ducts duct (which called here pronephric duct). The tubules and ducts lay in the dorsolateral mesoderm on either side of the mesentry that supported the gut. The tubules were segmentally arranged, connected with the near pronephric duct at its anterior end. The outer end of the tubules opens into the coelom by means of nephrostomes. The nephrostome and the part of the tubule near it are ciliated. Most forms are possessed internal glomeruli. These are knots of interarterial capillaries, each surrounded by a double wall structure called Bowman’s capsule, the two together are known as renal or Malpighian corpuscle. Sometimes, several glomeruli united to form a large glomus. In some cases, pronephric tubules expanded so as to form pronephric chambers or one large pronephric chamber. The uriniferous tubules of each pronephros open into a common pronephric duct which grows back to enter the embryonic cloaca. In some, there is a large pronephric chamber which surrounds the glomus (all glomeruli) and tubules.

13. All glomeruli project into the pronephric chamber where they may unite to form a single compound glomerulus called glomus. Pronephric chamber is derived from pericardial or pleuroperitoneal cavity. All the tubules of a pronephros open into a common pronephric duct opening posteriorly into the embryonic cloaca. Pronephros is replaced by mesonephros. In those vertebrates in which pronephroi become adult kidneys, they are called head kidneys due to its anterior position behind the head.

14. Fig.2. Schematic drawings of pro-, meso- and metanephros. A; aorta, B: bladder, C: coelom, Ca: caudal, Cr: cranial, DA: dorsal aorta, PD: pronephric duct, US: urogenital sinus, U: ureter, UB: ureteric bud, MD: mesonephric duct. C: coelom, G: glomerulus, G/G: glomerulus/glomus, L: lateral, M: medial, NS: ciliated nephrostome, which links the coelom or nephrocoel with the proximal tubule, PF: ciliated peritoneal funnel, which links the coelom to the encapsulated glomerulus (primitive Bowman’s capsule), T: Tubule.

15. MESONEPHROS Also called the Wolffian body. It is the functional kidney of the larvae as well as the adults of fish and amphibian and functional kidney in the embryonic stage of amniotes, i.e. reptiles, birds and mammals. The number of uriniferous tubules increases to thousands and glomeruli are enclosed in a cup-like Bowman’s capsule. Bowman’s capsule and glomeruli are called Renal corpuscle or malpighian body . In addition, there is also a nephrostome attached to the collecting tubule, which all meet the mesonephric duct that carries the wastes to the outside. Nephrostomes disappear in most of the higher elasmobranchs, teleosts and amphibians. In sharks and urodeles, mesonephros is elongated kidney that extends up to the posterior end of body and hence is called Opisthonephros .

16. METANEPHROS It is the adult amniote kidney The number of corpuscles is large; up to about 4.5 million is some species. Drained by a duct called the metanephric duct or ureter. Reptilian kidney is elongated, the body lumen is narrow and long. Bird kidney is trilobed and mammalian kidney is bean-shaped. Kidney is enclosed in a protective tunic that is made of fibrous and adipose layers. Kidney tissue is divided into an outer cortex that carries renal corpuscles and convoluted tubules and inner medulla which houses collecting ducts and loop of Henle. The number of nephrons (uriniferous tubules plus renal corpuscles) runs into millions, thereby increasing the efficiency of kidneys to extraordinary levels.

17. Migration After inducing the metanephric mesenchyme the lower portions of the nephric duct will migrate caudally (downward) and connect with the bladder, thereby forming the ureters. The ureters will carry urine from the kidneys to the bladder for excretion from the fetus into the amniotic sac. As the fetus develops, the trunk elongates and the kidneys rotate and migrate upwards within the abdomen which causes the length of the ureters to increase.

18. Blood supply: Kidney is supplied by 2 or more renal arteries in reptiles & birds, & by a single renal artery in mammals (below). Pathway of blood in mammalian kidney: renal artery > segmental arteries > interlobar arteries > arcuate arteries > interlobular arterioles. In mammals renal arteries bring blood into the kidneys and renal veins take it away, but reptiles and birds possess a renal portal system. Uriniferous tubule of mammals carries a loop of Henle that is designed to reabsorb all water from the filtrate by counter current mechanism. Reptiles do not have loop of Henle and birds have a reduced one, both groups being uricotelic do not excrete much water any way.

19. Urinary bladders Urinary bladders are found in all vertebrates except agnathans, snakes, crocodilians, some lizards, & birds (except ostriches). Fish - bladders are terminal enlargements of the mesonephric ducts called TUBAL BLADDERS. Amphibians through Mammals - bladders arise as evaginations of ventral wall of the cloaca. Value of tetrapod urinary bladder: release urine when desired rather than continuously as it is formed uses of urine: reproduction (e.g., providing males with information concerning the reproductive status of a female) behavioral (e.g., marking territories) moisten soil (some freshwater turtles use urine to soften the ground and make it easier to dig holes for egg-laying)

20. number of nephrons in mesonephric kidney= 40-44 Loop of Henle is absent in fishes, amphibians and reptiles (meso nephric kidney) number of nephrons in pronephric kidney = 2-4 renal corpuscular capsule, or Bowman's capsule is less developed number of nephrons in metanephric kidney = two million nephrons (approximately 1,000,000 per kidney )