The document discusses the urinogenital system in vertebrates. It begins by defining the urinogenital system and its components, which include the kidneys, urinary ducts, gonads, and genital ducts. It then describes the evolution and development of the kidney structures in vertebrates, from the primitive pronephros to the more advanced metanephros. Key points include that the kidney evolves from the intermediate mesoderm and progresses through pronephric, mesonephric, and metanephric stages. The metanephros is the definitive kidney structure in amniotes. The document also discusses kidney structure and blood supply in different vertebrate groups.

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
• reddysirr@gmail.com

4. 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.

5. 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.

6. 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.
 It 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

13. 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.

14. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.


21. 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.

22. 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.

23. 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)

24. 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)