Urogenital system in vertebrates: TYBSc course Semester-VI – USZ0601of University of Mumbai

Notes: Zoology- VI Semester, University of Mumbai, India

Comparative Anatomy of Chordates
Urogenital System in Vertebrates
PART-I: Development of Kidney in Vertebrates
Kidney tubules arise from the intermediate mesoderm. This is a ribbon of nephrogenic tissue
extending uninterrupted from the level of the heart to the cloaca. It lies just lateral to the
segmental/dorsal mesoderm. Almost the entire ribbon produces kidney tubules. The intermediate
mesoderm becomes segmented into units termed nephrotomes. The lateral portions of the
nephrotomes unite, forming a longitudinal duct on each side of the embryo. The anterior-most
tubules are always metameric, since one tubule develops from each mesodermal somite. Farther
back, numerous tubules develop in each segment and the metamerism is lost. The longitudinal ducts
of the basic patter appear first at the anterior end of the nephrogenic mesoderm as posteriorly
directed extensions of the first tubule. Each duct grows caudal until it achieves an opening into the
cloaca. At this time, it is known as the pronephric duct. The kidneys of myxinoid cyclostomes
closely resemble an archinephros.

Excretory organs in protochordates are very different from the higher vertebrates. Balanoglossus
(Hemichordata) 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.
A. Archinephros: It is believed that kidneys in all modern vertebrates evolved from a
hypothetical kidney known as Archinephros or Holonephros, which extended from anterior to
Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605


the posterior end of the body, with segmentally arranged
glomeruli and nephrostomes. Archinephros was found in
primitive vertebrate probably extended the entire length
of the body cavity & had external glomeruli that drained
the coelomic fluid. Each segment of the body contained
one pair of Uriniferous tubules with peritoneal funnels.
Malphigian bodies remained hanging in the coelom
collecting the waste matter from it and pouring in the
common archinephric duct. The archinephric duct
eventually opened into cloaca. Archinephros is found in
larval Myxine and some apodan amphibians. Most
vertebrates have embryonic transitory archinephros
commonly known as pronephros.
B. Pronephric duct: The development of the pronephric duct is a part of the development of the
urinary and reproductive organs. The initial kidney tissue of all vertebrates is positioned in the
outer part of the intermediate mesoderm, immediately under the ectoderm (in the region from the
V cervical segment to III thoracic segment) a series of short evaginations called nephrotomes,
from each segment grows dorsally and extends posteriorly, fusing successively forming the a
duct. This duct is known as the pronephric duct, mesonephric duct or Wolffian duct. This
primodium continues to grow posteriorly until it opens into the ventral part of the cloaca. In the
early stages of many vertebrate kidneys these first pronephric tubules or duct function for a brief
period as the pronephric kidneys (pronephros). Beyond the pronephros it is termed the
Wolffian duct now working as ureter. Thus, the Wolffian duct is what remains of the pronephric
duct after the atrophy of the pronephros.
C. Pornephros: Pronephros is the most basic of the three excretory organs that develop in
vertebrates, corresponding to the first stage of kidney development. Functional pronephros is
found in cyclostomes and as embryonic kidney in anamniotes. It is succeeded by the
mesonephros, which in fish and amphibians remains as the adult kidney. The pronephros develops
from the intermediate mesoderm, as do the later kidneys. It is a paired organ, consisting of a
single giant nephron that processes blood filtrate produced from glomeruli or glomera- large
embryonic glomeruli. The filtrate is deposited into the coelom. It then passes through thin ciliated


tubules into the pronephric nephron where
it is processed for solute recovery. The
organ is active in adult forms of some
primitive fish, like lampreys or hagfish
(cyclostomes). It is present at the embryo of
more advanced fish and at the larval stage
of amphibians (anamniotes) where it plays
an essential role in osmoregulation. In
others it degenerate but the pronephric duct
persists. In human, it is rudimentary,
appears at the end of the third week (day 20) and replaced by mesonephros after 3.5 weeks.
Despite this transient appearance in mammals, the pronephros is essential for the development of
the adult kidneys.
D. Opisthonephros:

The

opisthonephros

is

the

functional adult kidney in lampreys (cyclostomes), most
fishes, and amphibians. It is formed from the extended
mesonephros along with tubules from the posterior nephric
ridge. In 1949, Hyman wrote the opisthonephros “has used
up the mesomere tissue from which in amniotes both
mesonephros and metanephros come.” The mesonephros
and metanephros of amniotes are derived from different
parts of the anamniots opisthonephros. The mesonephros is
derived from anterior part of the opisthonephros whereas
the metanephros is derived from the posterior part of the
opisthonephros. In amniotes, most of the mesonephros
became the epididymis, and the archinephric duct became
the vas deferens. The mesonephros act as embryonic kidney and a more complex metanephros
acts as the adult kidney in amniotes. Some accounts call opisthonephros the ‘mesonephros’, but
the opisthonephros in anamniotes (lampreys, fish, and amphibians) differ considerably than
the mesonephros in amniotes (reptiles, birds, and mammals). Thus, the term mesonephros
is usually reserved for the embryonic kidney of amniotes.



E. Mesonephros: The mesonephros is one of three excretory organs that develop in vertebrates.
It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles,
birds, and mammals.
A long portion of the intermediate
mesoderm posterior to the pronephric
tissue develops to form the more advanced
mesonephric kidney. The development of
the pronephric duct proceeds in a cranialto-caudal

direction.

As

it

elongates

posteriorly, the pronephric duct induces
nearby intermediate mesoderm in the
thoraco-lumbar area to become epithelial
tubules called mesonephric tubules. Each
mesonephric tubule receives a blood
supply from a branch of the aorta, ending
in a capillary tuft analogous to the glomerulus of the definitive nephron. The mesonephric tubule
forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows
through the mesonephric tubule and is drained into the continuation of the pronephric duct, now
called the mesonephric duct or Wolffian duct. The nephrotomes of the pronephros degenerate
while the mesonephric duct extends towards the most caudal end of the embryo, ultimately
attaching to the cloaca. The mammalian mesonephros is similar to the kidneys of aquatic
amphibians and fishes. Once the more complex mesonephros forms the pronephros undergoes
apoptosis (degeneration) in amphibians. In fishes the nephron degenerates but the organ remains
and becomes a component of the immune system. Jawed fishes & amphibians - among males,
some anterior tubules of mesonephros conduct sperm from testis to mesonephric duct called as
sexual kidney while the rest is the uriniferous kidney. Amniote embryos mesonephros functions
for a short time after hatching or birth during a new kidney called the metanephros is developing.
The gonads, ovary or testis, also develop in the intermediate mesoderm. They originally form as
swellings that lie just ventral to the anterior mesonephric kidney. A mullarian duct also develops
in the intermediate mesoderm near the mesonephric duct.



F. Metanephros: Reptiles, birds, and
mammals develop a metanephric kidney
and lose the mesonephric kidney in the
process

of

early

development.

The

metanephric kidney begins as a small
outgrowth from the more posterior end of
the mesonephric duct. This is initially
called a metanephric bud. Later, as the
nephric tissue develops around this new
duct a new kidney structure forms. The
metanephric duct becomes the ureter. In
both males and females the ureter eventually becomes separated from the mesonephric duct. As
males develop the mesonephric duct makes connection with the testis as the primary sperm
conducting duct, and the mullerian duct is lost. In some fish species the testis secondarily grows a
separate sperm duct as the embryo develops. In females the mullerian duct becomes the passage
for eggs. The females of fish and amphibians retain the mesonephric duct as a urinary duct. In
reptiles, birds, and mammals (amniotes) the metanephric kidney replaces the mesonephric kidney.
The duct of the mesonephros forms the Wolffian duct and ureter of the adult kidney.
Subsequently, the mesonephric kidneys become small vestigial remnants.



PART-II: Structure of nephron, urogenital ducts,
Urinary bladder in vertebrates
Vertebrate kidneys consist of glomeruli,
tubules surrounded by peritubular capillaries, &
longitudinal ducts. Variations in kidney structure among
vertebrates are primarily in the number & arrangement of
the glomeruli & tubules.
Structure of nephron: In pronephros: 1st
embryonic tubules in all vertebrates; called pronephric
tubules because they are the 1st to develop & are anteriorly
located. Number - never very many (e.g., 3 in frogs, 7 in
human embryos, & 12 in chicken embryos). The duct that
drains the pronephros is called the pronephric duct. The



pronephros is temporary & functional only until mesonephric glomeruli & tubules further back
become functional. Glomeruli of some pronephros, lack Bowman’s capsule and peritoneal funnel and
hence known as external glomeruli. In certain examples the glomeruli unite to form a complex
glomus into the large cavities known as pronephric chambers. The uriniferous tubules open into a
common pronephric duct which runs posterior to join cloaca.
Structure of nephron: In mesonephros: In mesonephric kidney nephrons are formed by
corpuscles & tubules that develop caudal to pronephric region; form connections with existing
pronephric duct (which is now called the mesonephric duct). Paired segmental uriniferous tubules,
each with peritoneal funnel open into coelomic cavity. The glomerulus in mesonephros is enclosed in
bowman’s capsules (internal glomerulus). The mesonephric tubules later undergo secondary division
and form numerous tubules and thus the segmental arrangement is lost. The secondary mesonephric
tubules are lack of peritoneal funnel. In some fishes the kidney lacks the malpighian bodies and thus
called as aglomerular kidney. It is the functional adult kidney in fish & amphibians (& sometimes
called the opisthonephros) and embryonic kidney in reptiles, birds, & mammals.
Structure of nephron: In Metanephros:
It is an adult amniote kidney. It is formed from the
posterior most nephrotomes behind the embryonic
mesonephros. A tubular outgrowth arises near the cloaca
which gradually grows into a new metanephric duct
finally working as ureter. The distal part swells and
develops nephrotome of metanephros giving out
thousands of uriniferous tubules. The intermediate
mesoderm undergoes a series of branching to form the
collecting duct system of the kidney. It also forms the
major and minor calyces and the renal pelvis.
Mammalian kidneys are divided into the cortex (5),
medulla (6), & pelvis (4): Cortex - contains renal
corpuscles & lots of capillaries. Medulla - contains collecting ducts and loops of Henle; divided into
pyramids (7) & columns (2). Pelvis - hollow; receives the urine (which exits the kidney via the ureter
(3).



The metanephric tubules become long and much coiled structures. Each kidney tubule of mammals
is composed of the following parts:
 Proximal convoluted tubule
 Loop of Henle with ascending and
descending portions
 Distal convoluted tubule.
These have bowman’s capsules enclosing glomeruli.
Metanephric uriniferous tubules are lack of peritoneal
funnels and thus have no connection with coelomic
cavity. The number of corpuscles is large; up to about
4.5 million is some species. It is drained by a duct
called the metanephric duct or ureter. Tubules of
mammalian kidney have U-shaped Loops of Henle
(avian kidney has very short loops & reptilian kidney
has no loops). Metanephros indicates an isolation of
excretory organ from reproductive system and hence
exhibits the advancement in the evolution of amniotes.

Structure of Male Urogenital Ducts:
Some fishes (e.g., gar & sturgeon) & amphibians - mesonephric duct transmits sperm &
urine.
Sharks - mesonephric duct is used primarily for sperm transport; accessory urinary duct
develops.
Some amphibians - mesonephric duct transports only sperm; new accessory urinary duct
drains the kidney.
Teleosts - mesonephric duct drains kidney; separate sperm duct develops.
Amniotes - embryonic mesonephric ducts transport sperm in adults.



Intromittent organs:
Useful when fertilization is internal; introduce sperm into female reproductive tract.
Found in some fish, some birds, reptiles, & mammals.
Cartilaginous fish - appendages of pelvic fins called claspers direct sperm into female
reproductive tract.

Structure of Female Urogenital Ducts:
Typically consists of a pair of oviducts that extend from ostia to the cloaca.
Different segments of oviducts perform special functions including internal fertilization.
Cartilaginous fish - 2 ostia fuse to form single ostium (or osteum); shell gland secretes
albumen & a shell; uterus holds eggs until laying.
Teleosts - ducts are continuous with cavity of the ovary.
Lungfish & amphibians - oviducts long & convoluted; lining secretes jelly-like material
around each egg.
Crocodilians, some lizards, & nearly all birds- 1 coiled oviduct lined with glands that add
albumen, shells, &, sometimes, pigment.



Monotremes - tract is reptilian; caudal end secretes a shell before egg passes into the cloaca.
Placental mammals - embryonic ducts give rise to oviducts, uteri, & vaginas. Adult tract is
paired anteriorly & unpaired posteriorly (typically terminating as an unpaired vagina).


Oviducts (fallopian tubes) are relatively short, small in diameter, convoluted, & lined with
cilia; begin at ostium bordered with fimbria or cilia.



Uterus and Vagina:


Marsupials - no fusion of embryonic ducts so there are 2 tracts (duplex uterus)



Other placental mammals - varying degrees of fusion:


Bipartite uterus - 2 uterine horns, a uterine body (with 2 lumens), & a single vagina



Bicornuate uterus - 2 uterine horns, a uterine body (with a single lumen), & a single
vagina



Simplex uterus - no uterine horns & oviducts open directly into body of uterus



Vagina - fused terminal portion of oviducts that opens either into urogenital sinus or
to the exterior; receives male intromittent organ.



Structure of Urinary bladders & Cloaca:
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.
Tetrapod urinary bladder: Amphibians through Mammals - bladders arise as
evaginations of ventral wall of the cloaca. The ureters attach to the urinary bladder, which is a
derivative of the allantois in mammals, not an outgrowth from the cloacal wall. There is no
cloaca in therian mammals. The urinary bladder drains into the urethra to 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).

Cloaca: In vertebrates, common chamber and outlet into which the intestinal, urinary, and genital
tracts open. It is present in amphibians, reptiles, birds, some fishes (e.g., sharks), and monotreme
mammals but is absent in placental mammals and most bony fishes. Certain animals (e.g., many
reptiles and some birds, including ducks) have an accessory organ (penis) within the cloaca that is
used to direct the sperm into the female's cloaca. Most birds mate by joining their cloacas in a
“cloacal kiss”; muscular contractions transfer the sperm from the male to the female.
Receives digestive, reproductive, and urinary products and tracts
No cloaca
 Fish – may have three separate openings
 Lost in mammals above monotremes


Cloaca subdivisions:
 Coprodaeum : Receives alimentary canal
 Urodaeum : Receives urinary and reproductive products
 Proctodaeum : Associated with excretory

Figure: Subdivisions of cloaca shown in a lizard.

Figure: Subdivisions of cloaca shown in a Bird.
Prepared by
Mr. S. D. Rathod
Associate Professor
Department of Zoology
B. N. Bandodkar College of Science, Thane

Urogenital system in vertebrates: TYBSc course Semester-VI – USZ0601of University of Mumbai

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