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

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

Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605

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



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 cranial-

to-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

Education

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