Vertebrate Structure and Development
Typically when we think of respiratory structures we think of
the lungs, gills or whatever. The real work horse of the system
involves the microstructures - the capillaries. The only way
oxygen or carbon dioxide can get from air <-> cells is by
diffusion. This occurs in vertebrates via skin, lungs or gills.
Diffusion of gas can only occur across a moist membrane.
Therefore, the surface of the respiratory epithelium must be kept
moist. Amphibians keep the skin moist with skin glands, gills are
kept moist by being in water, and lungs are kept moist by being
To effectively get enough gas exchange requires a large thin
barrier between the air or water and the blood. It also requires
the blood to be in contact with air or water higher in oxygen and
lower in carbon dioxide.
All amphibians, some fishes and hibernating turtles get oxygen
via the skin. In the salamander family Plethodontidae it is the
only means of gas exchange.
The fishes and larval amphibians typically have gills for gas
exchange. The visceral arches and associated structures are
If we look at the pharyngeal region of a vertebrate embryo we
find that the wall of the pharynx develops pouches that go out.
The lining of these pouches is endodermal tissue. Between the
pouches we have a piece of tissue that is the visceral arch. It is
made of lateral plate mesoderm. The outer covering of the pharynx
is made from ectoderm. The ectoderm bulges in a bit to meet the
pouches. This forms a groove. The endoderm from the pouch induces
the ectoderm to form the groove. Vertebrates have 7 or fewer
pouches on each side. As we go up the taxonomic ladder the number
As development continues all or some of the connections
(closing plates) between the pouches and grooves disappear. The
slit that is left is the gill slit.
Inside the visceral arch we find one of the aortic arches.
This short vessel connects the ventral aorta to the dorsal aorta.
There are also nerves in the arches too.
The 1st visceral arch gives rise to the jaw of all verts
except Agnathans. The 1st closing plate is lost during development
in gilled fish and becomes the tympanum of tetrapods. The 1st
pharyngeal pouch becomes a gill chamber (Agnatha), a spiracle
(other fish) or the middle ear cavity (tetrapods).
The 2nd visceral arch becomes part of the jaw support or moves
into the middle ear (columella).
The other visceral arches support gills in fish and in
tetrapods they help support the tongue, and becomes parts of the
trachea and larynx.
In fishes the gills are derived from the visceral arches.
The mesodermal arch is surrounded by ectoderm on the outer surface
and endoderm on the other surfaces. The arch starts to develop
bone or cartilage skeletal structures and muscles. (Fig. 13-10)
The gill filaments sit close together and so water has to flow
over the filaments to get to the external gill chamber.
Water flows in with high oxygen content. It comes in contact
with blood that has lower oxygen content. By diffusion oxygen goes
from higher -> lower concentration.
Countercurrent exchange - works well if the water is well
aerated. Water holds less oxygen than air, but fish Hb binds to
oxygen better than tetrapod Hb.
Types of internal gills:
1) Pouched gills - Agnatha. Gills have pouches and water
flows through and exits via an external pore. Water goes in the
mouth -> gill pouch -> external pore or external pore <-> gill
pouch. The latter occurs when the mouth is involved in feeding.
Muscular contraction expels water.
2) Septal gills - Chondrichthyes. The gill chambers contact
the outside of the body through gill slits. Water enters either
the mouth or spiracle -> gills -> gill slits. After expelling
water the pharynx expands and the branchial muscles expand the gill
chambers. This draws in water (suction pump). The oral cavity and
pharynx are contracted and water flows over the gills (pressure
pump). When the pressure in the external gill chamber > water
pressure the gill slits open and water is expelled.
3) Opercular gills - Osteichthyes. A bony operculum covers
the gill filaments. It helps in pumping water. It is similar to
the mechanism found in Chondrichthyes. The operculum has an
internal flap that closes the external gill chamber. Ram
ventilation is found in some fish that continually swim with the
mouth open to force water through the gill apparatus. This is
associated with fish such as tuna that have a high oxygen demand.
Gills work well in cool well-aerated water, but in oxygen
poor, warm water other means are needed. This lead to the
evolution of the lung. Another structure is the gas bladder, but
it is not respiratory. It functions in buoyancy.
In the embryonic fish both lungs and gas bladders are
connected to the gut by a pneumatic duct. This is retained if it
becomes a lung and the fish is physostomous. It is lost if the
organ becomes a gas bladder and the fish is physoclistous.
Typically these are located dorsally to help keep the fish upright.
Only 1 lung or gas bladder is present.
Gas gets to the gas bladder via the bloodstream. The gas
gland puts air into the bladder. Air enters it by diffusion from
the rete mirabile - a capillary network. Gas exits the gas bladder
through the oval body.
The gas bladder is used for 1) changing density to allow the
fish to stay at a certain depth with little effort; 2) producing
sound for aggression, warning or courtship; 3) sound and pressure
reception. The bladder can't hear, but it picks up vibrations and
transmits them to the inner ear via little ossicles called the
Tetrapod lungs - These are normally paired and ventral. The
trachea connects the lungs to the pharynx. The trachea and bronchi
are ringed with cartilage to prevent collapse. The opening to the
trachea is the glottis.
Tetrapods have a larynx with 2 sets of cartilages - the
arytenoids and the cricoids.
Amphibians: Anurans: the lungs are short and fairly simple
without a lot of infolding. The trachea divides into 2 short
bronchi that go to the top of each lung. Anurans have vocal cords.
Apoda: only have a right lung as adults.
Caudata: have reduced or no lungs.
Amphibians use a positive pressure mechanism to force air into
the lungs (frog manual).
Reptiles: Snakes and amphisbaenians only have 1 full sized
lung. The other is reduced. The lungs of reptiles are more folded
to increase surface area. This is particularly true in larger
reptiles. Only a few lizards (geckos) have vocal cords. Lizards
and snakes may hiss by expelling air forcefully. Many rattle their
tails rapidly to produce a buzz or rattle. Crocodilians, a few
turtles, and the tuatara make vocalizations.
Reptiles have a negative pressure mechanism. In squamates it
relies on muscle movement. Muscles pull the ribs out enlarging the
chest cavity and the negative pressure in the lungs relative to
atmospheric pressure sucks air in. They use muscles to contract
the cavity. Crocodilians have a diaphragm that shoves the liver
backwards causing negative pressure in the lungs. Muscle
relaxation causes exhalation. In turtles leg muscles cause the
chest cavity to expand.
Mammals: These have very heavily infolded lungs creating a
great deal of surface area. The trachea -> primary bronchi ->
secondary bronchi -> tertiary bronchi -> bronchioles (some of which
are respiratory) -> alveoli.
The larynx has a thyroid cartilage and there is cartilage in
the epiglottis. The lungs are inflated by negative pressure caused
by the diaphragm contracting and moving down. Exhalation is
Birds: These vertebrates have the greatest lung surface area
to volume ratio in the subphylum. Trachea -> primary bronchi ->
laterobronchi -> posterior sacs * -> laterobronchi -> dorsobronchi
-> parabronchi where gas exchange occurs ** -> ventrobronchi ->
anterior air sacs *** -> ventrobronchi -> primary bronchi -> out
* - 1st inspiration; ** - 1st expiration
*** - 2nd inspiration; **** - 2nd expiration
This requires 2 cycles, but eliminates the problem of mixing
fresh and "used" air (handout).
The sternum is pulled downward to create negative pressure for
inspiration and swings back up for expiration.
As with the respiratory system, the real workhorse here is the
microscopic capillary bed. The vessels act merely to transport
blood, but the functions of the cardiovascular system are carried
out by capillaries.
The circulatory system has a number of functions:
1) transport oxygen and carbon dioxide; 2) transport nutrients; 3)
transport waste products; 4) transport hormones; 5) transport
antibodies; 6) maintain a constant internal environment - pH, ions,
blood gases, osmotic pressure; 7) remove toxins and pathogens; 8)
Vertebrate cardiovascular systems are closed in that the blood
is found only in the heart and blood vessels. This is a little
misleading because many components of blood freely leave through
the capillaries. White blood cells, ions, water, gases, nutrients,
hormones, antibodies etc can all leave the system via the
capillaries. If they couldn't life would cease.
When salts and water leave the blood vessels they go and bathe
the tissues and are called interstitial fluid. If too much
accumulates it results in edema - swelling. Some of this fluid
reenters the blood stream, but much of it does not. We have a 2nd
part of the circulatory system that handles the return. This is
the lymph system.
The heart is the main pump in the cardiovascular system.
Vessels leading from the heart are arteries and vessels leading to
it are veins. At the level of the organ where gas, nutrient etc
exchange occurs there is a capillary bed. Between trips to the
heart blood normally only passes through one capillary bed. There
are several exceptions known as portal systems where blood goes
through 2 capillary beds. This occurs in the digestive tract,
excretory tract and nervous system between the hypothalamus and
The heart propels blood through the arterial vessels, but
pressure drops along the way. When blood enters the venous
circulation there is very little pressure to drive it through.
Movements of muscles propel venous blood back to the heart. Valves
prevent back flow in veins and lymph of tetrapods. Some lower
vertebrates have lymph hearts to aid in lymph flow.
Because of the need to circulate wastes and nutrients
throughout the body the circulatory system develops very early -
within hours in some vertebrates. The heart actually starts out as
proteins that develop shortly after the mesoderm has passed through
the primitive streak. Cells with these proteins migrate to the
area where the heart will form. The cells cluster themselves into
regions that will become the different chambers of the heart. (Fig. 16-11)
The heart starts as an inner endocardium and an outer
epimyocardium which divides into the epicardium and myocardium. As
time continues these associate themselves into 4 chambers: sinus
venosus, atrium, ventricle, and truncus arteriosus. This tube over
time curls back on itself.
The protochordate amphioxus has no heart. All vertebrates
have a ventral heart.
Fishes: Blood enters from the common cardinal veins ->
sinuatrial valve atrioventricular valve
sinus venosus -------------------> atrium ---------------------->
ventricle --> truncus arteriosus (bulbus arteriosus in teleosts)
---->ventral aorta --> gills --> dorsal aorta --> organs.
Heart size and activity are correlate.
Amphibians and Reptiles: With 3 chambered hearts they
anatomically seem to mix oxygenated and deoxygenated blood.
Physiologically this is not really true.
Systemic blood Pulmonary blood
From organs R. atrium L. atrium
Conus arteriosus with spiral fold - divides into 2
The spiral fold allows separation of oxygenated and
deoxygenated blood. The deoxygenated blood goes to the lungs and
the oxygenated blood goes to the organs.
In lungless salamanders (Plethodontidae) the left atrium is
Systemic blood Pulmonary blood
R. atrium L. atrium
via cavum pulmonale via cavum venosum and cavum
Pulmonary trunk R and L systemic arch
goes anteriorly goes posteriorly
In air blood from the right side goes mainly to the lungs.
When the reptile is underwater and can't breathe most of the blood
goes to the systemic arteries because there is no functional reason
to take it to the lungs.
Reptiles have a lower metabolic demand (oxygen demand) than
birds and mammals. As a result they can go underwater for
extensive periods without any specializations because of their 3
chambered heart. It is not that they are primitive or less adapted
than birds and mammals rather they are adapted for a different
Birds and Mammals: These animals have a 4 chambered heart
used for keeping oxygenated and deoxygenated blood separate. The
left side contains deoxygenated blood and the right side has
oxygenated blood. The truncus arteriosus becomes split into the
aorta and pulmonary artery. The sinus venosus is incorporated into
the wall of the right atrium.
The hearts of mammals have trabeculae carneae which act to
strengthen the walls. Also we see chordae tendineae which attach
to the valves between the atria and ventricles. The other end of
the chordae tendineae attach to papillary muscles in the ventricle
Blood returns through the posterior and anterior vena cavae
and goes directly to the right atrium of mammals or to a small
sinus venosus -> right atrium.
Right atrium Left atrium
Right ventricle Left ventricle
Pulmonary trunk Aorta
Pulmonary veins Vena cava
The heart's function is to beat and move blood along. In
embryos and lower vertebrates the sinoatrial (SA) node of the sinus
venosus or right atrium initiates heart beat. The message spreads
out from the node and causes each chamber to beat as a unit. In
birds and mammals another node, the atrioventricular (AV) node
sends the message to the ventricles via the Purkinje fibers. The
atria contract first and then the ventricles.
In an embryo the ventricle beats first, next the atrium begins
to beat, but it beats faster than the ventricle and sets the
rhythm. Later the sinus venosus starts to beat. Its beat is still
faster and it becomes the pacemaker. The sinus venosus becomes
part of the right atrium. The atrium and ventricle split to form
double chambers (2 atria, and 2 ventricles). The truncus splits to
form the aorta and pulmonary trunk.
The heart can beat on its own without any stimulation from the
nervous system. The outcome is often an irregular beat.
Therefore, vertebrate hearts receive input from the nervous system
to keep the heart beating regularly and change its pace as needed.
The nervous system becomes attached to the heart in the embryo.
Once this happens they keep the pace. In tetrapods innervation
comes from 2 nerves. One acts to speed up the heart rate and the
other slows it down.
Blood cells are the most obvious component of blood
Erythrocytes (RBCs) contain hemoglobin (Hb) - a red pigment that
carts oxygen from the skin, lungs or gills to the organs and
returns carbon dioxide to the respiratory organs. RBCs are tiny
and in all classes, but mammalia, they contain a nucleus. They
stay in the blood vessels.
Leukocytes are found in blood and lymph vessels. They can
wiggle through capillaries to get to infections. They use
phagocytosis to engulf foreign stuff. There are 2 classes of WBCs
- the granulocytes and lymphoid leukocytes. They vary in function.
We have 3 classes of granulocytes and 2 classes of lymphoid
leukocytes. These cells vary in number from one class to another.
Thrombocytes are blood cells that fragment to form platelets.
These are involved in initiating clotting.
Hemopoiesis is the process of blood cell formation.
Plasma is the non-cellular portion of blood. It contains
proteins, ions, water, nutrients, wastes and hormones.
Physiologically this is very important because everything the blood
transports except blood gases are in the plasma.
In an early embryo pieces of mesoderm called blood islands
produce the 1st blood. Later, a number of organs produce blood.
In adult vertebrates the spleen produces blood cells. Red bone
marrow produces blood cells in tetrapods.
Blood vessels occur as arteries, veins or capillaries.
Capillaries are very tiny vessels that only allow 1 or so RBCs
through at a time. They are surrounded by endothelial cells in
connective tissue. This makes a very thin layer.
Arteries and veins have thicker walls. The endothelial layer
becomes the inner part of the wall and is the tunica interna
(intima). Around this is the tunica media which contains smooth
muscle. External to this is the tunica externa (adventitia) which
is made of connective tissue. The arteries are thicker walled and
contain blood under higher pressure than veins.
In a very early embryo we start to see blood vessels. The
first ones are derived from the yolk sac and will serve to take
nutrients to the embryo. Other vessels are associated with areas
that have high activity. These early vessels start as channels
that are excavated out of the tissue. The pattern of vessels that
forms is a combination of 2 things: 1) the pattern inherited
phylogenetically and 2) the need to start making adult vessels.
In a typical embryo blood leaves the heart through the ventral
aorta and runs anteriorly. Blood goes into the aortic arches which
run through the visceral arches. From the aortic arches blood runs
to the head via the internal carotid artery and to the posterior
body through the dorsal aorta. Blood returns to the heart via the
anterior and posterior cardinal veins which join to become the
common cardinal vein.
As development continues the liver forms right in the path of
the posterior cardinal vein. This vessel is draining blood from
the gut and yolk sac, so it is filled with nutrients. As the liver
continues to grow the vessels get cut off and end up draining into
capillary beds in the liver - the formation of the hepatic portal
The same arrangement occurs with the kidney and causes the
formation of the renal portal system.
Fate of the aortic arches: Embryos have 6 pairs. The 1st
aortic arch of all jawed vertebrates is lost. In fishes the 2nd
arch may be retained to serve the gill area or lost (in some
Osteichthyes). In tetrapods it is gone.
The 3rd - 6th aortic arches are retained in fishes. These
become the afferent and efferent branchial arteries. In tetrapods
the 3rd aortic arch is retained. The embryonic ventral aortae
become the common carotid arteries. The 3rd arch (carotid arch)
and part of the anterior aortae become the external and internal
carotid arteries. In birds there is no common carotid artery.
The 4th aortic arch becomes the right and left systemic
arches. Typically these join to become the dorsal aorta in
amphibians and reptiles. In birds only the right one remains and
in mammals only the left one. The 5th aortic arch is lost except
in adult gilled salamanders. The 6th aortic arches become the
Posterior to the heart there is a single median dorsal aorta.
The branch to the tail is the caudal artery. The celiac artery
branches off to the digestive tract. The subclavian artery
branches into the brachial artery which goes to the arms. The
iliac artery goes to the femoral and sciatic arteries of the legs.
The anterior cardinal veins drain the brain in fishes. We
call them the internal jugular veins in tetrapods. The external
head is drained by the inferior jugular veins in fish and external
jugular veins in tetrapods. The jugulars join to form the anterior
vena cava = precava.
The posterior cardinal vein of fish or embryos drains the
posterior part of the body. The anterior cardinal vein and the
posterior cardinal vein join to become the common cardinal vein
which drains into the sinus venosus.
The posterior vena cava (postcava) drains the posterior body.
The iliac veins drain blood from the legs and the subclavian veins
drain blood from the arms.
Portal systems: The hypothalamic portal system has its 1st
capillary bed in the hypothalamus. Here hormones that act on the
pituitary gland are released into the blood stream. They travel a
short distance to the pituitary gland where they are released.
This system enhances chemical communication.
The renal portal system is found from Chondrichthyes -> Aves.
The blood from the posterior body drains to capillaries in the
The hepatic portal system is found in all vertebrates. It
takes blood from capillaries in the intestine to capillaries in the
liver. In some fish it also receives blood from the tail area.
The heart needs to get freshly oxygenated blood to do its job.
In fish a coronary vessel comes off the efferent branchial artery.
In air breathers the coronary vessel comes off the systemic branch.
In vertebrates there is more blood vessel space than blood to
fill it. The major vessels are always full, but the arterioles
have little valves that shut the blood flow to the capillaries off
for short periods of time.
The lymph system is a low pressure system to return fluid to
the cardiovascular system. It contains a clear or milky fluid, not
blood. Many fishes have lymph propulsors to help return the fluid
to the cardiovascular system. Amphibians and reptiles have lymph
hearts that act to pump the lymph. Embryonic birds have lymph
hearts, but almost no adult species do. Mammals lack lymph hearts,
but along with birds have lymph nodes. These filter lymph and
remove foreign debris. They can also produce antibodies.
We see many examples of countercurrent heat exchangers in arms
and legs of vertebrates - especially endotherms.
The mammalian fetus does not breathe for itself, so there is
no need to send half of the blood from each heart beat to the
lungs. Also the embryo's needs are furnished by the mother, so
temporary links with her must be established.
Blood from the dorsal aorta passes into the umbilical
arteries. These go out through the umbilical cord to the placenta.
Gas exchange, nutrient exchange and so forth occur here. Blood
then returns to the embryo via the umbilical vein. It passes
through the falciform ligament and into the liver. The liver can
then process the nutrients. The blood continues through the
posterior vena cava and into the right atrium.
In an adult mammal the blood entering the right atrium will
pass on to the lungs. In amniote embryos we see a shunt that keeps
most of the blood from doing this. The walls between the 2 atria
have an opening (foramen ovale). Blood entering the right atrium
then can pass to the left atrium -> left ventricle and on to the
A great deal of the blood still goes from the right atrium to
the right ventricle. It then passes into the pulmonary trunk.
Where the pulmonary trunk passes over the aortic arch there is an
opening connecting the 2 vessels. This is the ductus arteriosus.
It shunts blood from the pulmonary route to the systemic route.
Some blood does reach the lungs and it provides nutrients and
oxygen to the developing tissue.
At birth a great many changes need to occur if the newborn is
to be healthy.
1) Nerve impulses close the ductus arteriosus. The closed
area becomes the ligamentum arteriosum.
2) The foramen ovale needs to close. The flap of tissue
between the 2 atria becomes sealed - either anatomically or
physiologically. The thin area between the atria becomes the fossa
3) The umbilical vessels are obliterated. The remainder of
the umbilical arteries become the lateral umbilical ligaments. The
remnants of the umbilical veins become the round ligament of the
liver and the ligamentum venosum.
In birds there is no placenta, but the ductus arteriosus and
foramen ovale do form. They serve the same function in the bird
heart as they do in the mammalian heart. The ductus arteriosus
closes about a day before the bird hatches which the bird breathes
air from the air space in the egg.
Blood flow: Trace the pathway of a glucose molecule from the
mouth to the liver and on to the brain of a turtle.
mouth -> pharynx -> esophagus ------------------>
stomach -------------------> small intestines ->
capillary bed in the intestinal wall -> mesenteric vein -->
hepatic portal vein -> liver ->
hepatic vein -> posterior vena cava ->
R AV valve
sinus venosus -> right atrium ----------->
ventricle (cavum pulmonale)------------------>
pulmonary trunk -> lungs -> pulmonary vein ->
L AV valve
L atrium -----------> ventricle (cavum arteriosum
-> cavum venosum)-------------> right systemic
artery -> common carotid artery -> internal
carotid artery -> capillary bed of brain
Blood flow of a fish from the tail to the brain.
Caudal Vein -> Renal portal vein ->
capillary bed in kidney -> postcardinal vein ->
common cardinal vein -> sinus venosus -> atrium ->
AV valve -> ventricle -> truncus arteriosus ->
ventral aorta -> afferent branchial artery ->
capillary bed in gills ->
efferent branchial artery -> dorsal aorta ->
internal carotid artery
Blood flow of a red blood cell from the skin of the cheek to the
top of your foot.
Facial vein -> external jugular vein ->
brachiocephalic vein -> precava -> right atrium ->
right AV valve -> right ventricle -> pulmonary
valve -> pulmonary trunk -> pulmonary artery ->
capillary bed in lungs -> pulmonary vein -> Left
atrium -> left AV valve -> left ventricle ->
aortic valve -> aorta -> common iliac artery ->
external iliac artery -> femoral artery ->
popliteal artery -> anterior tibial artery ->
dorsal pedis artery
Although we are going to cover the excretory and reproductive
tracts separately, they are often lumped together and called the
urogenital system. There actually is good reason to do this
because embryonically the 2 systems are closely linked.
In all tetrapods the kidney removes nitrogenous waste. They
are partially or totally responsible for water and salt balance.
The kidneys may be aided in water and salt balance by gills, lungs,
skin or salt glands.
Vertebrates are found in fresh and salt water as well as on
land. These present very different challenges for osmoregulation.
Therefore, it is no wonder that we see so much diversity among
In a saltwater existence the problem for living things is
dehydration and taking on too much salt. In these animals
conserving water and excreting salt is the role of the
In freshwater the animal has a problem of getting too much
water and losing salts. The osmoregulatory organs in this case get
rid of excess water and conserve salts.
A terrestrial animal lives in a very dry environment. In this
case conserving water and balancing salt intake and output is the
role of the osmoregulatory organs.
In general the kidney has glomeruli which are a rete mirabile
of capillaries. Coming off the glomeruli is a renal tubule. The
tubules are drained by excretory ducts. In adult vertebrates the
glomeruli are surrounded by the bowman's capsule.
The complexity and length of the renal tubules vary among the
different vertebrate groups. This is a good example of where the
form (anatomy) and function are related. In cases where the
kidneys are the only osmoregulatory organ they can be quite
Since conservation of salts and water is a main function of
the kidneys it makes sense that the kidneys must be well supplied
with blood vessels which take the salt or water back to the rest of
the body. The afferent glomerular arteriole takes blood to the
glomerulus. Fluid, salts, and other small molecules are then
filtered through the glomeruli and into the renal tubules. Blood
cells and other large structures aren't filtered and are returned
to the general circulation via the efferent glomerular arterioles.
The efferent glomerular arteriole then winds around the renal
tubule and is called the peritubular capillaries.
The kidney originates from intermediate mesoderm. It starts
as an outpocket of the coelom and initially retains this connection
to the coelom. We see a segmentation of the mesoderm into the
forerunners of the nephron, nephrotomes. Over time, little balls
of capillaries grow into the developing nephrotomes and become the
Because an animal needs functional kidneys during development
we see a change in functional structures during development. Our
best guess at what the original vertebrate kidney was is that it is
similar to the embryonic kidney of hagfish and caecilians. This is
called a holonephros. Early we see an anterior kidney with little
functional units called nephrotomes. This early kidney is called
a pronephros. All vertebrate embryos have it. A pronephric duct
takes the urine to the cloaca.
As development continues the posterior portion of the kidney
develops. If this whole structure becomes functional it is an
opisthonephros. We see this in adult fish and amphibians. The
opisthonephric duct takes the urine to the cloaca.
In some cases only the middle part of the kidney becomes
functional. It is called a mesonephros. Embryonic reptiles, birds
and mammals have this. The mesonephric duct becomes the sperm
duct in all male amniotes. In females it becomes incorporated into
the mesentery of the ovary and oviduct and is nonfunctional.
The posterior part of the kidney becomes functional in adults
of reptiles, birds and mammals and is the metanephros. It starts in
the posterior part of the embryonic kidney and moves laterally at
the same time it is developing. The metanephric duct is called the
If we look at the whole kidney the blood is brought to the
kidney via the renal artery. It goes through a series of arteries
the last of which in mammals is the interlobular artery. This
leads into the afferent arteriole. It is the blood supply for the
glomerulus - a capillary bed for filtering blood. Fluid then
enters the renal capsule and is processed. Blood exits the
glomerulus through the efferent arteriole and then enters the
When fluid is filtered from the glomerulus it enters the renal
capsule and then the proximal tubule. This is where sugars, amino
acids, vitamins, and electrolytes diffuse back into the blood
stream and leave the forming urine.
In some vertebrates the next segment is the intermediate
segment - this area is very specialized in mammals and is not the
same as the intermediate segment. It becomes known as the Loop of
Finally fluid enters the distal tubule. The urine gets
acidified and sodium and chloride are reabsorbed into the blood.
The tubules are under hormonal control. Antidiuretic hormone
causes water reabsorption and aldosterone causes Na+ reabsorption.
Vertebrate needs for salt and water balance vary with their
environment. Freshwater fish and amphibians have very salty body
fluids relative to the water. They tend to gain water through
osmosis at the gills or skin. These animals produce a lot of
dilute urine. Gills are used to get rid of nitrogenous waste. The
majority of their waste is ammonia (ammonotelic).
Chondrichthyes convert nitrogenous waste to urea (ureotelic).
It isn't toxic like ammonia so it can be kept in the bloodstream.
This makes them a little hyperosmotic. The excess monovalent ions
are discharged from the rectal gland. They also have a high
concentration of trimethylamine oxide (TMAO) which makes them
hypertonic to the water.
Marine Osteichthyes tend to lose water to the sea. They form
very little urine and get rid of ions through the gills and
kidneys. They drink sea water.
Reptiles, birds and mammals live in a dry environment and need
to drink or eat all their water. This may be hard to come by so
their kidneys conserve water. Birds and reptiles process
nitrogenous waste into uric acid (uricotelic). This is not water
soluble so little water is needed. Salt is excreted in the urine
or by salt glands. They don't make a watery urine, most water is
reabsorbed and a white paste is formed.
Salt glands are found in birds and reptiles that live in
marine or arid areas. These are found in the head and secrete
either NaCl or KCl. Control of the salt glands is via the
parasympathetic nervous system or via hormones. Birds can make
hypertonic urine, but reptiles can't. Reptiles can tolerate low
water levels much better than mammals and can survive and conserve
limited water because of the salt gland.
Mammals excrete nitrogenous waste as urea. Salts are also
excreted in the urine. Lots of fluid goes into the kidney tubules
but only about 1% actually ends up in the urinary bladder. These
kidneys can make a urine that is hypertonic to blood plasma. The
mammal nephron has a proximal and distal tubule, but no
intermediate segment. Between the tubules is a loop of Henle. It
is important in concentrating urine. The glomeruli are in the
cortex and the loops of Henle are in the medulla. The peritubular
capillaries surrounding the Loop of Henle are called the vasa
As mentioned earlier there are accessory osmoregulatory
organs. Chloride cells in fish gills can either take up or get rid
of excess NaCl. The rectal gland of sharks also has chloride
cells. Salt glands are found in marine reptiles and desert
reptiles and in marine birds. They secrete a very concentrated
solution of NaCl or KCl. Hagfish secrete a very salty mucous that
may be involved in salt balance - we don't know.
Osteichthyes have small urinary bladders. Agnathans and
Chondrichthyans lack them. Reptiles that produce watery urine have
urinary bladders. Birds and those reptiles that secrete a paste
lack bladders. All mammals have urinary bladders. Amphibians have
a large urinary bladder. It is used as a water storage area and
water can be reabsorbed. This is controlled by ADH.
The bladder starts as an evagination of the ventral wall of the
cloaca of an embryo.
The function of the reproductive system is strictly
reproduction. It produces gametes, brings gametes together,
nourishes the embryo, and releases eggs or sperm or young.
The gonad has an outer cortex and an inner medulla. It starts
as a ridge of mesoderm - the genital ridges. The very outer layer
of the ridge will produce the eggs or the cells that will produce
the sperm. This region is the germinal epithelium. Inside the
germinal epithelium lies the primary sex cords and later the
secondary sex cords.
The outer region can be called the cortex and the inner region
the medulla. Early in development the primordial germ cells can be
found in both the cortex and the medulla.
In very early development testes and ovaries are identical.
The trigger which causes them to become male or female is a
combination of genetic temperature, and hormonal influences. If
the medulla is induced to develop the gonad becomes a testis. If
the cortex is induced to develop the gonad becomes an ovary.
Interestingly, even though most vertebrates have chromosomal
sex determination you can take a germ cell from a male, put it into
a female and it will form an egg. The opposite also occurs. It's
not the cell itself, but rather the environment it is in that
determines what kind of gamete it makes.
If you think about sex determination from that perspective it
is not difficult to see why there are some seemingly bazaar sex
changes occurring in vertebrates. Remember the key as to what the
gonad becomes is whether the medulla or cortex is the dominant
There are a number of fish associated with coral reefs that
are well known to show sex reversal. All of the young are born
female. They grow up and become part of the harem of a male who is
larger than they are. If the male is removed from the harem, the
largest female becomes male and takes over the harem. This occurs
within a few days.
Menidia show the exact opposite. All the young are born male
and when they reach a certain size, they switch and become female.
Turtles, crocodiles and some lizards have temperature
determined sex determination. If the embryo is maintained at one
temperature the medulla is induced and if it is maintained at
another temperature the cortex is induced to develop.
In toads the anterior end of the testis sits next to a
Bidder's organ. If the testes are removed the Bidder's organs will
develop into functional ovaries and other female reproductive parts
Chickens have a similar situation. Only the left gonad
becomes an ovary and the right gonad is vestigial. If the ovary is
removed the vestigial gonad is induced to develop into a testis and
the chicken becomes a rooster.
Vertebrates normally have 2 testes, but in a few species there
is just a single gonad (lampreys, some teleosts) because the 2
fused or in some species only 1 of the 2 develops (some reptiles).
The testis is filled with seminiferous tubules (reptiles, birds and
mammals), seminiferous ampullae - a bag filled with developing
sperm which ruptures when sperm are mature, or something in-
The sperm precursors are spermatogonia. They sit along the
walls of the seminiferous tubules or ampullae. As they mature
(spermatogenesis) and move inward they develop from spermatogonia
to primary spermatocytes to secondary spermatocytes and eventually
to mature sperm.
Also associated with the spermatocytes are Sertoli cells which
support development and are involved in hormonal control of
spermatogenesis. The interstitial cells produce hormones involved
The mammalian testes are distinct in having an outer cover the
tunica albuginea. Septa divide the testes into lobes.
Spermatogenesis won't occur at temperatures over 36 C. Bird testes
are probably kept cool by the air sacs. In mammals the active
testes sit in the scrotum which is outside of the body cavity. In
some seasonal reproducers the testes move into the body when the
animal isn't reproducing.
The inguinal canal is the connection between the internal body
and the scrotum. The spermatic cord is all the arteries, veins,
lymph vessels, nerves and connective tissue that run from the body
cavity to the testes. The pampiniform plexus is a countercurrent
heat exchanger that is found in the spermatic cord. Its function
is to keep the testes cool.
All male vertebrates except Agnathans release sperm from the
testes into a set of tubes. These may be only used for sperm
transport or for sperm and urine transport. The sperm leave the
seminiferous tubules (or seminiferous ampullae) of the testes and
enter the efferent ductules. The efferent ductules are the
modified mesonephric tubules. These may lead into the epididymis
which serves for temporary sperm storage. It is also part of the
modified mesonephric tubule. In species without an epididymis it
goes directly to the ductus deferens = vas deferens = deferent
duct. The mesonephric duct is often called the Wolffian duct.
In many male vertebrates there is a copulatory organ. This is
normally found when fertilization is internal. Fertilization is
external and requires no copulatory organs in Agnathans and many
Osteichthyes and Anurans. Some Osteichthyes and Chondrichthyes
have internal fertilization and have fins modified into male
copulatory organs. Caudata have internal fertilization, but no
special organs. Caecilians have a male copulatory organ.
Male reptiles and mammals have a copulatory organ. Only
primitive male birds have a copulatory organ.
In the ovary the oogonia are located along the outer border of
the cortex. These become the egg cells of adults. As the oogonia
mature they move toward the inner part of the ovary. Oogenesis is
the process of egg maturation.
Each oocyte is surrounded by follicle cells and an outer layer
- the theca. The egg matures until ovulation. As the oogonia
matures it is known as a primary follicle and then a secondary
follicle. The mature follicle ruptures from the follicle and
leaves the ovary. The remaining follicle in mammals becomes the
corpus luteum. We have also found a corpus luteum-like structure
in ovoviviparous and viviparous vertebrates that aren't mammals.
The number of eggs that mature at any time is related to the
number of offspring produced by that species.
Most vertebrates have 2 ovaries, but in some they may be fused
(some fish) or 1 may be lost - chondrichthyes, birds.
In females the eggs and urine pass through completely
different ducts. Eggs leave the ovary and enter the oviducts. The
enlarged 1st portion of the oviduct is the ovarian funnel. The
opening is the ostium. These tubules form from the embryonic
muellerian ducts. Eggs pass through the oviducts by peristalsis.
In some cases the posterior end is very muscular so it can push
eggs and it's glandular and produces nutrients or an egg covering.
Chondrichthyes have shell glands to provide a soft coat or
even a hard shell. In some species the embryo stays in the
female's reproductive tract.
Amphibians coat the eggs with a gelatinous material while they
are in the female's body. Caecilians may retain young in the
oviducts until they are ready to be born.
Reptiles, birds and egg laying mammals 1st put egg proteins
(albumin) around the egg and then as it nears the cloaca they add
the outer shell.
Placental mammals send the fertilized egg from the oviduct to
the uterus where it implants in the lining. This inner lining is
the endometrium. The muscular part is the myometrium. During
pregnancy the uterus is closed off by a muscular cervix.
Marsupials have 2 completely separate uteri and 2 vaginas. This is
called a duplex uterus. Rats and other rodents have a uterus with
2 horns. This is a bicornuate uterus. We have just a single
uterus. This is a simplex uterus.
Most vertebrates have a cloaca which receives material from
the intestine, urinary tract, and reproductive tract. In some
cases this is a simple bag-like structure. In reptiles, birds, and
monotremes it is partially subdivided. In placental mammals it is
completely subdivided so that each system has its own passageway to
the outside of the body.
Most vertebrates are dioecious. Some Osteichthyes and hagfish
are monoecious. A few fish and lizards are parthenogenic and
produce young without males.
Many vertebrates are oviparous - egg layers. All classes have
some oviparous members.
The ovoviviparous vertebrates produce eggs but the female
keeps them in her body.
Viviparous vertebrates are found in all groups except Agnatha
and Aves. The young are retained within the female's reproductive
tract and exchange materials with her.
Last updated on 19 Jan 1999
Provide comments to Lynnette Sievert at firstname.lastname@example.org
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