Vertebrate Structure and Development
Lecture Notes

Respiratory System

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 internal.

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 important here.

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 decreases.

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 Weberian apparatus.

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 passive.

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.

Circulatory System

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) shuttle heat.

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 pituitary.

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
     From skin

          Conus arteriosus with spiral fold - divides into 2 
               body                   lungs
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 small.

          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 lifestyle.

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 wall.

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

     Lungs                         Organs

     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 system.

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 pulmonary artery.

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 kidney.

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 body.

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 ovalis.
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.
                              cardiac sphincter
mouth -> pharynx -> esophagus ------------------>

         pyloric sphincter
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 -----------> 
                              pulmonary valve
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 -> 
                                   SA valve
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
Excretory System

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 vertebrate kidneys.

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 osmoregulatory organs.

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 complex.

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 glomeruli.

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 ureter.

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 interlobular vein.

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 Henle.

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 recta.

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.

Reproductive System

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 part.

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 will develop.

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- between.

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 in reproduction.

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
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