COMPARATIVE ANATOMY HOME
COMPARATIVE ANATOMY TABLE OF CONTENTS
  OBL HOME OBL REFERENCES
NEPHRONS

THE URINARY SYSTEM

     When proteins are digested, individual amino acids are released.  In preparing amino acids for further degradation, the amino group is removed and this amino group subsequently forms ammonia and ammonium ions.  These byproducts of amino acid metabolism are potentially dangerous to living organisms.  The effect of ammonium ions on pH not only alters the activity of enzymes, it interferes with ion transport across membranes (such as sodium transport across cell membranes or hydrogen ion transport across mitochondrial membranes). Although this is potentially lethal for many organisms (such as humans), a number of animals have evolved mechanisms which allow them to tolerate ammonia. Ammonia tolerance is known in prokaryotes and primitive eukaryotes.  One prokaryote, Kakabekia umbellate, actually requires ammonia (and is similar to bacterial fossils of the Precambrian, a time in which ammonia levels are thought to have been higher). Most higher plants can assimilate at least some ammonium ions. Fish are particularly sensitive to ammonia while some mammals, such as the bat Tadarida brasiliensis, are tolerant of ammonia (Prosser, 1973). 

      Most animals convert ammonia to the less toxic form of urea.  While most animals excrete this urea, some retain large amounts of it, such as cartilaginous fish and coelocanths.  Retention of urea allows the body fluids to have the same osmolarity as seawater while still maintaining low ion concentrations comparable to other vertebrates.  (Griffith, 1991).

      The metabolic pathways involved in producing urea evolved long before mammals.  The main enzymes used to make urine are transanimases, gluatamate dehydrogenase, specific amino acid deaminases, and amino acid oxidases.   Transanimases ubiquitous in animals. In the liver, glutamate dehydrognease produces ammonia. The enzyme carbamylphosphate synthetase-I in mitochondria converts ammonia to carbamylphosphate.  In the mitochondria, ornithine transcarbamylase converts carbamylphosphate to L-citrulline which argininosuccinate (ASA) synthetase converts to l-argininosuccinate.  ASA lyase then converts this to l-arginine which arginase converts to urea.  Flatworms possess this pathway of arginine-urea synthesis and it became required for gnathostome embryonic development as embryos became larger. The loss of the enzyme uricase in the human lineage makes us more susceptible to gout but may have been important in creating conditions which fostered greater brain development (Prosser, 1973).

     The metabolic pathway that humans use to produce the urea excreted by the urinary system is called the arginine-urea pathway since it produces arginine for protein synthesis in addition to producing urea.  It has other uses in the excretion of wastes since the production of ammonia can combat acidosis since ammonia can bind hydrogen ions and be excreted from the kidneys.  Microorganisms use this pathway for the production of arginine only.  Birds and insects lack this pathway and must include arginine in their diets.  Thus the same metabolic pathway can be both nutritional and excretory.  Some aspects of the arginine-urea metabolic pathway crucial for the human urinary system can be found in at least some members of all living groups, including bacteria, some reptiles, and insects.  Some gastropods make use of a separate pathway to produce ammonia involving purine metabolism.  In other organisms, from bacteria through humans, this second pathway serves only to synthesize purines as components of nucleotides. (Prosser, 1973).

SPONGE

SPONGE

     In sponges (sponge cells are depicted above), mesozoans, coelenterates, and primitive flatworms (Acoela), there are no excretory structures.  Any wastes which are produced diffuse out of the body.  Most flatworms have primitive excretory tubules called protonephridia around the gut and into the head region.  These structures primarily function in osmoregulation and have little or no function in waste removal (Fretter, Hickman, Beklemishev, vol. 2).  In protonephridia, fluid is moved by ciliary action.  This osmoregulatory function apparently has advantages, since these higher flatworms can live in environments (such as brackish water) where acoela are not found (Fretter).  Protonephridia exist in flatworms, nemertine worms, annelids and Amphioxus (although they are absent in hemichordates, echinoderms, and urochordates) (Barrington, p. 239).

     Nemertine worms are the most primitive animals with blood vessels and are also the most primitive animals with excretory tubules associated with blood vessels (Hickman).   In lophophorates, excretory tubules (metanephridia) are used to transport water, waste, and gametes (Hickman, p. 289).

     The urinary system of hemichordates is similar to that of vertebrates in a number of characteristics.  In hemichordates, blood arrives under pressure (from heart contractions and contraction of the lining of the dorsal blood vessel) to thin walled sinuses.  Here, podocyte cells send their pedicel processes to form slit membranes which filter the blood (similar to the situation in higher vertebrates).  The filtrate, or primary urine, may be processed through the reabsorbtion of ions and other materials by podocytes and blood cells before becoming the secondary urine which is excreted from the body (Benito, form Harrison 1997, p. 64).  Echinoderms and urochordate pterobranchs also possess podocytes (Stach, 2000)..

     Amphioxus possesses podocytes (also called solenocytes, cryptopodocytes, or podosolenocytes) which are similar to vertebrate podocytes (Ruppert, from Harrison, 1997, p. 458 ).  The excretory structures exist in a segmental series (Romer, p. 403) and the nephridia are intermediate between those of invertebrates and vertebrate renal tubule (Ruppert, from Harrison, 1997).  The evidence suggests that the excretory system of nephridia found in invertebrates, or even that of lancelets, has little if any relationship to vertebrate excretory mechanisms.

 

NEPRHONS

       The functional unit of the vertebrate kidney is the nephron, which is composed of a capillaries surrounded by a glomerular (or Bowman’s) capsule (together called the renal corpuscle) and a tubule which can typically be divided into separate regions.  In the glomerular capsule, podocytes like those found in invertebrates help to filter blood.  In hagfish nephrons, the renal corpuscle includes afferent and efferent aterioles where the blood is filtered, as in the nephrons of higher vertebrates.  The glomerular capsule leads into the archinephric duct without the sections of the tubule observed in higher vertebrates.  The capsular space communicates with the pericardial cavity (Hoar, 1969, Vol. I).  There are no juxtaglomerular (JG) cells in hagfish or lamprey but these are present in bony fish and tetrapods (Hoar, 1969, Vol. I, p. 97). All craniate nephrons possess a glomerulus, neck, and proximal segment I.   Vertebrates added a collecting duct to these segments, gnathostomes added a proximal segment II and a distal segment; and bony fish and tetrapods added an intermediate segment (Hoar, 1969, Vol. I).    While some fish have external glomeruli, most possess internal glomeruli within a glomerular capsule (Weichert, 1970, p. 255).   In gnathostomes, there are fenestrations in the blood vessels of glomeruli (Hardisty 251) and urea is the most abundant nitrogenous waste (Romer, p. 398 ).   Amniotes reduced the size of the renal corpuscle to conserve water (Romer, p. 400).   In mammals and birds the tubule forms a loop of Henle which helps in both the reabsorbion of fluid and ions (Romer, p. 400). 

FISH RENAL CORPUSCLE

FISH RENAL CORPUSCLE

FISH RENAL CORPUSCLE

FISH RENAL TUBULE

FISH RENAL TUBULE

FISH RENAL TUBULE

FROG RENAL CORPUSCLE

FROG RENAL CORPUSCLE

 

FROG RENAL TUBULE

FROG RENAL TUBULE

TURTLE RENAL CORPUSCLE

TURTLE RENAL CORPUSCLE

TURTLE RENAL CORPUSCLE
TURTLE RENAL CORPUSCLE

TURTLE RENAL TUBULE

TURTLE RENAL TUBULE

TURTLE RENAL TUBULE

HUMAN MODEL

HUMAN MODEL

HUMAN MODEL

KIDNEYS

     Larval hagfish and the larvae of caecilians possess an archinephros, which is thought to represent the original form of the vertebrate kidney.  It is composed of two archinephric ducts which run along the coelom and receive wastes from archinephric tubules associated with each body segment.  Each archinephric tubule possesses an external glomerulus which produces the fluid to be excreted.  This type of kidney in embryos is referred to as the pronephros, although a few vertebrates retain part of it as the head kidney.  The rest of the embryonic kidney is called the opisthonephros, which can be divided into the mesonephros and metanephros (Weichert, 1970, p. 253).

     Adult hagfish possess both a pronephros and mesophephros which are separate because their joining region degenerates during development.  The pronephros is located in the walls of the pericardial cavity.  Only in hagfish and the teleosts Zoarces, Fierasfer, and Lepadogaster is the pronephros functional in adults as the head kidney.  (Weichert, 1970, p. 255)

     In lampreys, mesonephros and metanephros are not segmented after embryonic development (Romer, p. 405).and do not empty in archinephric ducts.  Some opistonephros use cilia to propel urine, such as those in frogs. Most species have lost connections between the coelom and urinary ducts (Weichert, 1970, p. 257).  In lampreys, the opistonephros is a long strip with no connections to the coelom and possesses an archinephric duct which runs along it.  A vestigial portion of the archinephric duct which was associated with the embryonic pronephros persists.   The two archinephric ducts fuse to form a urogenital sinus and the reproductive system empties here as well. (Weichert, 1970)

     In cartilaginous fish, the kidney is retroperitoneal.  One portion of the tubule is ciliated.   The opistonephros consists of long strips lateral to the cardinal veins and dorsal aorta (Weichert, 1970).   In bony fish, the kidneys may fuse across the midline.  Some teleosts lose their glomeruli and Bowman’s capsules. (Weichert, 1970)

     A small head kidney which retains connections to the coelom is present in some larval amphibians (Weichert, 1970, p. 259).  In Necturus, coelomic connections to the kidney tubules are present in adults.  In amniote embryos, segmentally arranged pronephric tubules with connections to the coleom form.  External glomeruli may be present.  Although higher vertebrates lack a pronephros and mesophros as adults, they develop these structures as embryos.

    In the fourth week of human development, intermediate mesoderm in the cervical region forms nephrotomes.  Nephric tubules develop which open into the intra-embryonic coelom.  Branches of the dorsal aorta enter these nephrotomes to form internal and external glomeruli.  The tubules and glomeruli form nephrons and longitudinal ducts form to connect the nephrotomes.  This urinary apparatus is never functional; it forms the vestigial pronephros (Sadler, p.261).

EMBRYO EMBRYO EMBRYO
     In humans each mesonephros may form 30-40 tubules and this number can be greater in other amniotes.  In reptiles, monotremes, and some marsupials, the mesonephros can persist after birth (Weichert, 1970, p. 263).  In most amniotes, the mesonephros is no longer functional after birth and the amniote kidney is composed of metanephros only (Romer, p. 406).

CHICK EMBRYO

CHICK EMBRYO

CHICK EMBRYO

FETAL CAT

FETAL CAT

PIG EMBRYO

PIG EMBRYO

PIG EMBRYO

Amniotes possess an increased number of kidney tubules.  Mammalian kidneys are organized into a renal pelvis, renal cortex and medulla.  In mammals, the kidney is usually bean shaped and there is no renal portal system (Romer, p. 412-4).  Kidneys in human infants are proportionally 3x the size that they represent in adults.

(Weichert, 1970, p. 269)

HAGFISH

HAGFISH

HAGFISH
HAGFISH

SHARK

SHARK

SHARK
SHARK

GAR

GAR

BOWFIN

BOWFIN

BOWFIN
BOWFIN

LUNGFISH

LUNGFISH

LUNGFISH