Excretory System

1. Excretory System Includes kidneys and associated ducts Excretory and Reproductive systems are closely associated morphologically, both in terms of mode of development and use of common ducts. This association is primarily due to a common derivation from mesomere and splanchnopleure. Paired kidneys are present in varied forms in all vertebrates, and serve as the major excretory organs. Paired excretory structures (nephridia, metanephridia, etc.) can be traced throughout all the coelomate phyla in Animalia.

2. Excretory System Functionally: Kidneys excrete nitrogenous wastes from protein breakdown and maintain osmotic and ionic balance within body fluids. Act by a filtration mechanism, with tubular reabsorption of the filtrate. Basic unit of the kidney = nephron. Millions of nephrons per kidney.

3. Excretory System SEQUENCE = Urine produced in kidneys, drains toward hilum ("dent" in kidney bean shape), entering renal pelvis where it leaves the kidneys. Upon exiting kidney, urine enters ureters which carry it to the urinary bladder (muscular-walled sac serving as reservoir for urine storage). Urinary bladder not present in birds as an adaptation to reduce weight for flight Fish urinary bladder has different formation than Tetrapods Urethra drains urinary bladder to exterior.

5. Mammalian Kidney As a gross structure, the kidney is surrounded by a dense CT capsule which gives off trabeculae extending toward the interior that produce a lobular organization. Divided into an outer cortex and an inner medulla. The medulla contains the loops of Henle (found only in mammals and some birds) and the collecting ducts. Medullary rays extend into cortex

6. Mammalian Kidney Basic structural unit = nephron (renal tubule or kidney tubule) Hilum = depression thru which urine exits and blood vessels enter (and exit) the kidney Renal Pelvis = expansion of upper part of ureter within the hilum, divided into large and small cups (major and minor calyces) Collecting Ducts = empty into calyces, these are structures into which nephrons drain (many nephrons empty into a single collecting duct)

8. Nephron Structure (Mammal) Renal Corpuscle = blind end of nephron; consists of a thin capsule of epithelial tissue (Bowman's Capsule) surrounding a ball of capillaries (glomerulus). Blood plasma filters from glomerular capillaries into Bowman's capsule, thereby entering the renal tubule. Filtration Barrier = capillary endothelium (fenestrated) + relatively thick glomerular basement membrane (functions in support of capillary network) + Bowman's Capsule epithelium (simple squamous cells = podocytes).

9. Nephron Structure (Mammal) Renal corpuscle leads into the remainder of the nephron Component parts: Proximal Convoluted Tubule Loop of Henle Distal Convoluted Tubule Distal tubules open into Collecting Ducts Parallel to nephrons lies a capillary network (vasa recta) which has freely permeable walls These capillaries passively participate in maintenance of the concentration gradient in the interstitium by removal of reabsorbed water and NaCl

11. Mechanism of Filtrate Concentration Countercurrent Multiplier System Each region has a special permeability for water, ions, and/or urea Interstitial spaces accumulate ions and urea to build up a concentration gradient that increases with depth into the medulla Vasa recta picks up reabsorbed materials and returns to body pool

13. Excretory System Development In the primitive kidney, the mesomere becomes segmented: each segment = nephrotome. Nephrotome contains a coelomic chamber (a nephrocoel) that opens to the general body coelom via a peritoneal funnel. The nephrotome is the forerunner of the nephron, both embryologically and (probably) phylogenetically.

14. Conversion of Nephrotome to Nephron Involves … Outgrowth of the lateral wall of the nephrotome as the principal tubule ? which eventually forms the nephron. The medial wall of the nephrotome then invests and encapsulates a ball of arterial capillaries which becomes the Bowman’s Capsule surrounding the glomerulus. Bowman’s capsule + glomerulus ? filtration apparatus Elongation and differentiation of the principal tubule. Closure of connection to the general body coelom.

17. Nephron Types 3 main types of nephrons may be found in adult kidneys of vertebrates: Type 1: Renal corpuscle (glomerulus + Bowman’s capsule) is of good size; amount of filtrate is high. This type is considered most primitive. Found in amphibians, freshwater bony fishes, elasmobranches, and cyclostomes. No Loop of Henle or comparable structure for concentrating wastes or reabsorbing water.

18. Nephron Types Type 2: Renal corpuscle is small or absent, amount of filtrate is very low. Found in marine teleosts and reptiles Type 3: Renal corpuscle is large, large amount of filtrate; Loop of Henle is interjected in the middle of the convoluted tubule. Much water is reabsorbed; and urine can be concentrated in this type of nephron Adaptation for relatively desiccating terrestrial habitat Found in mammals and in some birds This nephron is the only one capable of producing a urine with a higher solute concentration than that of the body fluids.

19. Kidney DevelopmentAncestral Conditions Presumptive conditions —soft tissues don’t fossilize well Chordate Ancestor = possessed segmented coelom. Kidney = series of paired nephrotomes which developed from segmented mesomere. Diffusion from coelom (nephrotome open to coelom), out through nephrotome to exterior via nephric duct.

20. Embryonic Development of Nephrons Basic Principles: Nephrons differentiate without segmental arrangement in a continuous cord of mesodermal tissue = nephrogenic cord Development occurs from cranial to caudal (anterior to posterior) along the cord; anterior tubules tend to degenerate before posterior tubules form Anterior Region = Pronephros Middle Region = Mesonephros Posterior Region = Metanephros

21. Embryonic Development of Nephrons Basic Principles: The middle and posterior regions together are termed the opistonephros, and a kidney in which all three regions are present and functioning is known as a holonephric kidney. The holonephric kidney is actually only a hypothetical ancestral structure that is not present in any living vertebrate, although similar structures (only the anteriormost tubules degenerate) are present in larval hagfish, elasmobrachs and caecilians. Nephrons increase in complexity from anterior to posterior. The nephric duct that drains the kidney does not have a consistent derivation in all vertebrates.

22. Embryonic Development of Nephrons Phylogeny: The initial portion to develop is the pronephros ("head kidney"). Forming pronephric tubules join to form a pronephric duct (a.k.a. archinephric duct) that extends back over the undifferentiated nephrogenic cord to empty into the cloaca Pronephric tubules maintain a connection with the coelom. The pronephric kidney is a transitory embryonic structure that is mostly nonfunctional. Exceptions: 1) portions persist throughout life in the hagfish and some teleosts; 2) pronephros functions in the early larvae/embryo of lamprey, fishes, and amphibians.

23. Embryonic Development of Nephrons Phylogeny: Remainder of nephrogenic cord posterior to the pronephros is the opistonephros. Derivation is similar to pronephros but tubules are longer and more complicated and generally lack a coelomic connection. They also generally lack segmentation, 2-10 tubules per body segment in mesonephros. Drained by opistonephric duct (= posterior pronephric duct) Occurs as functional adult kidney in most anamniotes

24. Embryonic Kidney Development in Amniotes Early embryo ? Pronephros begins to form but degenerates before complete Fetus ? Mesonephric kidney from middle region of nephrogenic cord Adult ? Metanephric kidney, mesonephric ducts become incorporated into the reproductive system of the male, degenerate in females

29. Overall Trends in Kidney Development Development of internal glomerulus (filtration) ? sole method for fluid entry into nephron. Closure of connection with general body coelom. Loss of segmental nature. Blood supply becomes exclusively arterial (mammals), in primitive kidneys there is blood supply from a renal portal system. Tubules become more complicated (e.g., Loop of Henle) and elongated. Progressive preferential utilization of posterior regions of nephrogenic cord; anterior region degenerates.

30. Nephron Types and Vertebrate Origins Attempts to use information on the distribution of nephron types among the vertebrates to answer the question … “Did the first vertebrates occur in freshwater or saltwater?”

31. Nephron Types and Vertebrate Origins Freshwater Problem: FW fish (and aquatic amphibians) have higher solute concentration than the environment, so they are constantly taking up water and losing salts. Freshwater Solution = kidney with well-developed renal corpuscles, which produce copious amounts of very dilute urine. Chloride cells at gills pump ions in (requires ATP) against a concentration gradient.

32. Nephron Types and Vertebrate Origins Saltwater Problem: SW fish have lower solute concentration than the environment, so they are constantly taking up salts and losing water. Saltwater Solution = Teleosts with aglomerular kidney, so have little urine production. Chloride cells at gills pump ions out (requires ATP) against a concentration gradient.

35. Nephron Types and Vertebrate Origins Vertebrate Origins: Distribution of nephron types among fish (well-developed renal corpuscle in primitive fishes) suggests a FW origin where this type of nephron would be useful. Alternate View = marine environment is more primitive; large renal corpuscle is the primitive condition for vertebrates.

36. Nephron Types and Vertebrate Origins Vertebrate Origins: Sharks are almost exclusively marine animals yet they have large renal corpuscles (which maintain elevated urea to deal with osmotic problems). Thus, sharks and marine teleosts (which have small corpuscles) show two very different methods of dealing with SW environment. This suggests a FW origin, and independent derivation on movement to SW.

37. Nephron Types and Vertebrate Origins Vertebrate Origins: Hagfish (most primitive vertebrate) has large renal corpuscle and fails to regulate ion content (most other vertebrates have salt content far below that of SW), so hagfish becomes isosmotic with SW in which it lives. This suggests that primitively, vertebrates did not regulate salt content and had a large corpuscle -or- that hagfish originated in FW and developed tolerance to high salt levels on moving to SW.

38. Nephron Types and Vertebrate Origins Vertebrate Origins: All known lower Chordates are marine and do not regulate salt content Suggests that osmoconformer strategy is the primitive condition for vertebrates = SW origin (since FW osmoconformers would have ion concentrations too low for proper physiological function for vertebrates) First fossil vertebrates come from marine deposits, which also suggests a SW origin for vertebrates CONCLUSION: A marine (SW) origin seems more probable although the question is far from resolved. Kidney tubule evidence is indecisive.