Lecture 22: Vertebrate Genitourinary System

Question 1

Give a brief background on the nephron

Answer 1

• The excretory system is typically a bilaterally symmetrical system consisting of a pair of kidneys and the ducts necessary to conduct the products of kidney function (urine) to the exterior.
• The functional unit of a kidney is the kidney tubule, or nephron.
• A collection of nephrons is a kidney.
• In amniotes, three kidneys develop throughout embryogenesis:
• Pronephros
• Mesonephros
• Opisthonephros
• A kidney tubule typically has three functions:
• Filtration:
• • Filtration involves the movement of particles (molecules, ions, etc.) across a filter.
• • Whether or not a particle can move across the filter is largely dependent on the size of the particle and the size of the pores in the filter.
• • The mechanical force for this movement is blood pressure.
• Selective reabsorption
• Secretion
• Materials that can cross the filter and enter the kidney tubule make up the filtrate.
• The filtrate, however, is not the final renal output, the urine.
• Urine is produced when the filtrate is modified by selective reabsorption and excretion.
• Kidneys develop from the mesodermal mesenchyme that forms the longitudinal mesonephric ridges in the roof of the coelom.
• These paired ridges are covered by somatic mesoderm which later becomes part of the parietal peritoneum.
• The kidneys, therefore, form behind the peritoneum and are considered retroperitoneal.
• The kidneys flank the dorsal aorta (abdominal aorta) and there develops a very intimate relationship between the kidneys and branches of the dorsal aorta.
• See Slides 5-6; Call that brief? Fuck!

Question 2

Describe the primitive nephron

Answer 2

• Found in elasmobranchs and in freshwater (and some marine) teleosts.
• The tubule is divided into proximal and distal convoluted regions.
• The corpuscle of this nephron is large.
• Because of the large corpuscle, this type of nephron allows a large amount of water to pass across the filter.
• Most of this water is not reabsorbed, and very dilute urine is formed.
• Fishes that possess this type of nephron typically excrete nitrogenous waste in the form of ammonia.
• Such fishes are referred to as ammonotelic
• Include freshwater teleosts and lungfishes.
• Glomeruli of the most primitive nephrons:
• Are supplied by arterial branches coming directly off the dorsal aorta
• Lie adjacent to the parietal peritoneum of the coelomic cavity. Filtration occurs into the coelom.
• Nephrons, lacking corpuscles, drain the coelomic fluid via ducts into the cloaca.
• There is no direct connection between the glomerulus and the renal corpuscle.
• Later, the glomeruli became incorporated within the renal corpuscles, forming a much more efficient filtration system

Question 3

Describe the marine teleost nephron

Answer 3

• Found in most marine teleosts
• Has a very small or reduced renal corpuscle
• Allows less water to be filtered
• Nitrogenous waste is excreted as urea (urotelic)
• Allows less water to be lost from the body compartments of the fish
• Excess salt from their environment cannot be eliminated by the kidneys because it would result in a large loss of body water in the process.
• Additional ways of eliminating salt:
• • Gill lamellae
• • Digitiform gland (shark)

Question 4

Describe the primitive nephrons of marine sharks

Answer 4

• Sharks have the primitive nephron which results in large amount of water loss.
• Sharks have digitiform gland which eliminates excess salt.
• Sharks can live with large concentrations of urea in their system.
• See Slide 11

Question 5

Describe Pronephros

Answer 5

• In all vertebrate embryos the first kidneys to develop are the pronephric kidneys.
• The pronephros is a collection of simple kidney tubules located near the cranial end of the future abdominal cavity.
• These differentiate from the anterior intermediate mesoderm at about 22 days of gestation in human embryos.
• Pronephric tubules are relatively simple structures both functionally and structurally and resemble in some ways the simple excretory structures seen in some invertebrates.
• The pronephric kidney in mammals is non-functional and the pronephric tubules are solid cords, but it is required in order for the rest of the excretory system to develop.
• The pronephros is a functional kidney only in the larvae of fishes and amphibians and in the adults of some very primitive fishes.
• Although it appears in all vertebrate embryos, it is non-functional and transitory in higher forms.
• In human embryos each pronephros consists of about seven nephrons.
• Each nephron develops from a single somite, from a region of the somite called the nephrotome.
• These amniote pronephric tubules are solid.
• Laterally they connect to a longitudinal duct called the pronephric duct.
• This duct extends from the pronephric tubules through the mesonephric ridge caudally to the cloaca.
• There are two pronephric ducts, one on each side.
• The pronephric duct will be renamed the mesonephric duct.
• In lower vertebrate embryos the proximal ends of the pronephric tubules open into the coelomic cavity.
• Running along the mesonephric ridge is a highly vascularized ridge called the glomus.
• Small molecules from the blood are filtered from the capillaries in the glomus into the coelomic cavity.
• From the coelom these materials move into the openings (nephrostomes) of the pronephric tubules and from there to the pronephric duct to the cloaca.
• See Slide 15-18

Question 6

Describe the Mesonephros

Answer 6

• Caudal to the pronephros the mesonephros develops. This is the functional kidney in most vertebrate embryos, including humans, and in most adult fishes and all amphibians.
• In salamanders the cranial part of the mesonephros does not function as a kidney in the adult. The functional caudal part is usually referred to as the opisthonephros (tail kidney).
• The mesonephrosconsists of mesonephric tubules. These tubules are longer than the pronephric tubules and are often sigmoid or convoluted in shape.
• Mesonephric tubules develop caudal to the pronephric tubules.
• Many mesonephric tubules form per somite segment.
• Mesonephric tubules tend to be longer than the pronephric tubules and are sigmoid in shape.
• The glomerulus associated with each mesonephric tubule is in direct contact with the renal corpuscle.
• Filtration, therefore, occurs directly between the glomerulus and the corpuscle.
• The mesonephric tubules grow into the existing pronephric duct, which is then renamed the mesonephric duct.
• The kidneys flank the dorsal aorta (abdominal aorta) and there develops a very intimate relationship between the kidneys and branches of the dorsal aorta.
• In human embryos there are about seventy pairs of these mesonephric tubules. They arise at the rate of several pairs per somite level.
• With a few exceptions, mesonephric tubules do not open into the coelom. Rather, the capillary aggregations from the dorsal aorta become more discrete and are enclosed within the expanded, double-walled saclike invaginations of the proximal ends of the tubules.
• These cup-like pockets are called renal corpuscles, and the enclosed capillary aggregations are called glomeruli. Because the mesonephric glomeruli are enclosed within the renal corpuscles, the vascular filtrate bypasses the coelom and goes directly into the nephron.
• The distal ends of the mesonephric tubules do not form their own excretory ducts. Instead, they connect with the existing pronephric ducts.
• The pronephric ducts are then renamed the mesonephric ducts or Wolffian ducts.
• After the mesonephric tubules begin to function, the pronephric tubules/cords degenerate.
• The development of the mesonephric tubules is dependent on the presence of the pronephric duct.
• If the pronephric duct is experimentally prevented from growing caudally, the mesonephric tubules will not develop.
• Just before it attaches to the cloaca, the mesonephric duct develops an evagination called the ureteric bud.
• The ureteric bud initiates a series of inductive interactions that will result in the formation of the final kidney—the metanephric kidney.
• See Slide 21-23

Question 7

Describe the metanephros

Answer 7

• Caudally, each mesonephric duct forms an evagination called the ureteric bud or metanephric diverticulum.
• This pocket-like expansion grows into the surrounding mesenchyme of the mesonephric ridge during week 5 of gestation.
• This metanephric diverticulum will give rise to the adult ureter, renal pelvis, and collecting tubules.
• As the metanephric diverticula expand, they induce the surrounding mesenchyme to condense into the metanephrogenic blastemata.
• These blastemata will form the renal tubules of the adult kidney.
• The metanephrogenic blastemata have a reverse inductive effect on the ureteric diverticula.
• Neither one can continue to develop in the absence of the other.
• The metanephric tubules will not form in the absence of the mesonephric duct.
• See Slides 29-30

Question 8

Describe the comparative anatomy of the urinary system in amphibians

Answer 8

• Characterized by the primitive nephron
• Nitrogenous wastes are eliminated in the form of ammonia in larval amphibians and urea in adult amphibians.
• The pronephrosis the functional kidney in larval amphibians. The mesonephros is the functional kidney in adult amphibians.
• Cloaca in amphibians is well developed.
• A urinary bladder develops from the cloaca, but it is a mesodermal structure and is not homologous to the endodermal mammalian urinary bladder.

Question 9

Describe the comparative anatomy of the urinary system in reptiles

Answer 9

• Reduction in size of renal corpuscle in living reptiles.
• Marine turtles have a salt gland in their orbit.
• Lizards and some turtles excrete their nitrogenous waste in the form of uric acid.
• An insoluble and nontoxic form of nitrogenous waste that does not require large amounts of water for dilution.
• Uric acid is more metabolically expensive to make than urea.
• Marine turtles also possess salt glands to rid their body fluids of excess salt without wasting water.
• Crocodiles eliminate nitrogen wastes via ammonia.
• Lizards and some chelonians eliminate nitrogen wastes via uric acid.

Question 10

Describe the comparative anatomy of the urinary system in amniotes

Answer 10

• Larval amniotes use the mesonephros as their functional kidney and the metanephros as the functional kidney in the adult (this is true of all amniotes)
• In all amniotes, urine conduction is completely excluded from the archinephric duct.
• The archinephric duct may be modified anteriorly to form the epididymis for sperm stimulation and ripening. Distally it may form sperm storage ampullae.
• The cloaca is a primitive vertebrate feature:
• Well developed in amniotes except mammals.
• A urinary bladder may develop from the cloaca in amphibians and reptiles but not in birds (except the ostrich).

Question 11

Describe the comparative anatomy of the urinary system in mammals

Answer 11

• The mammalian adult kidney is made up of metanephric nephrons.
• These have a large corpuscle
• Incorporate a specialized part of the tubule between the proximal and distal convoluted tubules called the loop of Henle.
• The loop of Henle creates an osmotic gradient that is responsible for most of the water reabsorption.