Embryology of the Kidneys & Urinary System Flashcards

1
Q

What structures make up the urinary system?

What system is it closely related to in development?

A
  • kidneys
  • ureters
  • urinary bladder
  • urethra
  • the urinary system and genital system both develop from a common mesodermal ridge (intermediate mesoderm) along the posterior wall of the abdominal cavity
  • the excretory ducts initially empty into a common cavity - the cloaca
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2
Q

How does the mesoderm become highly organised around day 17?

What structures does it go on to form?

A

Paraxial mesoderm:

  • majority of the skeleton
  • skeletal muscles
  • dermis of the skin

Intermediate mesoderm:

  • gonads
  • internal reproductive tracts
  • kidneys

Lateral plate mesoderm:

  • lining of body cavities
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3
Q

What are the kidneys derived from?

A
  • the kidneys are derived from intermediate mesoderm
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4
Q

What 3 kidney systems are formed during development of the kidney?

A
  • 3 slightly overlapping kidney systems are formed in a cranial-to-caudal sequence:
  1. pronephros
  2. mesonephros
  3. metanephros
  • the pronephros is rudimentary and nonfunctional
  • the mesonephros functions for a short time during the early foetal period
  • the metanephros forms the permanent kidney
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5
Q

When does the pronephros develop?

What does it develop from and when does it regress?

A
  • it develops during the 4th week
  • intermediate mesoderm in the cervical region condenses and reorganises to form nephrotomes
    • these are vestigial excretory units
  • the nephrotomes are epithelial buds that regress before more caudal ones are formed
  • the pronephros dissappears by day 25
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6
Q

When does the mesonephros develop?

What does it develop from?

A
  • the mesonephros develops from intermediate mesoderm in the upper thoracic to upper lumbar (L3) regions
  • the first excretory tubules of the mesonephros appear during the 4th week during regression of the pronephros
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7
Q

How does the mesonephros develop following formation of the first excretory tubules during week 4?

A
  • the excretory tubules lengthen rapidly, form an S-shaped loop and acquire a tuft of capillaries
  • the tuft of capillaries will form a glomerulus at their medial extremity
  • around the glomerulus, the tubules form a Bowman’s capsule
  • together these structures constitute a renal corpuscle
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8
Q

How does the mesonephric duct form?

How does this develop and what does it induce formation of?

A
  • intermediate mesoderm in the lower cervical region is induced to form a solid duct - the mesonephric or Wolffian duct
  • this develops caudally and fuses with the walls of the cloaca on day 26
  • canalisation commences from the caudal end
  • this induces formation of mesonephric buds
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9
Q

How are the mesonephric tubules formed?

What constitutes a renal corpuscle?

A
  • a renal corpuscle consists of:
  1. Bowman’s capsule
  2. glomerulus
  • the tubules are formed in a craniocaudal fashion
  • as the more caudal tubes differentiate then the cranial tubules regress
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10
Q

What are the functions of the mesonephric tubules?

Do they persist or regress?

A
  • while caudal tubules are differentiating, most cranial tubules and glomeruli have degenerated or fused with the mesonephric duct
  • by the end of month 2, the majority of mesonephric tubules have disappeared
  • in the female, all** of the mesonephric tubules **regress
  • in the male, some caudal tubules and the mesonephric duct persist and develop into reproductive structures
  • mesonephric tubules function between weeks 6-10 to produce small amounts of urine
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11
Q

What is the urogenital ridge and how does it develop?

A
  • the mesonephros forms a large ovoid organ on each side of the midline in the middle of month 2
  • as the developing gonad is on its medial side**, the ridge formed by both organs is the **urogenital (mesonephric) ridge
  • gonadal development takes place on the medial aspect of the mesonephric ridge
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12
Q

When does the metanephros begin to develop?

What does it develop from?

A
  • the metanephros begins to develop in week 5 (day 28) and forms the definitive kidneys
  • it has a dual origin:

the collecting portion / duct system develops from the ureteric bud

the excretory portion develops from the metanephric mesoderm

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13
Q

How does development of the metanephros begin?

A

formation of the ureteric bud

  • this is an outgrowth of the mesonephric duct at its caudal end close to its entrance to the cloaca
  • by day 32, the ureteric buds penetrate the metanephric mesoderm, which is molded over its distal end as a cap
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14
Q

What happens once the ureteric bud has penetrated the metanephric mesenchyme?

A
  • the ureteric bud penetrates the metanephric mesenchyme and then branches
  • the bud dilates to form the primitive renal pelvis
  • it then splits into cranial and caudal portions - the future major calyces
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15
Q

How do minor calyces and collecting tubules form following development of the major calyces from the ureteric bud?

A
  • each major calyx forms 2 new buds when penetrating the metanephric mesoderm
  • these buds continue to subdivide until 12 or more generations of tubules have formed
  • at the periphery, more tubules are forming until the end of month 5
  • tubules of 2nd order absorb those of the 3rd and 4th generations to form the minor calyces
  • collecting tubules of the 5th and successive generations elongate and converge on the minor calyx to form the renal pyramid
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16
Q

How are nephrons / excretory units formed from the metanephros?

A
  • each newly formed collecting tubule is covered by a metanephric tissue cap
  • under the inductive influence of the tubule, cells of the tissue cap form small renal vesicles
  • the renal vesicles expand to form small S-shaped tubules
  • capillaries grow into the pocket at one end of the S and differentiate into glomeruli
  • the tubules along with their glomeruli form nephrons
17
Q

What is formed by the proximal and distal ends of each nephron?

A
  • the proximal end forms the Bowman’s capsule, which is deeply indented by a glomerulus
  • the distal end forms an open connection with one of the collecting tubules, establishing a passage from Bowman’s capsule to collecting duct
    • there is tissue breakdown between the 2 embryological origins
18
Q

What happens to the nephron once the Bowman’s capsule and connection with the collecting system has formed?

A
  • there is continuous lengthening of the excretory tubule to form:
  1. proximal convoluted tubule
  2. loop of Henle
  3. distal convoluted tubule
19
Q

What are the 2 origins of the kidney?

What makes up a definitive nephron?

A
  • the metanephric mesoderm gives rise to the excretory units
  • the ureteric bud gives rise to the collecting system
  • a definitive nephron (excretory unit) consists of:
  1. renal corpuscle
  2. renal tubule
  3. collecting tubule
20
Q

What is duplication of the ureter and why does it occur?

A
  • occurs due to premature bifurcation of the ureteric bud, which can be partial or complete
  • this results in the presence of 2 ureters draining a single kidney
  • this can result in a bifid ureter or ectopic ureter
21
Q

What is renal agenesis and why does it occur?

A
  • caused by early degeneration of the ureteric bud or failed interaction between the ureteric bud and metanephric tissue cap
  • this leads to absence of one (unilateral) or both (bilateral) kidneys
22
Q

How does unilateral renal agenesis present?

A
  • it is generally asymptomatic
  • there may be hypertrophy of the remaining kidney
23
Q

How does bilateral renal agenesis present?

A
  • this results in oligohydraminos
  • the reduced volume of amniotic fluid causes the foetus to present with Potter sequence:
  1. clubbed feet
  2. pulmonary hypoplasia
  3. cranial anomalies
24
Q

What is congenital cystic kidney disease?

What are the 2 different types and how do they differ?

A
  • this describes a condition in which numerous cysts form
  • it can be inherited as an autosomal dominant (ADPKD) or autosomal recessive (ARPKD) condition
  • ADPKD and ARPKD are caused by mutations in genes that encode proteins localised in cilia that are important for ciliary function

ADPKD:

  • cysts form form all segments of the nephron
  • it usually does not cause kidney failure until adulthood
  • this is more common but less progressive than ARPKD

ARPKD:

  • this is a progessive condition in which cysts form from the collecting ducts
  • the kidneys become very large and renal failure occurs in infancy or childhood
25
Q

what are other non-genetic factors associated with congenital cystic kidney disease?

A
  • failure of induction between the ureteric bud and metanephric caps
  • nephrons fail to develop and the ureteric bud fails to branch
26
Q

How does the kidney relocate?

Why does this happen and when do the kidneys assume their adult position?

A
  • the kidneys develop in the pelvic region and later shift to a more cranial position in the abdomen
  • the ascent of the kidney is caused by dimunition of the body curvature and by growth of the body in the lumbar and sacral regions
27
Q

How does the blood supply to the kidney change as it ascends?

When does it assume its adult position?

A
  • the metanephros receives its blood supply from a pelvic branch of the aorta when it resides in the abdomen
  • during its ascent to the abdominal level, it is vascularised by arteries that originate from the aorta at continuously higher levels
  • the lower vessels usually degenerate but some may remain
  • the kidneys attain their adult position by week 9
28
Q

What is meant by horseshoe kidney and why does it occur?

A
  • during their ascent, the kidneys pass through the arterial fork formed by the umbilical arteries
  • as they pass through the fork they can be pushed so close together that their inferior poles fuse together
  • this forms a “horseshoe kidney” which resides in the lower lumbar region
    • its ascent is prevented by the root of the inferior mesenteric artery
  • this condition is usually asymptomatic
29
Q

When and how does the cloaca divide?

A
  • during weeks 4-7 the cloaca divides into:
  1. the urogenital sinus anteriorly
  2. the anal canal posteriorly
  • the urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus
30
Q

How can the urogenital sinus be divided into 3 distinguishable parts?

A

Upper part:

  • this is the largest part that will form the urinary bladder

Pelvic part:

  • this is a narrow canal that gives rise to the prostatic and membranous parts of the urethra in males

Phallic part:

  • this forms the penile urethra in males and the vestibule in females
  • it is flattened from side to side, and as the genital tubercle grows, this part of the sinus is pulled ventrally
31
Q

What is the bladder initially continuous with?

What happens when this structure obliterates?

A
  • the bladder is initially continuous with the allantois
  • when the lumen of the allantois obliterates, a thick fibrous cord called the urachus remains
  • the urachus connects the apex of the bladder with the umbilicus
  • this forms the median umbilical ligament in the adult
32
Q

During differentiation of the cloaca, hat happens to the mesonephric ducts?

A
  • the caudal portions of the mesonephric ducts become absorbed into the wall of the urinary bladder
  • the ureters, which were initially outgrowths from the mesonephric ducts, enter the bladder separately
  • as the kidney ascends, the orifices of the ureters move further cranially
  • the orifices of the mesonephric ducts move closer together to enter the prostatic urethra and become ejaculatory ducts in the male
33
Q

What is significant about how the lining of the trigone of the bladder changes over time?

A
  • both the mesonephric ducts are ureters are of mesodermal origin
  • the mucosa of the bladder formed by incorporation of the ducts - the trigone - is also mesodermal
  • over time, the mesodermal lining of the trigone is replaced by endodermal epithelium so that the inside of the bladder is completely lined with endodermal epithelium