Embryology of the Kidneys & Urinary System Flashcards
What structures make up the urinary system?
What system is it closely related to in development?
- 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
How does the mesoderm become highly organised around day 17?
What structures does it go on to form?
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
What are the kidneys derived from?
- the kidneys are derived from intermediate mesoderm
What 3 kidney systems are formed during development of the kidney?
- 3 slightly overlapping kidney systems are formed in a cranial-to-caudal sequence:
- pronephros
- mesonephros
- 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
When does the pronephros develop?
What does it develop from and when does it regress?
- 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
When does the mesonephros develop?
What does it develop from?
- 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
How does the mesonephros develop following formation of the first excretory tubules during week 4?
- 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
How does the mesonephric duct form?
How does this develop and what does it induce formation of?
- 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
How are the mesonephric tubules formed?
What constitutes a renal corpuscle?
- a renal corpuscle consists of:
- Bowman’s capsule
- glomerulus
- the tubules are formed in a craniocaudal fashion
- as the more caudal tubes differentiate then the cranial tubules regress
What are the functions of the mesonephric tubules?
Do they persist or regress?
- 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
What is the urogenital ridge and how does it develop?
- 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
When does the metanephros begin to develop?
What does it develop from?
- 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
How does development of the metanephros begin?
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
What happens once the ureteric bud has penetrated the metanephric mesenchyme?
- 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
How do minor calyces and collecting tubules form following development of the major calyces from the ureteric bud?
- 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
How are nephrons / excretory units formed from the metanephros?
- 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
What is formed by the proximal and distal ends of each nephron?
- 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
What happens to the nephron once the Bowman’s capsule and connection with the collecting system has formed?
- there is continuous lengthening of the excretory tubule to form:
- proximal convoluted tubule
- loop of Henle
- distal convoluted tubule
What are the 2 origins of the kidney?
What makes up a definitive nephron?
- 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:
- renal corpuscle
- renal tubule
- collecting tubule
What is duplication of the ureter and why does it occur?
- 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
What is renal agenesis and why does it occur?
- 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
How does unilateral renal agenesis present?
- it is generally asymptomatic
- there may be hypertrophy of the remaining kidney
How does bilateral renal agenesis present?
- this results in oligohydraminos
- the reduced volume of amniotic fluid causes the foetus to present with Potter sequence:
- clubbed feet
- pulmonary hypoplasia
- cranial anomalies
What is congenital cystic kidney disease?
What are the 2 different types and how do they differ?
- 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
what are other non-genetic factors associated with congenital cystic kidney disease?
- failure of induction between the ureteric bud and metanephric caps
- nephrons fail to develop and the ureteric bud fails to branch
How does the kidney relocate?
Why does this happen and when do the kidneys assume their adult position?
- 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
How does the blood supply to the kidney change as it ascends?
When does it assume its adult position?
- 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
What is meant by horseshoe kidney and why does it occur?
- 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
When and how does the cloaca divide?
- during weeks 4-7 the cloaca divides into:
- the urogenital sinus anteriorly
- the anal canal posteriorly
- the urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus
How can the urogenital sinus be divided into 3 distinguishable parts?
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
What is the bladder initially continuous with?
What happens when this structure obliterates?
- 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
During differentiation of the cloaca, hat happens to the mesonephric ducts?
- 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
What is significant about how the lining of the trigone of the bladder changes over time?
- 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
