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