Embryology Flashcards
Which germ layers are the…
-Kidney
-Bladder
… derived from?
- Kidney: intermediate mesoderm
- Bladder: mainly the endoderm (except the trigone which originates from the mesoderm)
What are the 3 nephric stages of kidney development?
Pronephros (the earliest nephric stage)
Mesonephros (is temporarily the main outflow tract for the embryo)
Metanephros (will become the permanent kidneys)
Which of the 3 nephric stages are functional?
Mesonephros and metanephros
What part of the intermediate mesoderm is the... -Pronephric system -Mesonephric system -Metanephric system ... derived from?
- Pronephric system: segmented intermediate mesoderm
- Mesonephric system: unsegmented intermediate mesoderm
- Metanephric system: unsegmented intermediate mesoderm
Describe the pronephric stage of kidney development
- Mesonephric tubules start to form craniocaudally along the length of the intermediate mesoderm at ~4 weeks
- The most cranial pairs regress by the end of week 5
Describe the mesonephric stage of kidney development
- ~week 5, 20 pairs of mesonephric tubules remain at L1-L3 level in the intermediate mesoderm
- These function as a temporary outflow tract for the embryo between weeks 6-10
- They will then regress
Describe the metanephric stage of kidney development
- Although present at ~week 5, the permanent kidney begins to function at ~weeks 9-11 when the mesonephric tubules are regressing
- It is formed from the ureteric bud and metanephric blastema which are found at the most caudal end of the intermediate mesoderm
- It begins to function ~weeks 9-11
What are the ureteric buds and the metanephric blastema?
Ureteric buds: outgrowths of the distal mesonephric ducts (sprout at ~day 28)
Metanephric blastema: a mass of intermediate mesoderm in the sacral region (penetrated by the ureteric buds at ~day 32)
Suggest two reasons why kidney agenesis (failure of the kidneys to develop) may occur
- The ureteric bud fails to grow from the mesonephric duct
- The ureteric bud is not long enough to reach the metanephric blastema
The ureteric bud will form the collecting/filtration system of the kidney
Collecting system
What structures are found in the collecting system of the kidney (and so are derived from the ureteric bud)?
Ureter Renal pelvis Major calyces Minor calyces Collecting ducts
How does the ureteric bud form the collecting system?
- The metanephric blastema gives off signals that cause the ureteric bud to bifurcate
- Four generations of bifurcations occur
- The branches then intussuscept to form the renal pelvis
- Another four generations of branching occurs, then the branches intussuscept to form the major calyces
- This repeats until the final wave of branching forms the collecting ducts
The metanephric blastema will form the collecting/filtration system of the kidney
Filtration system
What important structure will the metanephric blastema become?
The nephron
The metanephric blastema receives much of its development signals from…?
The ureteric bud
What is a nephron?
The functional unit for the filtration of blood and the production of urine
What are the 4 parts of the nephron?
Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Describe the formation of the nephron, remembering its 4 parts: Bowman's capsule Proximal convoluted tubule Loop of Henle Distal convoluted tubule
- Metanephric tissue caps are found at the ends of the collecting tubules formed by the branching of the ureteric bud
- These tissue caps differentiate to form renal vesicles (hollow cell clusters)
- The renal vesicle elongates and the Bowman’s capsule is formed - this is the beginning of the nephron
- The nephron continues to grow and becomes continuous with the collecting tubule
- Bowmann’s capsule connects with a collection of vessels known as the glomerulus
- The middle portion of the nephron twists to form the ascending and descending limbs of the Loop of Henle
- The nephron has now grown and twisted to form the Bowmann’s capsule, proximal convoluted tubule, loop of Henle, and the distal convoluted tubule which is connected to the collecting tubules
Where in the embryo do the kidneys develop and where do they end up?
They develop in the pelvic region and ascend to L1 level during weeks 6-9
Describe the blood supply to the developing kidneys
- Blood supply is initially from the common iliac arteries
- New vessels sprout from the abdominal aorta as the kidneys ascend and the earlier vessels regress
Why is variation in kidney vasculature common?
Many people have accessory renal arteries due to failure of earlier renal arteries to regress
Describe 2 positional abnormalities of the kidney
Pelvic kidney: one kidney fails to ascend so remains in the pelvic region, it can still function normally
Horseshoe kidney: the inferior poles of the kidneys meet and fuse before they ascend, resulting in a single horseshoe-shaped kidney which becomes trapped under the inferior mesenteric artery, the kidneys are both still functional
How is the bladder formed from the cloaca?
- The cloaca is an expansion of the hindgut endoderm
- The cloaca is divided into the primitive urogenital sinus superiorly and the rectum inferiorly
- The primitive urogenital sinus will develop to give the bladder, pelvic urethra, and definitive urogenital sinus
How do the ureters come to communicate with the bladder?
- The mesonephric duct attaches to the posterior wall of the bladder
- The ureteric bud forms as an outgrowth of the mesonephric duct at ~week 5
- The distal mesonephric duct migrates downwards to allow the ureter to connect directly to the posterior wall of the bladder too
The migrating mesonephric duct contributes to part of the bladder wall known as the…?
Trigone region (so this is derived from the intermediate mesoderm rather than the endoderm)
The mesonephric ducts will become what structure in the adult male?
The urethra
What is the…
-Allantois
-Urachus
…?
- Allantois: the connection between the bladder and the umbilicus for the removal of waste from the embryo
- Urachus: the embryological remnant of the allantois
Describe 3 possible bladder defects
Urachal fistula (failure of the allantois to close, may cause urine to leak from the umbilicus)
Urachal cyst (partial closure of the allantois resulting in cyst formation)
Urachal sinus (if only the posterior portion of the allantois closes)
