Session 2 Embryology Flashcards
Describe the beginning of urinary tract development? What is a nephrotome?
- During the third and fourth week, the primordium of the gut is formed.
- Folding of the cranial and caudal ends as well as lateral margins.
- The kidney and ureter both form from intermediate mesoderm at the urogenital ridge.
- The 3 systems develop sequentially.: pronephros => mesonephros => metanephros
- A nephrotome includes: a Glomerulus (Capillary tuft -> produces ultrafiltrate) which is enclosed by the Bowman’s capsule which is connected to the proximal and distal convoluted tubule (complex tubular system) which is connected to collecting duct (which drains out of kidney into ureter). It is the primitive nephron
Describe the pronephros
- Appears: Start of Wk 4
- Regresses: End of Wk4
- Functional: No
- Appears in the cervical region. Substantiatial differentiation of mesoderm into structures resembling nephrons occur with POTENTIAL renal function but is a vestigal structure. Its duct is very important – extends from the cervical region to the cloaca, parallel to the primitive gut tube and drives the development of the next developmental stage. The pronephros duct’s caudal end makes contact with the caudal end of the primitive gut tube. The pronephric duct becomes the mesonephric duct.
Describe the mesonephros
- Appears: End Wk 4
- Regresses: End of Wk8
- Functional: PRIMITIVE (yes, but no water conserving mechanism)
- The mesonephric tubules develop caudal to the pronephric region.
- Mesonephric tubules + mesonephric duct = embryonic kidney.
- Mesonephric duct has a very important role in the development of the duct system of the male reproductive tract.
- Mesonephric duct sprouts the “ureteric bud”.
Describe the metanephros
Appears: Wk5
Regresses: No
Functional: Yes, from end of first trimester
- The “definitive kidney” – ureteric bud induces development of the definitive kidney within intermediate mesoderm in the caudal region of the embryo.
- Collecting system develops from the ureteric bud and the excretory system develops from “metanephric tissue cap”
What is the urogenital ridge and metanephric blastema?
- The urogenital ridge is a region of intermediate mesoderm which develops and develops substantially to give rise to both embryonic kidney and the gonad.
- The ureteric bud contacts metanephric blastema. The bud expands and branches, forming the definitive kidney’s structure.
- The metanephic blastema is a particular zone of intermediate mesoderm which is under the inductive influence of the ureteric bud.
- The collecting system including the ureters is derived from the ureteric bud itself
- The excretory component is derived from the intermediate mesoderm, the metanephric blastema, under the influence of the ureteric bud.
Describe the migration/ascent of the kidney
During its development, the kidney undergoes a dramatic shift in position from the pelvis to the abdomen. This arises because of migration and expansion of the caudal region of the foetus. Migration includes
- cranio-caudal shift from L4 to L1/T12, crossing the arterial fork formed by vessels (umbilical artery) returning blood from the foetus to the placenta. This shift is largely due to the elongation of the trunk of the embryo (development is cranial -> caudal) driving the apparent shift (the kidneys don’t actually move). As the kidneys ascend, they require new arterial supply and the previous supply disappears. If it remains, they are accessory or supernumerary arteries. These arterials are end arteries, as the main renal artery will not branch to supply that area of the kidney if an accessory artery is present. This means there is no collateral supply.
- lateral displacement (meeting up with the adrenal glands in the process)
- a 90o rotation so the renal pelvis faces the midline
What happens when things go wrong? Include the two types of cystic disease
- When the ureteric bud fails to interact with intermediate mesoderm, this is renal agenesis (kidney fails to develop properly). Normally if only unilateral, it is compatible with life.
- Sometimes migration can go awry: if a kidney fails to cross the arterial fork (as the kidneys don’t actually move its more the fork snags the kidney) as it develops, pulling it down, it ends up much lower than it should be). During their ‘ascent’, the kidneys lie extremely close to one another. If they both get caught on the arterial fork, they can fuse and form a horseshoe kidney.
- Wilm’s tumour (tumour of kidney in children)
- Duplication defects
- Ectopic ureter
- Cystic disease:
Multicystic kidney disease – atresia (absence or abnormal narrowing of an opening or passage in the body) of the kidney (congenital structural defect)
Polycystic kidney disease – recessive, presents early poor prognosis (different pathogenesis to multicystic)
Explain about splitting of the ureteric bud
Duplication defects and ectopic ureteral orifices can be caused by splitting of the ureteric bud. This can be partial or complete so a spectrum of structural variations can occur. Symptomatic consequence is ectopic ureteral opening. The ureteral orifices can open in the urethra or female reproductive tract (vagina), bypassing the bladder. This can lead to incontinen
What is the urogenital sinus? What is the urachus?
Early in development, the gastrointestinal, urinary and reproductive tracts end at a single structure (the cloaca which is like a blind pouch).
The urogenital sinus (derived from endoderm) is created from the hindgut by the urorectal septum – it is the compartment of the cloaca, which will go on to develop into the urinary bladder and urethra. The urogenital sinus is continuous with umbilicus via the urachus. The urachus is initially open but eventually loses patency and becomes the median umbilical ligament in adults.
What does the urorectal septum do?
The urorectal septum separate the urinary tract from the GI tract.
NB: the kidney is a mesoderm-derived structure while the urethra and bladder are endoderm-derived structures.
- Divided into 3 parts. The larger upper part is will develop into the future bladder. The pelvic and phallic part will develop into the parts of the future urethra.
- Once the urorectal septum makes contact with the cloacal membrane, it ruptures.
Describe the development of the bladder
- The bladder is a hindgut derivative (derived from the caudal portion of the primitive gut tube formed during embryonic folding in the fourth week of development).
- The caudal portion is a dilated blind pouch called the cloaca, separated from the outside by the cloacal membrane (that and the buccopharyngeal membrane are the two mesoderm-less regions present after gastrulation).
- Also involved in the development of the bladder is the allantois, which is a superoventral diverticulum of the hindgut and extends into the umbilical cord.
- The lumen of the allantois becomes obliterated to become the urachus, which is the median umbilical ligament in adults.
- The cloaca becomes divided by the urorectal septum (separation from GI tract now) into the urogenital sinus (future bladder and urethra) and anorectal canal (future rectum and anal canal, covered in GI unit).
Describe the development of the male urethra
The urethra is formed by narrowing of the lower urogenital sinus. The development of its distal portion differs considerably between male and female.
The distal portion of the urethra develops as the external genitalia change from an indifferent starting point to the orphology dictated by the genotype of the embryo.
In the male, the distal urethra is elongated (becoming the spongy urethra) as the penis develops from fusion of the urethral folds under the influence of male Androgen sex homes.
[*] Mesonephric ducts reach urogenital sinus. They drain embryonic urine into the cloaca.
[*] Ureteric bud sprouts from mesonephric ducts. Ureteric bud will become ureter opening into the bladder.
[*] Smooth musculature begins to appear (develops into trigone of bladder) due to the inductive influence of the ureteric bud. The urogenital sinus begins to expand.
[*] Ureteric buds and mesonephric urogenital sinus.
[*] The mesonephric ducts are maintained by male hormone, forming the prostate and ducts of the male reproductive system.
[*] The prostate and prostatic urethra are formed.
Describe the development of the female urethra
In females urethra opens into the vestibule because the urethral folds do not fuse (no male sex hormones present).
[*] Mesonephric ducts reach urogenital sinus.
[*] Ureteric bud sprouts from mesonephric ducts
[*] Urogenital sinus begins to expand
[*] Mesonephric ducts begin to regress
[*] Mesonephric ducts regression continues, ureteric bud opens into the urogenital sinus (directly developing into bladder).
[*] No development of prostate
Compare the embryological structures of the male and female urethra
- The female urethra is formed by the pelvic part of the urogenital sinus (as it passes through the pelvic floor)
- Male urethra is divided into 4 parts:
Pre-prostatic (embryological structure: narrowed lower segment of urogenital sinus)
Prostatic (narrowed lower segment of urogenital sinus)
Membranous (narrowed lower segment of urogenital sinus)
Spongy (Phallic portion – enclosed by urethral folds)
Describe the development of the external genitalia in both sexes
Basic components:
Genital tubercle
Genital folds
Genital swellings
- In the male, the genital tubercle (phallic part) elongates and the genital folds + swellings fuse to form the spongy urethra – under the influence of male Androgen hormones.
- No fusion occurs in the female and urethra opens directly into the vestibule
- At 7 weeks in both sexes, external genitalia surrounds the opening of the urogenital sinus (where it drains into)
- At 9 weeks, there is a cross roads.
