The cloaca arises from the endoderm. It’s the blind end of the hindgut caudal to the origin of the allantois.
- The cloacal membrane is bilaminar with endoderm and ectoderm.
- Medial and cephalad migration of the intervening mesodermal cells causes the cloacal membrane to ‘retract’ into the perineum
- This mesenchymal migration is essential for formation of anterior abdominal wall, anterior bladder, pubic symphysis, genitals (failure associated with exstrophy)
Classically described, the descent of the urorectal septum separates the cloaca into the anterior urogenital sinus and posterior anorectal canal around the 5th – 6th week.
- During this, the cloacal membrane rotates caudally and posteriorly, driven by the development of the lower anterior abdominal wall and regression of the tail
- Where the urorectal septum joins the cloacal membrane becomes the perineal body
- Defects in the cloacal membrane and urorectal septum are related to bladder exstrophy and epispadias
Remember the ureteric buds sprouted from the distal ends of the mesonephric ducts – the mesonephric duct below/distal to the ureteric buds is called the common excretory ducts
- Around day 33 these common excretory ducts from the mesonephric ducts join the urogenital sinus and some tissue will fuse in the midline – forming the trigone
The end of the ureteric buds will enter the urogenital sinus also, around day 37, forming the ureteric orifices
- Therefore, by the end of the 7th week the ureters and mesonephric ducts have independent openings
- The ureteric orifices migrate cranially and laterally
- The openings of the mesonephric duct will migrate caudally and medially, becoming the ejaculatory duct
- Therefore, the trigone is derived from the intermediate mesoderm, whilst the remainder of the urogenital sinus/bladder is from the endoderm from the cloaca
Weigert and Meyer did a bit of work with duplex ureters:
- The lower pole ureter will be cranial and lateral in the bladder
- A very abnormal lower pole ureter may result from ureteric bud arising too low on the mesonephric duct, resulting in premature incorporation into bladder
- Therefore it may have an undeveloped intramural tunnel, leading to reflux
- The upper pole ureter will be caudal and medial
- A very abnormal upper pole ureter may have its ureteric bud arising too high on the mesonephric duct
- May drain into bladder neck or verumontanum in males (always proximal to sphincter), or even rarely into the mesonephric duct derivatives (namely vas deferens) – this abnormal insertion associated with obstruction
- In females, may also drain into mesonephric duct remnants (namely Gartner’s duct), or into the vaginal vestibule
- Anomalous development of the common excretory duct can lead to rare ectopic vas deferens draining into the ureter for example
From the 12th week or so onwards, the urogenital sinus starts to look a bit more like a bladder:
- Surrounding mesenchyme develops into recognisable smooth muscle layers
- Ventral part expands up to umbilicus to form bladder
- Pelvic part remains narrow and tubular, forming urethra
- Mature urothelial characteristics 13 – 17th week, by the 21st week normal urothelium
As the fetus grows the bladder is pushed down, and the allantois narrows and stretches to become obliterated by the 4th month – becoming the urachus