Development of kidney, urinary bladder and urethra

 

Kidney

• Intermediate mesoderm: kidneys, ureters and trigone of urinary bladder
• Endoderm: rest of urinary bladder, urethra

3 kidneys develop: Pronephros, mesonephros and metanephros

(I) Pronephros:

• Segmented cervical intermediate mesoderm
• 7-10 excretory tubules called pronephric tubules form – degenarate by end of 4th week
• Collecting duct called pronephric duct forms and opens down into cloaca – pronephric duct persists to form mesonephric duct

(II) Mesonephros:

• Segments of thoracic and upper lumbar region of intermediate mesoderm
• Each segment forms 2-3 ‘S’ shaped mesonephric tubules
• Lateral ends open in mesonephric duct and medial end invaginated by glomerulus

In males:

• Mesonephric tubules: upper degenerate, rest form efferent ducts of testis, head of epididymis, paradidymis
• Mesonephric duct: body and tail of epididymis, vas deferens, ejaculatory duct, seminal vesicle, ureteric bud and trigone of urinary bladder

In females:

• Mesonephric tubules: degenerate
• Mesonephric duct: ureteric bud and trigone of urinary bladder

(III) Metanephros: (kidneys)

(A) Development of collecting duct and ureter:

• Ureteric bud develops from mesonephric duct
• Ureteric bud grows cranially, and penetrates metanephric cap
• Upper end of ureteric bud enlarges ⇒ forms pelvis which divides into ⇒ 2-3 major calyces ⇒ where each divides into minor calyces ⇒ then collecting tubules ⇒ which join to nephrons 

(B) Development of nephrons:

• Caudal part of intermediate mesoderm forms a metanephric cap
• Which divides into renal vesicles
• Each renal vesicle surrounds the free end of a collecting tubule and forms a ‘S’ shaped nephron
• One end of nephron invaginated by glomerulus – Bowman’s capsule
• Other end joins collecting duct
• Each nephron elongates – forms proximal and distal convolutes tubules and loop of Henle

Further growth of kidney:

1. Lobulated grooves disappear – forms smooth surface
2. Ascends from pelvic region to adult level
3. Recieves blood supply from median sacral, common iliac, lower abdominal aorta. Then only from aorta
4. At first, hilum directed forwards, rotates 90 degrees so hilum becomes medial

Congenital anomalies of kidney:

1. Renal agenesis
2. Renal hypogenesis – small size
3. Congenital polycystic kidney – failure of fusion between nephrons and collecting tubules. Urine collects in nephrons, dilates and forms cysts, nephrons destroyed
4. Pelvic kidney – failure of ascent
5. Horseshoe shaped kidney – fusion of both kidneys, ureters kinked, this causes urinary stasis and so infection
6. Additional branches of aorta supplying kidney – cross infront of ureter and compress it – urinary stasis
7. Double ureter – 2 ureteric buds/ early splitting of ureteric bud. More liable to infection and stone formation

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Urinary bladder and urethra

A constriction appears in primitive urogenital sinus at site of entrance of mesonephric duct

Divides into:

1. Upper part – Vesico-urethral canal
2. Lower part – Definitive urogenital sinus

(I) In males

(A) Urinary bladder:

• From vesico-urethral canal
• Trigone from absorbed common stem of mesonephric duct and ureter. 

Differential growth of posterior bladder wall, therefore ureter moves upwards (posterior superior angle)

(B) Seminal vesicle:

Develops as a diverticulum from vas deferens. Part distal to it becomes ejaculatory duct.

(C) Urethra:

1. Prostatic urethra:

• Upper 1/2 – vesicourethral canal
• Lower 1/2 – Definitive urogenital sinus

2. Membranous urethra – Definitive urogenital sinus

3. Penile urethra – Definitive urogenital sinus forms a urethral plate that extends on the under surface of phallus (primitive penis) and is surrounded by 2 urethral folds – unite from back and front around urethral plate to form penile urethra. Lined by endoderm, terminal glandular part lined by ectoderm.

(D) Prostate gland:

• Develops from 15 to 20 buds from prostatic urethra
• Canalized to form alveoli and ducts
• Connective tissue and capsule from surrounding mesoderm

(II) In females

1. Vesicourethral canal – urinary bladder and urethra
2. Definitive urogenital sinus – lower 1/5 vagina and vestibule

Congenital anomalies:

1. Urachal fistula – unobliterated urachus. Urine drips from umbilicus
2. Urachal cyst – Incomplete obliteration
3. Bladder extrophy – urinary bladder opens into anterior abdominal wall
4. Hypospadius – external urethral meatus opens on under surface of penis
5. Epispadius –  external urethral meatus opens on upper surface of penis