ANHB2212 - Urogenital System Development

Urogenital System

organs concerned with reproduction and urinary excretion.

Urinary Tract

• consists of the kidneys, ureters, urinary bladder, and urethra.

• responsible for filtration of blood, and production, storage and elimination of urine.

Genital Tract

• consists of the penis, clitoris, vagina, testes, ovaries, and uterus.

• is involved in reproductive activity

What develops first? The urinary or genital system?

Urinary system

What does intermediate mesoderm form in the urinary system?

• adrenal gland cortex

• kidneys

• organs of posterior body wall

• CT, smooth muscles and blood vessels

What does splanchnopleure form in the urinary system?

Forms epithelial linings of terminal portions of urogenital system: cloaca & urogenital sinus

What does splanchnic form in the urinary system?

Forms mesodermal derivatives of organs lined by endoderm: CT, smooth muscle & blood vessels

What are the 3 main steps of kidney development?

1. pronephros

2. mesonephros

3. metanephros

How are nephrotomes formed?

Rapid growth of intermediate mesoderm which becomes segmented

How is the urogenital ridge formed?

A bulge formed by the nephrotomes forms that creates tubules

Describe the development of the pronephric kidney

1. develops in the cervical region of intermediate mesoderm called nephrotomes during week 4

2. forms pronephric tubules and bilateral pronephric ducts

3. bundles of blood vessels bulge into spaces created in the urogenital ridge.

4. As the pronephric tubules degenerate, the retained pronephric duct becomes the mesonephric duct as it grows caudally into the region of mesonephric tubule development.

What forms the first part of the second kidney’s drainage system?

pronephric duct

Describe the development of the mesonephric kidney

1. As the pronephros regresses in week 4, the mesonephros appears.

2. Forms from a long expanse of intermediate mesoderm, caudal to the pronephros (thoracic & lumbar nephrotomes)

3. Forms 30 pairs of mesonephric tubules at one time as cranial tubules degenerate (with 40 glomeruli) that open into bilateral mesonephric ducts (elongation of pronephric duct)

4. Grows rapidly until the ducts fuse with the cloaca

5. Degenerates by end of second month

6. Duct persists in males

When does the mesonephros create urine?

week 6 to 10 until permanent kidneys function

When do thoracic and lumbar mesonephros degenerate?

thoracic = week 5

lumbar = weeks 10-12

Describe the development of the metanephros (definitive) kidney

1. result of interactions between ureteric bud and metanephric blastema

2. ureteric bud induces the metanephric mesenchyme to condense and form metanephric tubules that will differentiate into nephrons

3. metanephric blastema induces the ureteric bud to elongate and branch repeatedly

4. tip of ureteric bud dilates to form renal pelvis and branches form 2-3 major calyces.

5. each calyx goes through 12+ more divisions: early divisions form minor calyces and final divisions form collecting ducts

6. metanephric cap mesenchymal cells condense to form first portion of nephron (glomerular capsule) then forms all epithelial excretory tubules from capsule in a proximal-distal sequence

7. nephrons continue to form during fetal life

8. collecting tubules fuse with distal convoluted tubules

9. glomerular capillaries are formed by lateral plate mesoderm which is induced to grow in and form blood vessels. These vessels connect to branches of the dorsal aorta

What does the metanephric mesenchyme form?

excretory parts:

glomerulus, proximal convoluted tubule, nephron loop (loop of Henle), and distal convoluted tubule.

What does the ureteric bud form?

urine collecting system:

collecting ducts, calyces, renal pelvis, and ureter.

When does urine production by the metanephric kidney begin?

10th week of gestation

Describe the development of the urogenital sinus

1. In week 4, the terminal portion of the hindgut endoderm dilates to form the cloaca

2. formation of the urorectal septum divides the cloaca into 2 chambers: urogenital sinus (ventral) and anorectal canal (dorsal)

3. both separated from the exterior by the cloacal membrane (eventually breaks down).

4. urogenital sinus divides into 3 parts: vesical, pelvic and phallic part

Describe the vesical part of the urogenital sinus

forms most of the bladder and is continuous with the allantois.

Describe the pelvis (urethral) part of the urogenital sinus

becomes the urethra in the neck of the bladder, the prostatic part of the urethra in males, and the entire urethra in females.

Describe the phallic part of the urogenital sinus

grows toward the genital tubercle (the primordium of the penis or the clitoris).

What does the endoderm of the urogenital sinus form?

the epithelial lining of the bladder and most of the urethra

What does the splanchnic mesoderm of the urogenital sinus form?

mesodermal components of the bladder and urethra

Describe the development of the ureters

1. Mesonephric ducts connect to the urogenital sinus.

2. Ureteric bud arises from the mesonephric duct close to this junction

3. Mesonephric duct opens into dorsal wall of urogenital sinus (> posterior bladder wall)

4. As the bladder grows its expanding wall incorporates first the mesonephric ducts then the ureteric buds resulting in these structures opening separately into the bladder.

5. As the kidneys ascend, the ureters elongate and open into the bladder more superiorly, while the roots of the mesonephric ducts are carried inferiorly.

Describe the development of the bladder

1. The bladder develops mainly from the vesical part of the urogenital sinus.

2. The trigone of the bladder is derived from the caudal ends of the mesonephric ducts.

3. The bladder is continuous with the allantois which constricts and becomes a thick, fibrous cord, th e urachus.

4. As the bladder enlarges, distal parts of the mesonephric ducts are incorporated into its dorsal wall

What is the remnant of the urachus in adults?

median umbilical ligament

Describe the development of the urethra

1. The epithelium of most of the male urethra and the entire female urethra is derived from the endoderm of the urogenital sinus.

2. The distal part of the urethra in the glans penis is derived from a solid cord of ectodermal cells that grows from the tip of the glans and joins the rest of the spongy urethra.

3. The connective tissue and smooth muscle of the urethra in both sexes are derived from splanchnic mesoderm.

Describe the ascent of the kidneys

1. Metanephric kidney initially develops in the caudal/sacral/pelvic region.

2. Kidneys relocate to abdomen and move further apart, rotating ~90 degrees medially.

3. Final position is in the cranial portion of abdominal cavity (under the suprarenal/adrenal gland).

4. Movement cranially due (partly) to degeneration of mesonephric kidneys (week 6).

5. Initially, the renal arteries are branches of the common iliac arteries .

6. Later the kidneys are supplied by lateral branches of the dorsal aorta.

7. During ascent of kidneys, several renal arteries supply the kidney in sequence.

8. The first renal arteries degenerate as the new ones form cranial to them.

Pelvic kidney

Failure of kidneys to ascend

Horseshoe kidney

If the inferior poles of the kidneys fuse before ascent, the resulting horseshoe kidney does not ascend to a normal position because of entrapment by the inferior mesenteric artery.

Function of fetal kidney

• Begins at start of second trimester.

• Does not mature until after birth

• Placenta does most removal of wastes

• Fetal kidney does some filtering of blood and processes a very dilute urine (excreted into the amniotic fluid).

• The fetal kidneys are subdivided into lobes. Lobes disappear during infancy as the nephrons increase in size.

• Functional maturation of the kidneys occurs after birth, but no additional nephrons are formed.

• Newborn kidneys are functionally and structurally immature and cannot concentrate urine well.

When is nephron formation complete?

week 36

Describe the development of the adrenal cortex

1. Neural crest cells migrate into the intermediate mesoderm

2. Become surrounded by intermediate mesodermal cells

3. Mesenchyme of urogenital ridge (intermediate mesoderm) forms fetal cortex that regresses after birth

4. Definitive cortex differentiates late in fetal life but is not fully formed until year 3.

Describe the development of the adrenal medulla

1. Neural crest cells migrate into the intermediate mesoderm

2. Become surrounded by intermediate mesodermal cells

3. Medulla develops from neural crest cells that differentiate into endocrine cells and become innervated by preganglionic sympathetic neurons (and release catecholamines).