Urogenital Development - Short Notes

1. Overview

2. Development of the Kidney (Three Successive Stages)

A. Pronephros

• Appears at day 22 of gestation (E8 in mice)
• Most rostral part of the urogenital ridge
• Consists of pronephric tubules and the pronephric duct (precursor of the Wolffian duct)
• Functional excretory organ in fish/amphibia larvae; vestigial in mammals
• Regresses by day 25

B. Mesonephros

• Appears at day 24 (E10 in mice); disappears by week 16
• Develops caudal to the pronephros in the midsection of the urogenital ridge
• Functional excretory organ in lower vertebrates; may filter during early embryonic life in mammals
• Before degeneration, endothelial, peritubular myoid, and steroidogenic cells migrate to the adrenogenadal primordium - abnormal migration causes gonadal dysgenesis
• In males: mesonephric (Wolffian) duct persists; in females: degenerates

C. Metanephros - The Definitive Kidney

• Begins at weeks 4-5 (day 28-32; E10.5 in mice)
• Results from reciprocal inductive interactions between:
  • Ureteric bud (UB) - outgrowth from the distal Wolffian duct
  • Metanephric mesenchyme (MM) - adjacent cluster of cells

Reciprocal Induction

1. MM signals (GDNF, HGF) cause UB to branch into a T-tubule (E11.5) and then undergo iterative dichotomous branching → forms the entire collecting duct system
2. UB sends reciprocal signals back to MM, inducing condensation
3. Condensed MM forms pretubular aggregates → undergo mesenchymal-to-epithelial transition → form the renal vesicle → elongates into S-shaped body → becomes the nephron (from Bowman capsule to distal convoluted tubule)

Key Molecular Regulators

Timeline of Nephron Formation

• Glomeruli increase from weeks 10-18, then rapidly to week 36
• Nephron formation is complete at birth: 200,000 to 2 million nephrons per kidney
• No new nephrons form after birth - limited nephron number has lifelong health consequences
• Glomerular filtration begins at ~9th fetal week
• Brenner and Rector's The Kidney; The Developing Human - Clinically Oriented Embryology; Langman's Medical Embryology

3. Ascent of the Kidney

• Metanephros initially lies in the pelvis; ascends to the lumbar region by week 9
• Ascent occurs as the lumbar/sacral region grows caudally
• Arterial supply changes as the kidney ascends (original pelvic arteries degenerate; new vessels arise from the aorta)
• Failure to ascend → pelvic kidney
• Fusion of the lower poles before rotation → horseshoe kidney (caught on the inferior mesenteric artery, preventing ascent above L3)
• Crossed fused renal ectopia: both kidneys fused and on the same side of the midline

4. Development of the Urinary Bladder and Urethra

Cloaca and Urorectal Septum

• By week 5, the fetal cloaca is identifiable (common chamber receiving hindgut, allantois, and vestigial tailgut)
• The urorectal septum grows caudally (weeks 4-6) and divides the cloaca into:
  • Ventral urogenital sinus
  • Dorsal anorectal canal
• The urogenital sinus is then divided into three parts:

Bladder

• Epithelium: endoderm of the vesical part of the urogenital sinus
• Wall musculature: adjacent splanchnic mesenchyme
• Initially continuous with the allantois, which constricts → becomes the urachus (fibrous cord from bladder apex to umbilicus) → in adults = median umbilical ligament
• Distal mesonephric ducts are absorbed into the dorsal bladder wall → form connective tissue of the trigone
• Ureters open separately into the bladder; traction from ascending kidneys causes orifices to move superolaterally (oblique course)

In Males

• Distal mesonephric duct ends → become ejaculatory ducts
• Multiple endodermal outgrowths from prostatic urethra → prostate gland (glands from endoderm, stroma from mesenchyme)
• Campbell-Walsh-Wein Urology; The Developing Human; Grainger & Allison's Diagnostic Radiology

5. Development of the Gonads

Origin of Gonads (Three Sources)

1. Mesothelium lining the posterior abdominal wall (coelomic epithelium)
2. Underlying mesenchyme
3. Primordial germ cells (PGCs)

Primordial Germ Cells (PGCs)

• First recognizable at day 24 in the endoderm of the umbilical vesicle near the allantois
• Migrate along the dorsal mesentery of the hindgut to the gonadal ridges
• Enter gonadal cords by the 6th week
• Migration regulated by genes stella, fragilis, and BMP-4

Indifferent (Bipotential) Gonad

• Forms during week 5: mesothelium thickens on medial side of mesonephros → gonadal ridge with finger-like gonadal cords growing inward
• Consists of an outer cortex and inner medulla
• Transcription factors FOG2, WT1, NR5A1 required for bipotential gonad development
• Gonads remain morphologically identical until week 7 (indifferent stage)

Sex Determination

• Chromosomal sex determined at fertilization
• Morphological differentiation begins at week 7
• SRY gene (short arm of Y chromosome) encodes testis-determining factor (TDF)

Testis Development

• SRY activates Sox9 → gonadal cords become seminiferous cords
• Seminiferous cords: contain spermatogonia (from PGCs) + Sertoli cells (from mesenchyme)
• Cords enter medulla → rete testis
• Tunica albuginea forms, separating cords from surface epithelium
• Leydig cells (interstitial) produce testosterone from week 8

Ovary Development

• No TDF → cortical cords extend from surface epithelium
• PGCs within cortical cords → oogonia
• Follicular (granulosa) cells derived from surface epithelium
• Rete ovarii degenerates
• The Developing Human - Clinically Oriented Embryology

6. Development of the Genital Ducts

Indifferent Stage

• Mesonephric (Wolffian) ducts - drain the mesonephros
• Paramesonephric (Müllerian) ducts - form as evaginations in coelomic epithelium lateral to Wolffian duct (~week 6)

Male Differentiation (requires active hormonal signals)

• Proximal portion → head, body, tail of epididymis
• → Vas deferens
• Lateral outgrowth → seminal vesicle
• Distal segment → ejaculatory duct
• Multiple endodermal urethral outgrowths → prostate

Female Differentiation (default pathway - no active hormonal induction required)

• Absence of AMH → Müllerian ducts persist
• Absence of testosterone → Wolffian ducts degenerate

• Cranial portions remain separate → fallopian tubes (fimbriae from müllerian tunnels)
• Midportions fuse → fundus and corpus of uterus
• Distal fused portions → cervix + upper 1/3 of vagina (uterovaginal primordium)
• Lower 2/3 of vagina + external female genitalia → from urogenital sinus
• Medical Physiology (Boron & Boulpaep); Campbell-Walsh-Wein Urology; Color Atlas of Human Anatomy

7. Development of the Adrenal Glands

• Cortex: mesenchyme of the urogenital ridge (6th week); cells aggregate between dorsal mesentery root and developing gonad
• Medulla: neural crest cells migrating from adjacent sympathetic ganglion
• Fetal adrenal glands are 10-20x larger relative to body weight than adult glands
• Zona glomerulosa and fasciculata present at birth; zona reticularis not recognizable until year 3
• Key regulators: SF1, DAX1, Pbx1 for cortical development

8. Key Congenital Anomalies (Summary)

9. Quick Summary Timeline

Development of the Heart - Short Notes

1. Overview and Timeline

2. Origin - Heart Fields

Two Heart Fields

• Neural crest cells also contribute, migrating into the outflow tract region for septation and into the aortic arches
• Laterality (left-right patterning) is established simultaneously; the PITX2 transcription factor (master gene for left-sidedness) programs heart cells in both fields

Key Molecular Signals for Heart Induction

• BMPs (from endoderm and lateral plate mesoderm) + inhibition of WNT by CRESCENT → induce NKX2.5 in PHF
• NKX2.5 (homolog of Drosophila tinman) - master cardiac transcription factor
• TBX5 - expressed later; critical for septation
• HAND1/HAND2 - downstream of NKX2.5; expressed in future left and right ventricles respectively
• Looping regulated by: PITX2, NKX2.5, HAND1/HAND2, SONIC HEDGEHOG (SHH)

3. Formation of the Single Heart Tube

1. PHF cells form blood islands in a horseshoe-shaped region (cardiogenic region) above the oropharyngeal membrane
2. Islands unite → a horseshoe-shaped endothelial-lined tube surrounded by myoblasts
3. With embryonic folding (cranial and lateral), the paired tubes fuse in the midline at the cephalic end (caudal ends remain initially separate)
4. Result: a single primitive heart tube in the pericardial cavity

Layers of the Primitive Heart Tube

1. Endocardium - inner endothelial lining
2. Myocardium - thickens and secretes cardiac jelly (extracellular matrix rich in hyaluronic acid)
3. Epicardium (visceral pericardium) - from the proepicardial organ (mesenchymal cells at caudal border of dorsal mesocardium); also gives rise to coronary artery endothelium and smooth muscle

Regions of the Heart Tube (Cranial → Caudal)

1. Truncus arteriosus → roots/proximal portions of aorta and pulmonary artery
2. Bulbus cordis (conus cordis) → outflow tracts of both ventricles
3. Primitive ventricle → trabeculated portion of left ventricle (mainly)
4. Primitive atrium → trabeculated portions of both atria
5. Sinus venosus → smooth-walled right atrium (sinus venarum) + coronary sinus + oblique vein of left atrium

4. Cardiac Looping (Days 23-28)

• Cephalic portion bends ventrally, caudally, and to the RIGHT
• Atrial (caudal) portion shifts dorsocranially to the LEFT
• Creates the cardiac loop (D-loop) - complete by day 28

Looping is the first morphological sign of left-right asymmetry. Abnormal looping → dextrocardia or situs inversus.

5. Development of the Sinus Venosus

• Mid-week 4: Sinus venosus drains blood from both right and left sinus horns via three veins each: vitelline, umbilical, and common cardinal veins
• Left-to-right shunts during weeks 4-5 → entrance of sinus shifts to the right
• Right umbilical vein + left vitelline vein obliterate
• Left common cardinal vein obliterates at week 10 → left sinus horn remnants become:
  • Oblique vein of the left atrium
  • Coronary sinus
• Right sinus horn incorporated into the posterior right atrial wall → smooth-walled sinus venarum
• Right and left venous valves form; right valve contributes to crista terminalis and the Eustachian (inferior vena caval) valve

6. Atrial Septation

Formation of Interatrial Septum (End of week 4 through week 6)

• A sickle-shaped crest grows from the roof of the common atrium downward toward the endocardial cushions
• The gap between its lower rim and the cushions = ostium primum (allows R→L flow)
• Endocardial cushion extensions grow up to close ostium primum
• Before closure: apoptosis in the upper septum primum creates perforations → coalesce into ostium secundum (maintains R→L flow)

• A new crescent-shaped fold (septum secundum) descends from the roof of the right atrium, to the right of septum primum
• Never completes as a full partition
• Its free concave edge overlaps the ostium secundum → leaves the foramen ovale (oval foramen)
• Upper part of septum primum gradually disappears → remaining portion = valve of the foramen ovale

AV Canal Septation

• End of week 4: 4 endocardial cushions appear in the AV canal (superior, inferior, and 2 lateral)
• Superior and inferior cushions fuse → divide AV canal into right and left AV orifices (by end of week 5)
• AV canal enlarges to the right → blood now has access to both primitive ventricles

AV Valve Formation

• After cushion fusion, surrounding tissue is hollowed out from the ventricular side
• Forms the valve leaflets, attached to ventricular wall by muscular cords
• Muscular cords degenerate → chordae tendineae (dense connective tissue)
• Connected to papillary muscles
• Left AV canal → bicuspid (mitral) valve (2 leaflets)
• Right AV canal → tricuspid valve (3 leaflets)

Left Atrium

• Primitive left atrium expands
• A pulmonary vein appears from the dorsal mesocardium (not as an outgrowth of the atrium itself)
• Stem of pulmonary vein incorporated into left atrium → becomes the smooth-walled portion of the left atrium
• Original embryonic left atrium becomes only the trabeculated left atrial appendage

7. Ventricular Septation

1. Two primitive ventricles expand by myocardial growth + diverticulation/trabeculation
2. Medial walls appose and merge → muscular interventricular septum (grows upward from apex)
3. Gap between free rim and fused endocardial cushions = interventricular foramen (allows communication)

• Outgrowth of tissue from inferior endocardial cushion grows along top of muscular septum
• Fuses with parts of the conus (conotruncal) septum
• Forms the membranous part of the interventricular septum

8. Outflow Tract (Conotruncal) Septation

• Spiral conotruncal ridges (formed by neural crest cells + SHF-derived mesenchyme) appear in the conus cordis and truncus arteriosus
• These ridges spiral 180° and fuse to form the aorticopulmonary septum
• Spiral nature ensures: pulmonary artery wraps around the aorta
• Aorta → exits from left ventricle; Pulmonary trunk → exits from right ventricle

Semilunar Valves

• Small tubercles appear on the truncus swellings when partitioning is nearly complete
• Each of the two channels (aortic and pulmonary) gets 3 tubercles
• Tubercles hollow out on their upper surface → form 3 semilunar valve cusps each
• Neural crest cells contribute to formation of these valves

9. Congenital Heart Defects Summary

10. Key Timeline