Development of Thoracic Diaphragm

Question 1

Bilaminar Germinal Disc

Answer 1

• Appears in week 2 of development
• Formed from embryoblast (from inner cell mass)
• Composed of epiblast (blue) and hypoblast (yellow)

Question 2

Gastrulation

Answer 2

• Process by which 3 primary germ layers are formed and axial orientation (cranial-caudal ends as well as left and right sides)
  
  • Ectoderm (skin and nervous system)
  • Mesoderm (muscle, kidneys, heart)
  • Endoderm (gut tube)

Question 3

Bilaminar Disc and Primitive Streak

Answer 3

• Week 3: Primitive Streak (at the caudal end of the embryo)
  
  • Thickened band of epiblast
    
    • ​Epiblast cells invaginate and replaced hypoblast to form endoderm
    • Migrating cells form intraembryonic mesoderm
    • Remaining cells are ectoderm
    • Formation of trilaminar embryonic disc
  • The craniocaudal axis is established

Question 4

Mesodermal Differentiation

Answer 4

• Mesoderm divided into:
  
  • Paraxial Mesoderm
  • Intermediate Mesoderm
  • Lateral Plate Mesoderm
    
    • ​Cotinuous with extraembryonic mesoderm
    • Divided by intaembryonic coelom (embryonic cavity)
    • Somatic Mesoderm (body wall)
    • Splanchnic Mesoderm (GI tract)

Question 5

Folding of Embryonic Disc

Answer 5

Embryo and amnion grow more than the yolk sac

• Results in overgrowth on cranial and caudal ends

Craniocaudal Folding

• Forms future cranial and caudal regions
• Septum Transversum
  
  • Forms central tendon of diaphragm
  • Divides intraembryonic coelum into:
    
    • Pericardial Cavity
    • Peritoneal Cavity

Lateral Folding

• Occurs as sides of the embryo fuse
• Pinch off yolk sac except for vitelline duct (connection between yolk sac and gut)

Question 6

Thoracic Diaphragm

Answer 6

• Separates thorax and abdomen
• Primary muscle of respiration
• Innervation:
  
  • L and R Phrenic Nerves
    
    • ​C3, C4, C5

Question 7

Thoracic Diaphragm Embryological Development

Answer 7

• Develops from 4 structures that come together to form neonatal diaphragm:
  
  • Septum Transversum: mesoderm
    
    • Separates pericardial and peritoneal cavities
    • Forms central tendon
  • (2) Pleuroperitoneal Membranes:
    
    • Forms as lungs develop
    • Separates lungs from abdomen
  • Dorsal Mesentery of the Esophagus:
    
    • ​Holds esophagus in place
    • Forms crura of diaphragm
  • Lateral Body Walls:
    
    • Forms lateral muscular walls

Question 8

Septum Transversum Development

Answer 8

• Originally at level of C3-C5 somites
  
  • ​Nerve components of 3-5th cervical segments of the spinal cord grow into the septum
• Due to rapid growth of the dorsal part of the body in weeks 5-7, the dorsal diaphragm migrates to level of L1 vertebra

Question 9

Congenital Diaphragmatic Hernia

Answer 9

• Result of pleuroperitoneal membranes failing to completely fuse on one side
  
  • Protrusion of abdominal contents
• More common on left side
  
  • Liver helps facilitate closure of pleuroperitoneal membrane on right side
• Usually leads to respiratory failure if not treated in utero
  
  • Abdominal herniation pushes lungs out of way leading to hypoplastic lungs (underdevelopment)