Day 13, Lecture 4: Human Development 4: Weeks 4-8

Question 1

Mesododerm is a germ layer not a cell type may be composed of ____ or ___ throughout the embryonic disk except at

Answer 1

• Mesenchymal or epithelial cells
• throughout the embryonic disk except at the oropharyngeal and cloacal membranes

Question 2

_____ results in the formation of the three germ layers

Answer 2

• Gastrulation

Question 3

Mesoderm is organized into 1 midline and 3 paired regions, name them?

Answer 3

• Midline
  
  • Notochord
• Paired regions
  
  • Paraxial mesoderm
  • Intermediate mesoderm
  • Lateral plate mesoderm

Question 4

What major events occur in the fourth week of development

Answer 4

• Organization of mesoderm into groups of tissue
  
  • Notochord
  • Paraxial Mesoderm
  • Intermediate Mesoderm
  • Lateral Plate Mesoderm
• Segmentation of paraxial mesoderm into somites
• Body folding
  
  • 3-D establishment of body plan

Question 5

Organization of Paraxial Mesoderm

Answer 5

• Cranio-Caudal sequence of development initially organize into partially segmented structures termed somitomeres
• More caudal paraxial mesoderm completely separates into segments termed somites
  
  • ​​7 cephalic somitomeres
  • 42-44 somites (occipital + trunk)

Question 6

What are somites

Answer 6

cuboidal blocks of mesoderm compsed of epithelial cells formed from the paraxial mesoderm

Question 7

Somites disperse to form

Answer 7

• sclerotome
  
  • forms from ventromedial side of somite
  • forms axial skeleton (vertebrae and ribs)
• dermamyotome
  
  • ​forms from dorsolateral side of somite
  • Disperses to form two groups of cells
    
    • Dermatome
      
      • Contributes to dermis
    • Myotome
      
      • Forms all skeletal muscle

Question 8

Question 9

Intermediate mesoderm forms

Answer 9

• Urogenital system

Question 10

Answer 10

Question 11

cervical spinal nerves are numbered by vertebrae below with the exception of

Answer 11

• Spinal nerve C8
• (note the rest of spinal nerves are numbered by vertebrae above)

Question 12

Lateral Plate Mesoderm

Answer 12

• Lateral plate mesoderm splits:
  
  • Forms embryonic coelom
  • Somatic and Splanchnic mesoderm, which flank the embryonic coelom
• Somatopleure
  
  • Somatic mesoderm + ectoderm
    
    • body wall
• Splanchnopleure
  
  • Splanchnic mesoderm + endoderm
    
    • Visceral wall
• Embryonic coelom
  
  • Forms body cavities
    
    • Pleural
    • Pericardial
    • Peritoneal

Question 13

Medial Folding

Answer 13

• occurs in the 4th week of development
• Head folding
  
  • Cardiogenic region and oropharyngeal membrane “swing” ventrally and caudally
• Tail Fold
  
  • Cloacal membrane “swings” ventrally and cranially
• Part of yolk sac incorporated into embryo as primitive gut tube

Question 14

Embryonic Folding

Answer 14

• Occurs during 4th week of development
• Establishment of body plan
• 3-layered disk transformed into cylindrical embryo by median and horizontal folding
  
  • Median folding (head and tail folding)
  • Horizontal folding (lateral folds)
• Purse-string closure mechanism
  
  • put string around periphery of embryonic disk
  • tighten string around ventral side of embryo
  • closure region is umbilical ring

Question 15

Horizontal folding (lateral folds)

Answer 15

• Occurs in the 4th week of development
• Lateral plate mesoderm moves ventrally and medially
• participates in forming body wall

Question 16

Lateral folding in week 4 of developement

Question 17

Molecular Regulation of Somite Differentiation

Answer 17

• Sclerotome formation
  
  • SHH produced by neural tube and notochord induce ventromedial portion of somite to become sclerotome
  • ​Sclerotome expresses PAX-1 initiates cartilage and bone formation
• Dermamyotome formation
  
  • WNT from dorsal neural tube and epidermis induces dorsolateral portion to become dermamyotome
  • Medial portion expresses MYF5 and becomes epaxial musculature
  • lateral portion induced by BMP4 expresses MYOD and becomes hypaxial musculature

Question 18

Craniocaudal Patterning of embryo

Answer 18

• Hox genes organized in specific spatial pattern on chromosomes
• Hox gene expression occurs in specific spatial pattern based on gene organization
• Spatial expression of Hox genes determines pattern of somite segmentation
  
  • Clusters of genes are on separate chromosomes
  • Genes at the 3’ end of the cluster are expressed cranially while genes at the 5’ end are expressed caudally
• Hox6 normally expressed in thoracic region where ribs form

Question 19

Which hox gene expression regulates Rib formation

Answer 19

• Hox6 normally expressed in thoracic region where ribs form
• Ectopic expression of Hox6 in lumbar region promotes abnormal rib formation

Question 20

Holoprosencephaly

Answer 20

• Insult in 3rd week of development
  
  • Development occuring during week after 1^st missed menses
• Kills cells in anterior midline
• single brain ventricle, cyclopia
• Possible reasons:
  
  • may be caused by alcohol
  • Genetic defect in SHH

Question 21

Caudal Dysgenesis (Sirenomelia)

Answer 21

• Insufficient mesoderm forms in caudal-most region
• Effects all caudal mesoderm derived structures
  
  • lower limbs
  • kidneys
  • genital organs
  • rectum/anus
  • bladder

Question 22